L, upon it, then the lines of force and the equipotential planes will be distorted, as shown in Fig. 3. If the hill or building be so high as to make the distance H h or L l equal to e f (Fig. 2), then we shall again have disruptive discharge.
If instead of a hill or building we erect a solid rod of metal, G H, then the field will be distorted as shown in Fig. 4. Now, it is quite evident that whatever be the relative distance of the cloud and earth, or whatever be the motion of the cloud, there must be a space, g g’, along which the lines of force must be longer than a’ a or H H’; and hence there must be a circle described around G as a center which is less subject to disruptive discharge than the space outside the circle; and hence this area may be said to be protected by the rod, G H. The same reasoning applies to each equipotential plane; and as each circle diminishes in radius as we ascend, it follows that the rod virtually protects a cone of space whose height is the rod, and whose base is the circle described by the radius, G a. It is important to find out what this radius is.
[Illustration: Fig. 5]
Let us assume that a thunder-cloud is approaching the rod, A B (Fig. 5), from above, and that it has reached a point, D’, where the distance. D’ B, is equal to the perpendicular height, D’ C’. It is evident that, if the potential at D be increased until the striking-distance be attained, the line of discharge will be along D’ C or D’ B, and that the length, A C’, is under protection. Now the nearer the point D’ is to D the shorter will be the length A C’ under protection; but the minimum length will be A C, since the cloud would never descend lower than the perpendicular distance D C.
Supposing, however, that the cloud had actually descended to D when the discharge took place. Then the latter would strike to the nearest point; and any point within the circumference of the portion of the circle, B C (whose radius is D B), would be at a less distance from D than either the point B or the point C.
_Hence a lightning-rod protects a conic space whose height is the length of the rod, whose base is a circle having its radius equal to the height of the rod, and whose side is the quadrant of a circle whose radius is equal to the height of the rod._
I have carefully examined every record of accident that was available, and I have not yet found one case where damage was inflicted inside this cone when the building was properly protected. There are many cases where the pinnacles of the same turret of a church have been struck where one has had a rod attached to it; but it is clear that the other pinnacles were outside the cone; and therefore, for protection, each pinnacle should have had its own rod. It is evident also that every prominent point of a building should have its rod, and that the higher the rod the greater is the space protected.
* * * * *
PHOTO-ELECTRICITY OF FLUOR-SPAR CRYSTALS.
Hantzel has communicated to the Saxon Royal Society of Science some interesting observations on the production of electricity by light in colored fluor-spar. The centers of the fluor-spar cubes become negatively electric by the action of light. The electric tension diminishes toward the edges and angles, and frequently positive polarity is produced there. With very sensitive crystals a short exposure to daylight is sufficient; by a long exposure to light the electric current increases. The direct rays of the sun act much more powerfully than diffused daylight, and the electric carbon light is more powerful even than sunlight. The photo-electric action of light belongs principally to the “chemically active” rays; this is shown by the fact that the production of electricity is extremely small behind a glass colored with cuprous oxide, and behind a film of a solution of quinine sulphate; while it is not appreciably diminished by a film of a solution of alum. The photo-electric excitability of fluor-spar crystals is increased by a moderate heat (80 deg. to 100 deg. C.).
* * * * *
THE AURORA BOREALIS AND TELEGRAPH CABLES.
