Popular Science Monthly/Volume 19/July 1881/Recent Advances in Electric Lighting
|RECENT ADVANCES IN ELECTRIC LIGHTING.|
By W. H. PREECE.
ADVANCES have been made, not so much in electric lighting, itself as in the popular favor with which it is regarded. The public is becoming more accustomed to its use, and is acquiring more confidence in it. The result of trials during the last year or two has been to make the defects of the electric light better known. It has been taken out of the experimental stage, and brought within reach of the practical stage. The principal fact which has brought the electric light to the front has been the substitution of machinery for the direct conversion of mechanical energy into electricity for the expensive batteries which were the only sources a few years ago. Machines, working with high velocity, great steadiness, and uniform pressure, have solved the problem of cheap electricity. The amount of coal required to produce one horse-power has been reduced from seven and eight pounds to three and even two pounds. The gas-engine—a very economical source of energy—has been successfully applied to electric lighting in many places. Such an engine has been used at the docks in Newport, South Wales, to produce a light of eleven times the power that the same gas would give if used directly. Here is a sphere in which gas companies may maintain their dividends. Water furnishes a convenient source of energy wherever it can be found available. Sir William Armstrong makes his brook light his house, producing from it, by the aid of a turbine, a force giving six horse-power. The caloric-engine at the Lizard Lighthouse has been found to be economical, useful, and very suitable for an isolated place where it is hard to provide water.
The difference in the value of the several excellent machines employed for generating the current is not great. Each is especially adapted for its own particular work, either by a variation in velocity or by a variation in the manner in which the wire is wound, so as to produce a variation in the current produced to suit the particular light required. In both the Siemens and the Gramme machines ninety per cent, of the power is converted into useful current. It is easily demonstrable that there is economy in the use of small machines. Trials made for the Trinity House have shown that more efficiency is obtained by joining small machines in multiple arc than by using a larger machine, or joining the same small machines in series.
For conducting-wires the preference is given to copper, the purest that can be got, and wire of the largest dimensions consistent with economy, so as to keep the resistance as low as possible and avoid waste of energy. When it can be carried overhead, facility is given for the radiation of the heat into the air, and the wire is kept cool and conveys more electricity. Since the currents to be carried over these wires are three thousand times larger than those used in telegraphy, the difficulties to be encountered in their safe transmission are greatly magnified. The disturbing effects produced by the inductive influence of such currents are so serious that apprehensions are entertained that it will be impossible to maintain electric light and telegraph currents close together.
The electric light is coincident with electric heat; the art of producing a brilliant light is the art of producing a high temperature. No greater illusion is extant than the idea that the electric light is a cold light, for the electric arc is the greatest source of heat known. This heat can be produced either by causing the electricity to fly across an air-space, in which case we have light by the arc, or an arc light, or by causing it to flow through a small wire or a carbon filament, which offers obstruction to the flow and produces light by incandescence, or the incandescent light. The forms of arc-lamps are very numerous. In every case carbon rods are opposed to each other, and are disintegrated and consumed in the fierce blast to which they are subjected. The lower pole—the negative—acquires a temperature of 3,150° C. (5,702° Fahr.), and is broken up and fired in a fierce bombardment of white-hot molecules across the air against the upper pole—the positive—which is beaten up by incessant impacts into a higher temperature of 3,900° C. (7,052° Fahr.), the arc itself being 4,800° C. (8,672° Fahr.). A number of ingenious appliances have been adopted to obtain steadiness and uniformity in the action of the arc, which is liable to variations arising from the irregularity in the character and consequently in the consumption of the carbon, and from variations in the strength of the current. AVe want brilliancy combined with absolute steadiness, and a durability equal to the length of a winter's night. All the improvements that have been made in the arc have not given a silent and steady light.
The incandescent light is free from many of the defects of the arc light. In it we have something that is beautifully soft, absolutely noiseless—a light that brightens up Nature in all her true colors and purity. It, however, requires a considerable expenditure of power, and is at present an expensive luxury. Sir William Armstrong finds that six horse-power will supply thirty-seven lights, giving altogether the illumination of nine hundred and twenty-five candles. The same power applied to arc-lights would give more than six thousand candles. But rapid progress has been made in this field. Maxim, Edison, etc., in America—Swan, Lane-Fox, and others, in England—are working hard; while Gordon and Joel are working in an intermediate field, in which a prospect appears of a happy compromise being effected between the arc and incandescence.
Some wild statements, involving wonderfully divergent estimates, have been made about the light-giving power of the different lights. A standard sperm-candle, although it may be a good unit to measure gas by, is a very poor standard for the electric light. None of the various modes of measurement in use seem to apply exactly to this light, and the standard of measurement of the future has yet to be found. Much is said about the subdivision of the electric light by certain gentlemen, who hope to distribute it throughout our houses from one central spot, and furnish it cheaply and abundantly in our cities. I am one of those who do not believe in the impossible, but I say that, with our present knowledge, this problem is unsolvable. Sir William Armstrong can only keep thirty-seven lamps going; Lane Fox could only show twelve lights; Professor Adams could only produce from the most powerful dynamo-electric machine, by calculation, one hundred and forty lamps. Where is the subdivision? The advocates of subdivision assume an inexhaustible source of electricity. Their opponents reply that there is but a very limited source of energy in every dynamo-electric machine. It may be that more powerful machines and lamps of lower resistance may enable us to light up a greater number on one circuit, but this is not subdivision, it is multiplication.
