Part 28

We have now to consider how light suitable for purposes of illumination may be obtained from the electric current. Hitherto we have considered only light such as might be used for special purposes, where a bright and very intense light was required, where expense and complexity of construction might not be open to special objections, and where in general the absolute steadiness of the light was not an essential point. But those who have seen the electric light used even by the most experienced manipulators for the illustration of lectures will know that the light as so obtained, though of intense brilliancy, is altogether unsuited for purposes of ordinary illumination.

If we consider a few of the methods which have been devised for overcoming the difficulties inherent in the problem of electric lighting, the reader will recognise at once the nature of these difficulties, and the probability of their being effectually overcome in the future, for though much has been done, much yet remains to be done in mastering them.

Let us consider first the Jablochkoff candle, the invention of which brought about, in July 1877, the first great fall in the value of gas property.

The Jablochkoff candle consists of two carbons placed side by side (instead of one above the other in a vertical line). Thus placed, with a slight interval between them, the carbon rods would allow the passage of the electric current at the place of nearest approach, and therefore of least resistance to its passage. A variable and imperfect illumination would result. M. Jablochkoff, however, interposes between the separate carbon rods a slip of plaster of Paris, which is a non-conducting material. The upper points of the carbon rods are thus the only parts at which the current can cross. They are connected by a little bridge of carbon, which is necessary for the starting of the light--just as in the case of the ordinary electric light, the two carbons must, in order to start the light, be brought into contact. When the current flows, the small bridge of carbon connecting the two points is presently consumed, but the arc between the points is still maintained: for the plaster becomes vitrified by the intense heat of the two carbon points on each side, and melts down as the carbons are consumed. If the light is in any way put out, however, a small piece of carbon must be set again, to form a bridge between the carbon points. Throughout the burning of the Jablochkoff candle the fused portion of the insulating layer forms a conducting bridge between the carbon points; and hence there is a considerable loss of electric force (probably about thirty per cent.), which in the ordinary arrangement would increase the intensity of the light. The great advantage of the candle consists in the circumstance that throughout its consumption the carbon ends are at a constant distance from each other without any mechanical or other arrangement being necessary to maintain them in due position.

One point should be noticed here. In the ordinary arrangement of carbon points, the positive carbon, as we have already said, is much more intensely heated, and consumes twice as fast as the negative carbon. Now, if one carbon of the Jablochkoff candle were connected with the positive, and the other with the negative pole of the battery or of a machine, the former side would consume twice as fast as the latter, and the two points would no longer remain at the same horizontal level, which is essential to the proper burning of the Jablochkoff candle. By using a machine which produces alternating currents, M. Jablochkoff obviates this difficulty, the carbons being alternately positive and negative (in extremely rapid succession), and therefore consuming at the same rate.

The Jablochkoff candle lasts only about an hour and a half. But four, six, or more candles may be used in the same globe or lantern, and automatic arrangements adopted to cause a fresh candle to be ignited at the moment when its predecessor is burnt out.

In Paris and elsewhere (as in Holborn, for instance), each Jablochkoff lamp is enclosed in an opal glass globe. Mr. Hepworth remarks on this, that in his opinion the use of the opal globe is a mistake, as it shuts off quite 50 per cent. of the light without any corresponding advantage, except the correction of the glare. 'This wasteful disadvantage will no doubt be remedied in the future,' he says, by the use of some less dense medium. 'Mr. Shoolbred states that from a series of careful photometric experiments carried out by the municipal authorities with the Jablochkoff lights, each naked light is found to possess a maximum intensity of 300 candles. With the opal globe this was reduced to 180 candles, showing a loss of 40 per cent., while during the darker periods through which the light passed the light was as low as 90 candles. It may be mentioned here that Mr. Van der Weyde, who has long used the electric light for photographic purposes, has given much attention to the important problem of rendering the electric light available as an illuminator without wasting it, and yet without throwing the rays directly upon the object to be illuminated. The rays are intercepted by an opal disc about four inches in diameter, and the whole body of the rays is gathered up by a concave reflector (lined with a white material), and thrown out in a flood of pure white light, in which the most delicate shades of tint are discernible. He can use any form of electric candle in this way. Only it should be noticed, before the employment of his method is advocated for street illumination, that there is a difference between the problems which the photographer and the street-lighter have to solve. The Jablochkoff candle, for instance, must be screened on all sides, and even above, when used to illuminate the streets. If its direct light is allowed to escape in any direction, there will be a mischievous and unsightly beam, and from every point along the path of the beam, the intensely bright light of the candle will be directly visible. Again: it is essential that whatever substance is used to screen the light should be dense enough to cause the whole globe to seem uniformly bright or nearly so. The only modification which seems available (when these essential points have been secured) is that the tint of the globe should be such as to correct any colour which the light may be found to have in injurious excess. We may, however, remark that the objection which has been often raised against the colour of the electric light can hardly be just--the injury to the eyes in certain cases arising probably from the strong contrast between the light and the background on which it is projected. For, as to colour, the electric light derived either from the glowing carbon or from incandescent metal is appreciably the same as sunlight.