The January and February numbers of the _Elektrotechnische Zeitschrift_ contain a number of articles on this interesting subject by several eminent electricians. Professor Foerster, director of the observatory in Berlin, points out the great importance of the careful study of earth currents, first observed at Greenwich, and now being investigated by a committee appointed by the German Government. He further points out, according to Professor Wykander, of Lund, in Sweden, that a close connection exists between earth currents, the protuberances of the sun, and the aurora borealis, and that the nearly regular periodical reappearance of protuberances in intervals of eleven years coincides with similar periods of excessive magnetic earth currents and the appearance of the aurora borealis. The remarkable disturbing influences on telegraph wires and cables of the aurora borealis observed from the 11th to 14th of August, 1880, have been carefully recorded by Herr Geh. Postnath Ludwig in Berlin, and a map of Europe compiled, showing the places affected, with the extent to which telegraph wires and cables were influenced and disturbed. Although the aurora was but faintly visible in England and Germany, and in Russia only as far as 35 deg. north, disturbing influences were reported from all parts of Europe, the Mediterranean, and Africa, and even Japan and the east coast of Asia. As far south as Zanzibar, Mozambique, and Natal disturbances were also noticed. They were in Europe most intense on the morning of August 12, when they lasted the whole day, and increased again in intensity toward eight o’clock in the evening, while they suddenly ceased everywhere almost simultaneously. Scientific and careful observations were only taken at a few places, but the existence of earth currents in frequently changing direction and varying intensity, was noticed everywhere. Long lines of wires were more affected than short ones, and although some lines–for instance the Berlin-Hamburg in an east-west direction–were not at all influenced, no general law was noticed according to which certain directions were freed from the disturbing influence. While, for instance, the Red Sea cable was not noticeably affected, the land line to Bombay, forming a continuation of this cable, was materially disturbed. The Marseilles-Algiers cable, so seriously influenced in 1871, showed no signs at all, but as may be expected, the north of Europe suffered more than the south, and in Nystad, Finland, the galvanometer indicated an intensity of current equal to that of 200 Leclanche cells.
Since thunderstorms are generally local, it is only natural that their effect upon telegraph cables should also be confined to one locality. Numerous careful observations, carried out over considerable periods of time, show that the disturbing influences of thunderstorms on telegraph lines are of less duration and more varying in direction and intensity than those of the aurora borealis. Long lines suffer less than short lines; telegraph wires above ground are more easily and more intensely affected than underground cables. It is, however, possible, that this is mainly due to the fact that in the districts where strict records were kept, in the German Empire, most of the long lines are underground cables, while most of the short local lines are overground wires. The results of the disturbances varied; in Hughes’s apparatus the armatures were thrown off, lines in operation indicated wrong signs, dots became dashes, and the spaces were either multiplied in size or number, according to the direction of the earth currents induced by the thunderstorms. Since these observations extended over nearly 2,000 cases, some conclusions might fairly be drawn from them. For the purpose of a more complete knowledge on this subject, Dr. Wykander recommends a series of regular observations on earth currents to be carried out at different stations, well distributed over the whole surface of the globe, these observations to be made between six and eight A.M., and at the same time in the evening. Special arrangements to be made at various stations to record exceptionally intense disturbances during the phenomena of the aurora borealis, notice to be taken of time, direction, intensity, and all further particulars. Since this question appears to bear a considerable amount of influence on underground cables, it is one that deserves serious attention before earth cables are more generally introduced; there can, however, be little doubt that they are not nearly so much exposed as overhead wires to disturbing influences of other kinds, such as snow, rain, wind, etc., while they certainly do suffer, though perhaps in a less degree, by electrical disturbances.–_Engineering_.
* * * * *
THE PHOTOGRAPHIC IMAGE: WHAT IT IS.
[Footnote: A communication to the Sheffield Photographic Society in the _British Journal of Photography_.]
It is quite possible that in the remarks I propose making this evening in connection with the photographic art I may mention topics and some details which are familiar to many present; but as chemistry and optical and physical phenomena enter largely into the theory and practice of photography, the field is so extensive there is always something interesting and suggestive even in the rudiments, especially to those who are commencing their studies. Although this paper may be considered an introductory one, I do not wish to load it with any historical account, or describe the early methods of producing a light picture, but shall at once take for my subject, “The Photographic Image: What It Is,” and under this heading I must restrict myself to the collodion and silver or wet process, leaving gelatine dry plates, collodio-chloride, platinum, carbontype, and the numerous other types which are springing up in all directions for future consideration.
Now, in an ordinary pencil, pen and ink, or sepia sketch we have a deposit of a dark, non-reflecting substance, which gives the outline of a figure on a lighter background. The different gradations of shade are acquired by a more or less deposit of lead, ink, or sepia. In photography–at least in the ordinary silver process–the image is formed by a deposition of metallic silver or organic oxide in a minute state of division, either on glass, paper, or other suitable material. This is brought about by the action of light and certain reagents. Light has long been recognized as a motive power comparable with heat or electricity. Its action upon the skin, fading of colors, and effect on the growth of vegetable and animal organisms are well known; and, although the exact molecular change in many instances is not clearly understood, yet certain salts of silver, iron, the alkaline bichromates, and some organic materials–as bitumen and gelatine–have been pretty well worked out.
