The largest and most extensive application of electric energy at the present time is to lighting, but, considering how much has of late been said and written for and against this new illuminant, I shall here confine myself to a few general remarks. Joule has shown that, if an electric current is passed through a conductor, the whole of the energy lost by the current is converted into heat; or, if the resistance be localized, into radiant energy comprising heat, light, and actinic rays. Neither the low heat-rays nor the ultra-violet of highest refrangibility affect the retina, and may be regarded as lost energy, the effective rays being those between the red and violet of the spectrum, which in their combination produce the effect of white light.
Regarding the proportion of luminous to non-luminous rays proceeding from an electric arc or incandescent wire, we have a most valuable investigation by Dr. Tyndall, recorded in his work on "Radiant Heat." Dr. Tyndall shows that the luminous rays from a platinum wire heated to its highest point of incandescence, which may be taken at 1,700° C., formed 1 part of the total radiant energy emitted, and 1 part in the case of an arc-light worked by a battery of 50 Grove's elements. In order to apply these valuable data to the case of electric lighting by means of dynamo-currents, it is necessary in the first place to determine what is the power of 50 Grove's elements of the size used by Dr. Tyndall, expressed in the practical scale of units as now established. From a few experiments lately undertaken for myself, it would appear that 50 such cells have an electromotive force of 98·5 Volts, and an internal resistance of 13·5 Ohms, giving a current of 7·3 Ampères when the cells are short-circuited. The resistance of a regulator such as Dr. Tyndall used in his experiments may be taken at 10 Ohms, the current produced in the arc would be Ampères (allowing one Ohm for the leads), and the power consumed Watts; the light power of such an arc would be about 150 candles, and, comparing this with an arc of 3,308 candles produced by 1,162 Watts, we find that , i.e., 7·3 times the electric energy produce i.e., 22 times the amount of light measured horizontally. If, therefore, in Dr. Tyndall's arc 1 of the radiant energy emitted was visible as light, it follows that in a powerful arc of 3,300 candles, , or fully 1, are luminous rays. In the case of the incandescent light (say a Swan light of twenty-candle power) we find in practice that nine times as much power has to be expended as in the case of the arc-light; hence part of the power is given out as luminous rays, as against 1 in Dr. Tyndall's incandescent platinum—a result sufficiently approximate considering the wide difference of conditions under which the two are compared.