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great field which has been opened by the transformations of energy accomplished by the dynamo machine. We can congratulate ourselves, however, that the electric light is more efficient than other forms of light. It has been computed that the energy consumed in producing a light of sixteen-candle power by kerosene is 42.86 watts (a watt is of a horse power). An Argand gas burner of twenty-two-candle power consumes 68.8 watts; an incandescent electric light, 3.5 watts per candle; the arc light, 0-8 watt per candle.

We fail, thus, in utilizing the energy in the coal, and when we produce a light we convert most of the small amount of energy we obtain from the coal into nonluminous heat waves. It has been computed that ninety-five per cent of the energy expended in producing a light goes to the production of waves of the ether which do not affect the eye. Lodge remarks that we are in the condition of an organist who, in order to sound certain high notes of his instrument, is compelled to sound all those of the keyboard. Two great practical questions in the transformation of energy thus confront us: How to utilize to a greater degree the energy stored up in the coal, and how to produce a light rich only in those rays which appeal to our senses as light.

Ebert has constructed an economical lamp in the following manner: A is an exhausted glass globe (Fig. 29) with an inner glass stem, B, on the end of which is a paste of phosphorescent paint-"Grün blaue Leucht farbe." E, and E, are tin-foil rings, which are glued to the glass globe. These rings are connected to the two terminals of the finer coil of the step-up transformer. Under the action of the to-and-fro currents of the trans

* Ann. der Physik und Chemie, No. 9, 1894.

former the plates E, and E, are electrified, and the lines of force fluctuating through the rarefied globe raise the phosphorescent paint to a high degree of luminosity. Ebert calculates that such a lamp consumes from 1,500

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to 2,000 times less energy than the amylacetate unit lamp, and that this amount of energy is in the neighbourhood of one millionth of a watt, the watt being of a horse power. In order to avoid the inevitable losses which arise in conveying such high-tension effects any distance, Ebert suggests that a little stepup transformer can be put in the base of each little lamp. The necessity of electrical turning in the experiments of Ebert is perhaps the most interesting fact in regard to the endeavour to obtain an econòmical lamp by means of to-and-fro high-tension currents. He found that unless the circuit in which the lamp was placed was in resonance with the exciting circuit, the lamp did not light up to its full brilliancy.

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FIG. 29.

In an interesting paper on the cheapest form of light, Prof. Langley calls attention to the fact that there is an enormous waste of energy in the ordinary methods of producing illumination. In the ordinary Argandburner gas flame this waste for illumination purposes can be shown to be something over ninety-nine per cent of the radiant energy emitted by the lamp. As Prof. Langley points out, "this waste comes from the necessity of expending a large amount of heat in invisible forms, and each increase of light represents not only the small

amount of heat directly concerned in the making of the light itself, but a new indirect expenditure in the production of invisible calorific rays. Our eyes recognise heat mainly as it is conveyed in certain rapid ethereal vibrations associated with high temperatures, while we have no usual way of reaching these high temperatures without passing through the intermediate low ones, so that if the vocal production of a short atmospheric vibration were subject to analogous conditions, a high note could never be produced until we had passed through the whole gamut, from discontinuous sounds below the lowest bass, up successively through every lower note of the scale till the desired alto was attained." *

The phenomena of phosphorescence as it is manifested in fireflies, seems to form an exception to this rule. The light emitted by this insect can be produced artificially by raising a body to 2,000° F. No sensible heat, however, accompanies the firefly's light, and indeed this can also be said of the light in Geissler tubes. It is assumed that the firefly's light is produced without the invisible heat that accompanies our usual processes, for the spectrum of the firefly's light falls off more rapidly toward the red end than the spectrum of a candle, for instance. Prof. Langley, in his Memoir, which we have quoted, gives in detail the delicate measurements with his bolometer, by which he obtained the following results with the Cuban firefly (Pyrophorus noctilucus), an insect about one inch and a half long and half an inch wide. He concludes that Nature produces this cheapest light with about the four hundredth part of the cost of the energy which is expended in the candle flame.

* American Journal of Science, vol. cxl, 1890.

CHAPTER XVI.

LIGHTNING.

By means of the alternating-current dynamo and by the Ruhmkorff coil we have been able to transform the energy of coal into the energy manifested by lightning, for the long sparks of Thomson are lightning discharges. Electro-magnetic waves from the sun produced the coal, and we shall see later that from the coal, and by means of the dynamo and the step-up transformers, we can obtain again electro-magnetic waves. After one hundred and fifty years we have come back to the study of sparks and the behaviour of Leyden jars. We have returned to the domain of science, which Benjamin Franklin may be said to have suddenly illumined when he drew lightning from the clouds.

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We return, however, after a long study of the mechanical equivalent of heat and with scientific knowledge greatly increased by exact measurements. are conscious of the great truth that whenever we can test our electrical theories by heat experiments, following out the line of work indicated by Count Rumford, we are certain to obtain a residuum of truth. The study of the transformations of energy by means of instruments which measure the equivalence in heat of the electrical

actions we observe is the final method we must adopt to test any physical theory of electricity.

Before the epoch of Count Rumford the amount of exact experimentation in the subject of physics was extremely small, and it is not to be wondered at that men attributed to mysterious effluvia, to caloric, and to phlogiston the cause of the various transformations of energy which they witnessed. They had no measures of comparison. Benjamin Franklin's experiments have stood the test of time, but his theory of electricity has long since ceased to have value in the scientific world, largely because it had not the weight of quantitative measurements behind it. Let us look at his theory for a moment, and then examine our present conceptions of the lightning flash, which was of such absorbing interest to him.

The following account of Franklin's fluid theory of electricity was presented to the Royal Society in 1851 by William Watson, F. R. S.:

"This ingenious author (Franklin), from a great variety of curious and well-adapted experiments, is of opinion that the electrical matter consists of particles extremely subtile, since it can permeate common matter, even the densest metals, with such ease and freedom as not to receive any perceptible resistance; and that if any one should doubt whether the electrical matter passes through the substance of bodies or only over and along their surfaces, a shock from an electrified large glass jar taken through his own body will probably convince him.

"Electrical matter, according to our author, differs from common matter in this, that the parts of the latter mutually attract, and those of the former mutually repel each other, hence the divergency in a stream

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