It is a remarkable and well-known fact that the chloride, iodide, and bromide of silver–called “sensitive salts” in photography–are not susceptible (at least only slowly) to change when exposed to the yellow, orange, and red rays. The longer wave lengths of the spectrum, as you know, form, with violet, indigo, blue, and green, white light. The diagram on the wall shows this dispersion and separation of the primitive colors. These–the yellow, orange, and red– are called technically “non actinic” rays, and the others in their order become more actinic until the ultra violet is reached. The action of white light, or rays, excluding yellow, orange, and red, has the effect of converting silver chloride into a sub-chloride; it drives off one equivalent of chlorine. Thus, silver chloride, Ag_2Cl_2=Ag_2Cl+Cl. When water is present the water is decomposed. Hydrochloric acid, HCl, hypochlorous acid, HClO is formed.
The iodide of silver in like manner is changed into a sub-iodide; but with water hydriodic acid is formed unless an iodine absorbent be present–then into hypoiodic acid. The silver bromide undergoes a similar change. When with light alone, a sub-bromide, Ag_2Br_2=Ag_2Br+Br, and with water hypobromous acid. It is important to bear this in mind, as one or other, and frequently both iodide and bromide of silver, is the sensitive salt requisite or used in producing the invisible image.
The theory regarding these sensitive salts of silver is that, being very unstable, _i. e._, ready to undergo a molecular change, the undulations produced in the ether, which pervades all space, and the potential action or moving power of light is sufficient to disturb their normal chemical composition; it liberates some of the chlorine, iodine, or bromine, as the case may be. This action, of course, applies to light from any source–the sun, electricity, or the brighter hydrocarbons, also flame from gas or candle, whether it comes direct as rays of white light or is reflected from an object and conducted through a lens as a distinct image upon the screen of a camera.
I have no time to speak on the subject of lenses, only just to mention that they are, or ought to be, achromatic, so as to transmit white light and of perfect definition, and the amount of light passed through should be as much as possible consistent with a sharp image–at least when rapid exposure is attempted.
I shall touch very lightly on the manipulative part of photography, as that would be unnecessary; but a brief account of the chemicals in use is essential to a right appreciation of the theory of developing the image. In the first place, our object is to get a film of some suitable material coated with a thin layer of a sensitive salt of silver–say a bromo-iodide. By mixing certain proportions of ammonium iodide and cadmium bromide, or an iodide and bromide of cadmium with collodion–which is pyroxyline, a kind of gun-cotton dissolved in ether and alcohol–a plate of glass is coated, and before being perfectly dry is immersed in the nitrate of silver bath. The silver nitrate solution, adhering and entering to a slight extent the surface of the collodion, becomes converted by an ordinary chemical action of affinity into silver iodide and bromide.
The ammonium and cadmium play a secondary part in the process, and are not absolutely necessary in forming the image. The plate is now extremely sensitive to light. When we have entered it into the dark slide and camera, and then exposed to light, the change I mentioned has taken place. The film is transformed into different quantities of sub-iodide and sub-bromide of silver, according to brilliancy of light. In addition, there is on the plate an amount of unchanged silver nitrate which becomes useful in the second stage, or development. The image is not seen as yet, being latent, and requiring the well-known developing solution of sulphate of iron, acetic acid, alcohol, and water. Practically we all recognize the effect of a nicely-balanced wave of developer worked round a plate. The high lights are first to appear as a darker color, till the details of shadow come out; when this is reached the developer is washed off. The chemical action is briefly thus, and it can be shown by solutions without a photographic plate, as in a test tube: Pour into this glass a solution of silver nitrate, AgNO, and add a solution of ferrous sulphate, FeSO_4. The ferrous sulphate combines with the nitric acid, forming two new salts–ferric nitrate and ferric sulphate. The silver is deposited. Any other substance which will remove oxygen from silver nitrate without combining with the silver would do the same, and metallic silver would be thrown down. The formula, as shown on the diagram, explains the interchange.