The Rapieff burner, employed in the 'Times' office, consists of four carbon pencils, arranged thus [Symbol] (except that the two v's are not in the same plane, but in planes at right angles to each other). The spark crosses the space between the points of the v's, and arrangements are made for keeping the two points at the right distance from each other, and also for keeping the ends of the two pencils which form each point in their proper position. If the current is from any cause interrupted, an automatic arrangement is adopted to allow the current to pass to the other lamps in the same circuit. There are six lamps in circuit at the 'Times' office; and M. Rapieff has exhibited as many as ten. The advantages claimed for this light are the following:--'First, its production by any description of dynamo-electric machine with either alternating or continuous currents; secondly, great diversibility and complete independence of the several lights, and long duration without change of carbons; and lastly, the extreme facility with which any ordinary workman or servant can renew the carbons when necessary, without extinguishing the lights.' The last-named advantage results, it need hardly perhaps be said, from the use of two carbons to form each point. One can be removed, the other remaining to keep the voltaic arc intact until a new carbon has been substituted for its fellow; then it in turn can be replaced by a new carbon, the new carbon already inserted keeping the voltaic arc intact.

The six lamps at the 'Times' office thoroughly illuminate the room, and give light for working the eight Walter presses used in printing the paper. The light has been thus used since the middle of last October, and it is said that other rooms in the building are shortly to be illuminated in the same manner. 'Each lamp is enclosed in an opal globe of about four inches in diameter, and so little heat is given off, that the hand can be placed on the globe without inconvenience, even after the light has been burning for some time.'

In the Wallace lamp there are two horizontal plates of carbon, about nine inches in diameter, instead of mere carbon points. When the current is passing, these carbon plates are separated by a suitable small distance which remains unchanged. The electric arc, being started at the point along the edge of the carbons where there is least resistance to the passage of the current, gradually passes along the edge of the carbons as combustion goes on, changing the position of the place of nearest approach and consequently of least resistance. The light will thus burn for many hours (even for a hundred with large carbon plates), and any number of lights up to ten can be worked from the machine. The objection to the Wallace lamp is, that the light does not remain at one point, but travels along the whole extent of the carbons. It will not be easy to design a glass shade which will be suitable for a light thus changing in position.

The Werdermann regulator is on an entirely new plan; but it has not yet been submitted to the test of practical working outside the laboratory. The positive carbon, which is lowest, ends in a sharp point, which strangely enough retains its figure, while the carbon burns away at the rate of about two inches per hour. The negative carbon is a block having its under side, against which the positive carbon presses, slightly convex. The positive carbon is pressed steadily against the negative by the action of a weight. The increased resistance to the passage of the current, at the sharp point of the positive carbon, generates sufficient heat to produce a powerful light. The light resembles a steadily radiant star, but 'with all its softness and purity of tint, it is so intense, that adjacent gas-flames are thrown on the wall as transparent shadows.' The light will last for fifteen hours without attention, the positive carbon rod being used in lengths of three feet. The carbon block hardly undergoes any change. When the lamp has been burning a long time, a slight depression can be seen at the place where the positive carbon touches it, but by shifting the carbon in its holder this is easily remedied. Mr. Werdermann lately exhibited a row of ten small lamps burning side by side at the same time. 'The two wires from the machine,' says Mr. Hepworth, were carried one on either side of this row of lamps, branch wires being led from them for the service of each lamp. Mr. Werdermann says that his perfected lamps will be furnished with keys, by which the current can be turned on or off, as in the case of gas. We may say in fact, that in the nature of its connections and various arrangements, it ("the Werdermann lamp") most nearly comes up in convenience to the use of gas.'