For application to external illumination, we have, first, the centralized system of Dr. Siemens, in which one machine works one powerful light, raised like a small moon on the top of a high mast; and, secondly, the distributed system of the Brush Company, who utilize the existing street lamp-posts, one machine working many lights. The former system appears to be the best for symmetrical spaces and large areas, the latter for long and narrow streets and thoroughfares; so, in internal illumination, the central system is preferable for lofty, spacious rooms, the distributed for long and low ones.
An eventful feature in practical lighting is the proper scattering or diffusion of light, by shades, screens, and reflecting surfaces. We want to emulate the diffusion of daylight. It is marvelous how whitewashed surfaces do this. Well-selected globes act as though they were self-luminous; they scatter light and produce shadows. The power of the light, however, to penetrate fogs does not appear to be any greater than that of gaslight. This is because the shorter-waved rays that give the light its violet tint are checked by the vapors, in the same manner as the like rays in the sun are checked, and it is made to appear red. The same cause operates to give the electric light a greater illuminating power in its immediate neighborhood, for the checked rays are reflected back to add their intensity to that of the direct rays.
Nearly three hundred Gramme machines are in use in England generating light; there are many more Siemens machines, and the Brush people have installed many machines and lights. Nearly all the ironclads in the navy are supplied with the electric light. In libraries, while reading by gaslight is irksome, reading by the electric light is simply delightful. Railway-stations are gradually adopting the lights; seaside resorts are illuminating their parades with them. It would be impossible to make any summary of the numerous manufacturing establishments that have been supplied with lights worked successfully.
Notwithstanding these great advances in its use, it must not be forgotten that the electric light has its defects and its disadvantages. The intense shadows that it occasions are troublesome. The unsteadiness of the light is at times wearisome. The hissing which impurities in the carbon and irregularities in the current produce is tantalizing, and the light has an unfortunate habit of misbehaving itself when it is most wanted. Moreover, the problem of durability remains yet to be solved. Many have tried the light and abandoned it. In some cases its economy is unquestionable, but there are places where careful persons have shown that gas, as regards economy, surpasses it. It is questionable whether, in some cases, the electric light does not affect the eye. The arc-light produces, also, nitrous acid and other deleterious gases, but the incandescent lamp is free from this trouble. The powerful currents that it requires can not be carried over buildings and rooms without incurring danger from fire and to life. Nevertheless, the light has great and manifold advantages. The brilliancy of a well-lighted room is simply enchanting. The purity of the light for the transaction of business, the selection of colors, and the ordinary daily avocations of life, is simply superb. Its cleanliness is one of its great merits. It emits no smoke. Probably its greatest advantage is to be found in the influence it exerts on health. At Glasgow, where the light has been applied, all causes of trouble arising from the vitiation which was occasioned by other lights have ceased. Health has been engendered, and more work has been got out of the men; and experience has shown that the electric light will pay for itself in the superior return it makes in this point alone.
Mr. Preece has published in a separate paper some facts bearing upon the economy of the electric light. The Loan Court of the South Kensington Museum, a room not favorably arranged for the diffusion of light, has been lighted for nine months with sixteen Brush lamps, at a cost for working of 3s. 10d. per hour of light. Had gas been used, a consumption of sixteen hundred cubic feet per hour would have been required, at a cost of 16s. Considering that the museum is lighted up) for seven hundred hours every year, the total saving effected by the use of electricity is at the rate of £426 or $2,130 a year. It is fair, however, to add something for the use of the capital, wear and tear, etc., to the annual expense. Reckoning this at five per cent, all around, the annual saving is still £316 10s., or $1,580. The reading-room of the British Museum is lighted by the Siemens electric light, at a cost of 5s. 6d. per hour, one third of what would be required for gas, were it used. A shed at the sugar-refinery of Messrs. Henry Tate & Sons, Silvertown, is lighted by a Crompton lamp in the ceiling, assisted by a canvas reflector. The whole of the shed is well lighted—four or five times more strongly than with gas—and the light penetrates an adjoining shed. The cost for fourteen hours of illumination is 1s. 9d., or 1½d. an hour; the cost of illumination by gas was 3s. 6d., or 3d. an hour. At the ship-building dock, Barrow-in-Furness, a work-shed is lighted by Brush lights at £4 14s. a week, where oil blast-lamps were formerly used at £8 9s. a week; and the erecting-shop, formerly dimly lighted by gas at £22 a week, is now efficiently lighted by electricity at half the cost.
- Abstract of a lecture before the London Society of Arts.