When the developer is poured over the plate it attacks first the free silver nitrate, and causes it to deposit extremely fine particles of metallic silver. The question arises: How is it these particles arrange themselves to form an image? This is explained by the physical movement known as molecular attraction or affinity. These particles are attracted first to the portions of the plate where there is most sub-iodide and sub-bromide. In the shady parts less silver is deposited. When the image is once started it follows that particles of silver produced by the iron developer will cause more to fall down on the face of those already present, and the image is, of course, built up if the silver nitrate be all consumed on the plate. The developer then becomes useless or injurious. The presence of acetic acid checks the reduction of the silver, and the alcohol facilitates the flow when the bath becomes charged with ether and spirit.
The molecular attraction just mentioned is made plainer by reference to the simple lead tree experiment. We have here in this bottle a piece of zinc rod introduced into a solution of acetate of lead. A chemical change has taken place. The zinc has abstracted the acetic acid and the lead is deposited on the zinc, and will continue to be so until the solution is exhausted. The irregularities of surface and arborescent appearance are well shown. If the change were rapidly conducted the lead particles would from their weight sink directly to the bottom instead of aggregating together like ordinary crystals. I have constructed a diagram of colored card, which will perhaps more clearly demonstrate the relation of the different constituents. The lower portion (Fig. a) represents a section of the glass plate or support, the collodion film (Fig. b) having upon its surface a thin layer of bromo-iodine silver (Fig. c), which, when exposed to a well-lighted image, as in a camera, changes into different gradations of sub-bromide and sub-iodide, as indicated by irregular, dark masses in the film. The dotted marks immediately above these are intended for the silver deposit (Fig. d)–clusters of granules, more abundant in the well lighted and less in the shaded parts of the picture, corresponding to the amount of sub-bromide and iodide beneath.
[Illustration: SECTION OF SENSITIVE PLATE AFTER EXPOSURE AND DURING DEVELOPMENT.
d Silver deposit–Image, c Sub-bromide and sub-chloride (gradations of), b Collodion film–Substratum, a Section of glass plate–Support.]
The next point to consider is that of intensification–a process seldom required in positive pictures, and would not be needed so often in negatives if there was enough free silver nitrate on the plate during development. The object, as we all know, in a wet-plate negative is to get good printing density without destruction of half-tone. It is a rule, I believe, in an over-exposed picture to intensify after fixing the image, and in an under-exposed picture to intensify before fixing. Whichever is done the intention is similar, namely, to intercept in a greater degree the light passing through a negative, so as to make a whiter and cleaner print. The usual intensifier–and, I suppose, there is no better–is pyrogallic acid, citric acid, water, and a few drops of silver nitrate solution. Pyrogallic is the most active agent, and might be used alone with water; but for special reasons it is not desirable. As a chemical it has a great affinity for oxygen, and will precipitate silver from a solution containing, for instance, nitrate of silver. It also combines with the metal, forming a pyrogallate–a dark brown, very non-actinic material. The use of a few drops of AgNO_3 solution is very evident. A deposit is added to the image already formed. Citric acid is the retarder in this case. Alcohol is unnecessary, as the film is well washed with water before the intensifier is used, consequently it flows readily over the plate.
As regards fixing, or, more properly, clearing the image: it is the simple act of dissolving out or from the film all free nitrate, chloride, iodide, or bromide. Cyanide of potassium does not attack the metallic deposit unless very strong. It has then a tendency to reduce the detail in the shadows.
THOMAS H. MORTON, M.D.
* * * * *
GELATINE TRANSPARENCIES FOR THE LANTERN.
[Footnote: A communication to the Photographic Society of Ireland.]
Few of those who work with gelatine dry plates seem to be aware of the great beauty of the transparencies for lantern or other uses which can be made from them by ferrous oxalate development with the greatest ease and certainty.
I think this a very great pity, for I hold the opinion that the lantern furnishes the most enjoyable and, in some cases, the most perfect of all means of showing good photographic pictures. Many prints from excellent negatives which may be passed over in an album without provoking a remark will, if printed as transparencies and thrown on the screen, call forth expressions of the warmest admiration; and justly so, for no paper print can do that full justice to a really good negative which a transparency does. This difference is more conspicuous in these days of dry gelatine plates and handy photographic apparatus, when many of our most interesting negatives are taken on quarter or 5 x 4 plates the small size of which frequently involves a crowding of detail, much of which will be invisible in a paper print, but which, when unraveled or opened out, as it were, by means of the lantern, enhances the beauty of the pictures immensely.