We do not yet know certainly what arrangement Mr. Edison employs to obtain the light of which so much has been heard. It is asserted that his light is obtained from the incandescence of an alloy of iridium and platinum, which will bear without fusion a heat[26] of 5,000 degrees Fahrenheit. It would be unsafe, however, to assume that this account is trustworthy, or to infer (as we might in the case of almost any other inventor), that such being the nature of his plan, it could lead to no result of practical value. As has been well remarked by a contemporary writer, whatever Edison's invention may be, 'it is certain to be something to command respect, even if it does not quite come up to the glowing accounts which have reached us in advance.'

The following passage from one of these accounts, which appeared in the 'New York Herald,' will be read with interest, and may be accepted as trustworthy so far as it goes. 'The writer last night saw the invention in operation in Mr. Edison's laboratory. The inventor was deep in experimental researches. What he called the apparatus consisted of a small metal stand placed on the table. Surrounding the light was a small glass globe. Near by was a gas jet burning low. The Professor looked up from his work, to greet the reporter, and in reply to a request to view the invention, waved his hand towards the light, with the exclamation, "There she is!" The illumination was such as would come from a brilliant gas jet surrounded with ground glass, only that the light was clearer and more brilliant. "Now I extinguish it and light the gas, and you can see the difference," said Mr. Edison, and he touched the spring. Instantly all was darkness. Then he turned on the gas. The difference was quite perceptible. The light from the gas appeared in comparison tinted with yellow. In a moment, however, the eye had become accustomed to it, and the yellowish tint disappeared. Then the Professor turned on the electric light, giving the writer the opportunity of seeing both, side by side. The electric light seemed much softer; a continuous view of it for three minutes did not pain the eye; whereas looking at the gas for the same length of time caused some little pain and confusion of sight. One of the noticeable features of the light, when fully turned on, was that all the colours could be distinguished as readily as by sunlight. "When do you expect to have the invention completed, Mr. Edison?" asked the reporter. "The substance of it is all right now," he answered, putting the apparatus away and turning on the gas. "But there are the usual little details that must be attended to before it goes to the people. For instance, we have got to devise some arrangement for registering a sort of meter, and again, there are several different forms that we are experimenting on now, in order to select the best." "Are the lights to be all of the same degree of brilliancy?" asked the reporter. "All the same!" "Have you come across any serious difficulties in it as yet?" "Well, no," replied the inventor, "and that's what worries me, for in the telephone I found about a thousand;[27] and so in the quadruplex. I worked on both over two years before I overcame them."'

Other methods, as the Sawyer-Man system, and the Brush system, need not at present detain us, as little is certainly known respecting them. In the former it is said that the light is obtained from an incandescent carbon pencil, within a space containing nitrogen and no oxygen, so that there is no combustion. In the latter the carbon points are placed as in the ordinary electric lamp, but are so suspended in the clasp of a regulator, that they burn 14 inches of carbon without adjustment, the carbons lasting eight hours, and producing a flood of intense white light, estimated as equivalent to 3,000 candles.

