1 2 ELE C 7 RIC LIGH'l 'ING. 



des Arts, 40. These apparatus consist in the more or less 

 complete action of resistance coils arranged like a set of 

 weights, and in rheometres graduated in ohms and volts ; so 

 that byasimple reading, without calculation, the electro-motive 

 force of a battery, its resistance, and that of the external circuit 

 can be ascertained. The indications are not perhaps rigo- 

 rously exact, but they are quite sufficient for the ordinary 

 applications that have to be made of electric currents. 



WHAT THE ELECTRIC LIGHT IS. 



An artificial light is usually the result of a combustion, 

 and we rarely think of a luminous effect without the inter- 

 vention of a combustible substance. With the electric light, 

 however, this is not the case, for it can be displayed in a 

 vacuum, in water, and in gases incapable of supporting com- 

 bustion. Whence arises this difference ? From the fact that 

 in one case the heat (which always accompanies the produc- 

 tion of light), by effecting the decomposition of the combus- 

 tible substance, gives rise to a disengagement of its con- 

 stituent hydrogen gas that feeds the flame; while in the 

 other case, the luminous effect is the result of a mere 

 transformation of the physical forces. This transformation 

 shows itself when the conditions of the passage of the 

 electricity are such that, the free development of the electric 

 action being impeded, an abrupt elevation of temperature 

 takes place at one point in the circuit, and there manifests 

 itself by incandescence without any combustion being re- 

 quired for the production of the incandescent effect. This 

 phenomenon follows from the law which requires that, 

 whatever may be the nature of the parts of a circuit, every 