When I last had the pleasure of bringing this subject before the members of our society, it may be remembered that I demonstrated the ease and simplicity with which those beautiful results maybe obtained, by printing in an ordinary printing frame by the light of my petroleum developing lamp, raising one of its panes of ruby glass for the purpose for five seconds, and then developing by ferrous oxalate until I got the amount of intensity requisite. On that evening, in the course of a very just criticism by one of our members, Mr. J. V. Robinson, he pointed out what was undoubtedly a defect, viz., a slightly opalescent veiling of the high lights, which should range from absolutely bare glass in the highest points. He showed that, in consequence of this veiling, the light was sensibly diminished all over the picture. This veiling of the high lights was a serious disadvantage in another important particular, inasmuch as it lessened the contrast between the lights and shadows of the picture, thereby robbing it of some of its charm and deteriorating its quality.
Since that evening I have endeavored, by a series of experiments, to find out some means by which this opalescence might be got rid of in the most convenient manner. Cementing the transparency to a piece of plain, clear glass with Canada balsam, as suggested by Mr. Woodworth, I found in practice to be open to two formidable objections. One of these was that Canada balsam used in this manner is a sticky, unpleasant substance to meddle with, and takes a long time–nearly a month–to harden when confined between plates in this manner. The other objection was of extreme importance, namely, that, in consequence of commercial gelatine plates not being prepared on perfectly flat glasses in all cases, I found that, after squeezing out the superfluous balsam and the air bubbles that might have formed from between the two plates, they are liable to separate at the places where the transparency is not flat, causing air bubbles to creep in from the edges, as you may see from these examples. I, therefore, have discarded this method, although it had the effect desired when successfully done.
I have hit, however, upon another way of utilizing Canada balsam, which, while retaining all the good qualities of the former method, is not subject to any of its disadvantages. This consists in diluting the balsam with an equal bulk of turpentine, and using it as a varnish, pouring it on like collodion, flowing it toward each corner, and pouring it off into the bottle from the last corner, avoiding crapy lines by slowly tilting the plate, as in varnishing. If the plate be warmed previously, the varnish flows more freely and leaves a thinner coating of balsam behind on the transparency. When the plate has ceased to drip, place it in a plate drainer, with the corner you poured from lowest, and leave it where dust cannot get at it for four or five days, when it will be found sufficiently hard to be put into a plate box. The transparency may be finished at any time afterward by putting a clean glass of the same size along with it, placing one of the blank paper masks sold for the purpose–either circular or cushion-shaped to suit the subject–between the plates, and pasting narrow strips of thin black paper over the edges to bind them together. This method is very successful, as you may see from the examples. It renders the high lights perfectly clear, and leaves a film like glass over all the parts of the transparency where the varnish has flowed.
In order to avoid the risk of dust involved in this process, I tried other means of arriving at similar results and with success, for the plates I now submit to you have been simply rubbed or polished, as I may say, with a mixture of one part of Canada balsam to three parts of turpentine, using either a small tuft of French wadding or a small piece of soft rag for the purpose, continuing the rubbing until the plate is polished nearly dry. This method is particularly successful, rendering the clear parts of the sky like bare glass. I have here a plate which is heavily veiled–almost fogged, in fact–one half of which I have treated in this way, showing that the half so treated is beautifully clear, while the other half is so veiled as to be apparently useless.
I have tried to still further simplify this necessary clearing of those plates, and find that soaking tor twelve hours in a saturated solution of alum, after washing the hypo out of the plate, is successful in a large number of cases; and where it is successful there is no further trouble with the transparency, except to mount it after it becomes dry. Where it is not entirely successful I put the plate into a solution of citric acid, four ounces to a pint of water, for about one minute, and have in nearly all cases succeeded in getting a beautifully-clear plate. The picture must not be left long in the citric acid solution, or it will float off; neither do I like using citric acid until after trying the alum, for a similar reason.
I may mention that I recommend a short exposure in the printing-frame and slow development, in order to get sufficient intensity. Of course the exposure is always made to a gas or petroleum light. I also still prefer the old method of making the ferrous oxalate solution, pouring it back into the bottle each time after using, and using it for two or three months, keeping the bottle full from a stock bottle, and occasionally putting a little dry ferrous oxalate into the bottle and shaking it up, allowing it to settle before using next time. By treating it in this way it retains its power fairly well for a long time; and as it becomes less active I give a little longer exposure, balancing one against the other. Making the ferrous oxalate solution from two saturated solutions of iron sulphate and potassium oxalate has not succeeded so well with me for transparencies. The tone of the picture is not so black as when developed by the old method; and I do not like gray transparencies for the lantern. I also recommend very slow gelatine plates, about twice as sensitive as wet collodion–not more, if I can help it.
