SCIENCE IN RELATION TO THE ARTS. 61 



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 con- 

 fine myself to a few general remarks. Joule has shown that, if an elec- 

 tric 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 com- 

 bination produce the effect of white light. 



Regarding the proportion of luminous to non-luminous rays pro- 

 ceeding from an electric arc or incandescent wire, we have a most 

 valuable investigation by Dr. Tyndall, recorded in his work on " Ra- 

 diant Heat." Dr. Tyndall shows that the luminous rays from a plati- 

 num wire heated to its highest point of incandescence, which may be 

 taken at 1,700 C, formed fa part of the total radiant energy emit- 

 ted, and y 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 ele- 

 ments of the size used by Dr. Tyndall, expressed in the practical scale 

 of units as now established. From a few experiments lately under- 

 taken for myself, it would appear that 50 such cells have an electro- 

 motive force of 98"5 Volts, and an internal resistance of 13 - 5 Ohms, 

 giving a cm-rent of 7*3 Amperes when the cells are short-circuited. 

 The resistance of a regulator such as Dr. Tyndall used in his experi- 

 ments may be taken at 10 Ohms, the current produced in the arc would 



98"5 

 be Tir^ T7, r 4 Amperes (allowing one Ohm for the leads), and the 

 13'o + lO + i 



power consumed 10 X 4 3 = 1G0 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 f -y - J, i. e., 73 times 



(OOAQv 

 TT-TrJi- e., 22 times the amount of light 



measured horizontally. If, therefore, in Dr. TyndalPs arc yV of the radi- 

 ant energy emitted was visible as light, it follows that in a powerful arc 



1 22 - 

 of 3,300 candles, T^ v ~7*7<r> or fully i, 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 J- x = fa part of the power is 

 given out as luminous rays, as against fa in Dr. Tyndall's incandes- 

 cent platinum a result sufficiently approximate considering the wide 

 difference of conditions under which the two are compared. 



