Rough Ways Made Smooth: A series of familiar essays on scientific subjects

by Richard A. Proctor

In the cases here considered it is in reality the sudden presentation of the coil (twice at each rotation) before the positive and negative poles of the magnet, which induces a momentary but intense current of electricity. The rotation being exceedingly rapid, these currents succeed each other with sufficient rapidity to be appreciably continuous. A similar principle is involved in the use of what is called the inductive coil, except that in this case the sudden beginning and ceasing of a current in one coil (and not magnetic action) induces a momentary but strong current: matters are so arranged that the current induced by the starting of the inducing current, immediately causes this to cease; while the current induced by the cessation of the inducing current immediately causes this current to begin again: so that by a self-acting process we have a constant series of intense induced currents, succeeding each other with great rapidity, so as to be practically continuous, as with those produced by magneto-electric and dynamo-electric machines.

All that I have said about the voltaic arc, the incandescence resulting from resistance to the current's flow, and so forth, in relation to electricity generated by galvanic batteries, applies to electricity generated by induction coils, or by magneto-electric and by dynamo-electric machines. Only it is to be noticed that in some of these machines the currents alternate in direction with each revolution of the swiftly turning coil, in others the currents are always in the same direction, and in yet others the currents may be made to alternate or not, as may be most convenient.

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