I have demonstrated, I hope to your satisfaction, the possibility of producing lantern slides from commercial gelatine plates of a most beautiful quality–ranging from clear glass to deep black, and giving charming gradation of tones, showing on the screen a film as structureless as albumen slides, without the great trouble involved in making them. You must not accept the slides put before you this evening as the best that can be done with gelatine. Far from it; they are only the work of an amateur with very little leisure now to devote to their manufacture, and are merely the result of a series of experiments which, so far as they have gone, I now place before you.–_Thomas Mayne, T. C., in British Journal of Photography._
* * * * *
AN INTEGRATING MACHINE.
[Footnote: Read at a meeting of the Physical Society, Feb. 26.]
By C.V. BOYS.
All the integrating machines hitherto made, of which I can find any record, may be classed under two heads, one of which, Ainslee’s machine, is the sole representative, depending on the revolution of a disk which partly rolls and partly slides on the paper, and the other comprising all the remaining machines depending on the varying diameters of the parts of a rolling system. Now, none of these machines do their work by the method of the mathematician, but in their own way. My machine, however, is an exact mechanical translation of the mathematical method of integrating y dx, and thus forms a third type of instrument.
The mathematical rule may be described in words as follows: Required the area between a curve, the axis of x and two ordinates; it is necessary to draw a new curve, such that its steepness, as measured by the tangent of the inclination, may be proportional to the ordinate of the given curve for the same value of x, then the _ascent_ made by the new curve in passing from one ordinate to the other is a measure of the area required.
The figure shows a plan and side elevation of a model of the instrument, made merely to test the idea, and the arrangement of the details is not altogether convenient. The frame-work is a kind of T square, carrying a fixed center, B, which moves along the axis of x of the given curve, a rod passing always through B carries a pointer, A, which is constrained to move in the vertical line, ee, of the T square, A then may be made to follow any given curve. The distance of B from the edge, ee, is constant; call it K, therefore, the inclination of the rod, AB, is such that its tangent is equal to the ordinate of the given curve divided by K; that is, the tangent of the inclination is proportional to the ordinate; therefore, as the instrument is moved over the paper, AB has always the inclination of the desired curve.
The part of the instrument that draws the curve is a three-wheeled cart of lead, whose front wheel, F, is mounted, not as a caster, but like the steering wheel of a bicycle. When such a cart is moved, the front wheel, F, can only move in the direction of its own plane, whatever be the position of the cart; if, therefore, the cart is so moved that F is in the line, ee, and at the same time has its plane parallel to the rod, AB, then F must necessarily describe the required curve, and if it is made to pass over a sheet of black tracing paper, the required curve will be _drawn_. The upper end of the T square is raised above the paper, and forms a bridge, under which the cart travels. There is a longitudinal slot in this bridge in which lies a horizontal wheel, carried by that part of the cart corresponding to the head of a bicycle. By this means the horizontal motion communicated to the front wheel of the cart by the bridge, is equal to that of the pointer, A; at the same time the cart is free to move vertically.
The mechanism employed to keep the plane of the front wheel of the cart parallel to AB is made clear by the figure. Three equal wheels at the ends of two jointed arms are connected by an open band, as shown. Now, in an arrangement of this kind, however the arms or the wheels are turned, lines on the wheels, if ever parallel, will always be so. If, therefore, the wheel at one end is so supported that its rotation is equal to that of AB, while the wheel at the other end is carried by the fork which supports F, then the plane of F, if ever parallel to AB, will always be so. Therefore, when A is made to trace any given curve, F will draw a curve whose ascent is (1/K) f y dx, and this, multiplied by K, is the area required.
[Illustration: AN INTEGRATING MACHINE.]
Not only does the machine integrate y dx, but if the plane of the front wheel of the cart is set at right angles instead of parallel to AB, then the cart finds the integral of dx / y, and thus solves problems, such, for instance, as the time occupied by a body in moving along a path when the law of the velocity is known.