I have little space to consider the cost of electric lighting, even if the question were one which could be suitably dealt with in these pages. Opinions are very much divided as to the relative cost of lighting by gas and by electricity; but the balance of opinion seem to be in favour of the belief that in America and France certainly, and probably in this country, where gas is cheap, electric lighting will on the whole be as cheap as lighting by gas. It should be noticed, in making a comparison between this country and others in which coal is dearer, that the cheapness of coal here, though favourable in the main to gas illumination, is also favourable, though in less degree (relatively) to electric lighting. Machines for generating electricity can be worked more cheaply here than in America. Nay, it has even been found advantageous in some cases to use a gas engine to generate electricity. Thus Mr. Van der Weyde used an Otto gas engine driven at the cost of 6_d._ an hour for gas, to produce the light which he exhibited publicly on the night of November 9. So that the cheapness of gas may make the electric light cheaper. Then it is to be remembered that important though the question of cost is, it is far from being all-important. The advantages of electric lighting for many purposes, as in public libraries, in cases where many persons work together under conditions rendering the vitiation of the air by gas lighting exceedingly mischievous, and in cases where the recognition of delicate differences of tint or texture is essential, must far more than compensate for some slight difference in cost. The possibility (shown by actual experience to be real) of employing natural sources of power to drive machines for generating electricity, is another interesting element of the subject, but could not be properly dealt with save in greater space than this here available.

FOOTNOTES:

[Footnote 23: It is supposed by many, that when the spark is long enough we can note the direction in which it travels; and observations of the motion of lightning from the earth to the cloud have been collected, as showing that the usually observed direction of the flash is sometimes reversed. In reality, no one has ever seen a lightning flash travel either one way or the other. If the attention is fixed on the storm cloud, as usual when a lightning storm is watched, every flash appears to pass from the cloud to the earth. If, on the contrary, at the moment when the attention is fixed on some terrestrial object the lightning flashes near that particular object, the flash will seem to pass from the object to the cloud. In either case the motion is apparent only. If there is motion at all, the passage of the electric spark occupies less than the 100,000th part of a second, and of course it is utterly impossible that any eye could tell at which end of its track the flash first appeared. In every case the flash seems to travel from the end to which attention was more nearly directed. The apparent motion corresponds to the chance direction of the eye.]

[Footnote 24: The extremity of the wire connected with the metal least affected by the acid solution is called the positive pole, that of the wire connected with the metal most affected by the solution is called the negative pole.]

[Footnote 25: So called, though in reality the best magnets gradually lose force.]

[Footnote 26: My occasional use of the word 'heat' where in scientific writing 'temperature' would be the word used, has exposed me to peevish, not to say petulant comments from Professor P.G. Tait, who has denounced half the mathematical world for using the word 'force,' in the sense in which Newton used it, and has spoken of an eminent physicist as one deserving universal execration and opprobrium for not explaining, when speaking of work done against gravity, that terrestrial gravity was meant, and not gravity on the sun, or Jupiter, or Mars, or anywhere in the heavens above or in the earth beneath, but only at the earth's surface. Where there is no risk of confusion, the word 'heat' may be used either to signify temperature, as when in ordinary speech and writing we talk of blood-heat, fever-heat, summer-heat, and so forth. Science, indeed, very properly forbids the use of the word in any sense save one. But outside the pages of scientific treatises, there is no inaccuracy in using a word in a sense popularly attributed to it, when no mistake can possibly arise. No one can suppose, when I speak of a heat of so many degrees Fahrenheit or Centigrade, that I mean anything but such and such a degree of heat, any more than if I spoke of the intense heat of that _savant entêté_, Professor P.G. Tait, any one would imagine that I referred to his calorific condition.]

[Footnote 27: The comments made by one of Mr. Edison's assistants on this point are interesting and instructive. 'Mr. Batchelor, the Professor's assistant, who here joined in the conversation,' proceeds the report of the _Herald_, 'said, "Many a time Mr. Edison sat down almost on the point of giving up the telephone as a lost job; but at the last moment, he would see light." "Of all things that we have discovered, this is about the simplest," continued Mr. Edison, "and the public will say so when it is explained. We have got it pretty well advanced now, but there are some few improvements I have in my mind. You see, it has got to be so fixed that it cannot get out of order. Suppose when one light only is employed it got out of order once a year, where two were used it would get out of order twice a year, and where a thousand were used you can see there would be much trouble in looking after them. Therefore, when the light leaves the laboratory, I want it to be in such a shape that it cannot get out of order at all, except of course by some accident."']

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Trascribers Note: Original spelling has been retained.

End of Project Gutenberg's Rough Ways Made Smooth, by Richard A. Proctor