Some modifications of the machine already described will enable it to integrate squares, cubes, or products of functions, or the reciprocals of any of these.
Of the various curves exhibited which have been drawn by the machine, the following are of special physical interest.
Given the inclined straight line y = cx, the machine draws the parabola y = cx squared / 2. This is the path of a projectile, as the space fallen is as the area of the triangle between the inclined line, the axis of x, and the traveling ordinate.
Given the curve representing attraction y = 1 / x squared the machine draws the hyperbola y = 1 / x the curve representing potential, as the work done in bringing a unit from an infinite distance to a point is measured by the area between the curve of attraction, the axis of x, and the ordinate at that point.
Given the logarithmic curve y = e^x, the machine draws an identical curve. The vertical distance between these two curves, therefore, is constant; if, then, the head of the cart and the pointer, A, are connected by a link, this is the only curve they can draw. This motion is very interesting, for the cart pulls the pointer and the pointer directs the cart, and between they calculate a table of Naperian logarithms.
Given a wave-line, the machine draws another wave-line a quarter of a wave-length behind the first in point of time. If the first line represents the varying strengths of an induced electrical current, the second shows the nature of the primary that would produce such a current.
Given any closed curve, the machine will find its area. It thus answers the same purpose as Ainslee’s polar planimeter, and though not so handy, is free from the defect due to the sliding of the integrating wheel on the paper.
The rules connected with maxima and minima and points of inflexion are illustrated by the machine, for the cart cannot be made to describe a maximum or a minimum unless the pointer, A, _crosses_ the axis of x, or a point of inflexion unless A passes a maximum or minimum.
* * * * *
UPON A MODIFICATION OF WHEATSTONE’S MICROPHONE AND ITS APPLICABILITY TO RADIOPHONIC RESEARCHES.
[Footnote: A paper read before the Philosophical Society of Washington. D. C., June 11, 1881.]
By ALEXANDER GRAHAM BELL.
In August, 1880, I directed attention to the fact that thin disks or diaphragms of various materials become sonorous when exposed to the action of an intermittent beam of sunlight, and I stated my belief that the sounds were due to molecular disturbances produced in the substance composing the diaphragm.[1] Shortly afterwards Lord Raleigh undertook a mathematical investigation of the subject and came to the conclusion that the audible effects were caused by the bending of the plates under unequal heating.[2] This explanation has recently been called in question by Mr. Preece,[3] who has expressed the opinion that although vibrations may be produced in the disks by the action of the intermittent beam, such vibrations are not the cause of the sonorous effects observed. According to him the aerial disturbances that produce the sound arise spontaneously in the air itself by sudden expansion due to heat communicated from the diaphragm–every increase of heat giving rise to a fresh pulse of air. Mr. Preece was led to discard the theoretical explanation of Lord Raleigh on account of the failure of experiments undertaken to test the theory.
[Footnote 1: Amer. Asso. for Advancement of Science, August 27, 1880.]
[Footnote 2: _Nature_, vol. xxiii., p. 274.]
[Footnote 3: Roy. Soc., Mar. 10, 1881.]
[Illustration: Fig. 1. A B, Carbon Supports. C, Diaphragm.]
He was thus forced, by the supposed insufficiency of the explanation, to seek in some other direction the cause of the phenomenon observed, and as a consequence he adopted the ingenious hypothesis alluded to above. But the experiments which had proved unsuccessful in the hands of Mr. Preece were perfectly successful when repeated in America under better conditions of experiment, and the supposed necessity for another hypothesis at once vanished. I have shown in a recent paper read before the National Academy of Science,[1] that audible sounds result from the expansion and contraction of the material exposed to the beam, and that a real to-and-fro vibration of the diaphragm occurs capable of producing sonorous effects. It has occurred to me that Mr. Preece’s failure to detect, with a delicate microphone, the sonorous vibrations that were so easily observed in our experiments, might be explained upon the supposition that he had employed the ordinary form of Hughes’s microphone shown in Fig. 1, and that the vibrating area was confined to the central portion of the disk. Under such circumstances it might easily happen that both the supports (a b) of the microphone might touch portions of the diaphragm which were practically at rest. It would of course be interesting to ascertain whether any such localization of the vibration as that supposed really occurred, and I have great pleasure in showing to you tonight the apparatus by means of which this point has been investigated (see Fig. 2).
[Footnote 1: April 21, 1881.]
[Illustration: Fig. 2. A, Stiff wire. B, Diaphragm. C, Hearing tube. D, Perforated handle.]
The instrument is a modification of the form of microphone devised in 1872 by the late Sir Charles Wheatstone, and it consists essentially of a stiff wire, A, one end of which is rigidly attached to the center of a metallic diaphragm, B. In Wheatstone’s original arrangement the diaphragm was placed directly against the ear, and the free extremity of the wire was rested against some sounding body–like a watch. In the present arrangement the diaphragm is clamped at the circumference like a telephone diaphragm, and the sounds are conveyed to the ear through a rubber hearing tube, c. The wire passes through the perforated handle, D, and is exposed only at the extremity. When the point, A, was rested against the center of a diaphragm upon which was focused an intermittent beam of sunlight, a clear musical tone was perceived by applying the ear to the hearing tube, c. The surface of the diaphragm was then explored with the point of the microphone, and sounds were obtained in all parts of the illuminated area and in the corresponding area on the other side of the diaphragm. Outside of this area on both sides of the diaphragm the sounds became weaker and weaker, until, at a certain distance from the center, they could no longer be perceived.
At the point where we would naturally place the supports of a Hughes microphone (see Fig. 1) no sound was observed. We were also unable to detect any audible effects when thepoint of the microphone was rested against the support to which the diaphragm was attached. The negative results obtained in Europe by Mr. Preece may, therefore, be reconciled with the positive results obtained in America by Mr. Tainter and myself. A still more curious demonstration of localization of vibration occurred in the case of a large metallic mass. An intermittent beam of sunlight was focused upon a brass weight (1 kilogramme), and the surface of the weight was then explored with the microphone shown in Fig. 2. A feeble but distinct sound was heard upon touching the surface within the illuminated area and for a short distance outside, but not in other parts.
In this experiment, as in the case of the thin diaphragm, absolute contact between the point of the microphone and the surface explored was necessary in order to obtain audible effects. Now I do not mean to deny that sound waves may be originated in the manner suggested by Mr. Preece, but I think that our experiments have demonstrated that the kind of action described by Lord Raleigh actually occurs, and that it is sufficient to account for the audible effects observed.
* * * * *
A catalogue, containing brief notices of many important scientific papers heretofore published in the SUPPLEMENT, may be had gratis at this office.
* * * * *
THE SCIENTIFIC AMERICAN SUPPLEMENT.
PUBLISHED WEEKLY.
TERMS OF SUBSCRIPTION, $5 A YEAR.
Sent by mail, postage prepaid, to subscribers in any part of the United States or Canada. Six dollars a year, sent, prepaid, to any foreign country.
All the back numbers of THE SUPPLEMENT, from the commencement, January 1, 1876, can be had. Price, 10 cents each.
All the back volumes of THE SUPPLEMENT can likewise be supplied. Two volumes are issued yearly. Price of each volume, $2.50, stitched in paper, or $3.50, bound in stiff covers.
COMBINED RATES–One copy of SCIENTIFIC AMERICAN and one copy of SCIENTIFIC AMERICAN SUPPLEMENT, one year, postpaid, $7.00.
A liberal discount to booksellers, news agents, and canvassers.
MUNN & CO., PUBLISHERS,
37 PARK ROW, NEW YORK, N. Y.
* * * * *
PATENTS.
In connection with the SCIENTIFIC AMERICAN, Messrs. MUNN & Co. are Solicitors of American and Foreign Patents, have had 35 years’ experience, and now have the largest establishment in the world. Patents are obtained on the best terms.
A special notice is made in the SCIENTIFIC AMERICAN of all Inventions patented through this Agency, with the name and residence of the Patentee. By the immense circulation thus given, public attention is directed to the merits of the new patent, and sales or introduction often easily effected.
Any person who has made a new discovery or invention can ascertain, free of charge, whether a patent can probably be obtained, by writing to MUNN & Co.
We also send free our Hand Book about the Patent Laws, Patents, Caveats. Trade Marks, their costs, and how procured, with hints for procuring advances on inventions. Address
MUNN & CO., 37 PARK ROW, NEW YORK.
Branch Office, cor. F and 7th Sts., Washington, D. C.