ELECTRIC LIGHTING. 299 



effects which are indicated to us by the resulting heat and 

 light. 



A powerful galvanic battery is capable of producing light 

 of intense brilliancy. For this purpose, instead of taking 

 sparks between the two metallic poles, each of these is con- 

 nected with, a piece of carbon (which is nearly as good a 

 conductor as the metal), and the sparks are taken between 

 these two pieces of carbon, usually set so that the one con- 

 nected with the negative pole is virtually above the one 

 connected with the positive pole, and at a distance of a tenth 

 of an inch from each other or more, according to the strength 

 of the battery. Across this gap between the carbons an arc 

 of light is seen, which in reality results from a series of 

 electric sparks following each other in rapid succession. 

 This arc, called the voltaic arc, is brilliant, but it is not from 

 this arc that the chief part of the light comes. The ends of 

 the carbon become intensely bright, being raised to a white 

 heat. Both the positive and negative carbons are fiercely 

 heated, but the positive is heated most. As (ordinarily) 

 both carbons are thus heated in the open air, combustion 

 necessarily takes place, though it is to be noticed that the 

 lustre of the carbons is not due to combustion, and would 

 remain undiminished if combustion were prevented. The 

 carbons are thus gradually consumed, the positive nearly 

 twice as fast as the negative. If they are left untouched, 

 this process of combustion soon increases the distance be- 

 tween them till it exceeds that which the electricity can pass 

 over. Then the light disappears, the current ceasing to flow. 

 But by bringing the carbon points near to each other (they 

 must, indeed, be made to touch for an instant), the current 

 is made to flow again, and the light is restored. 



The following remarks by M. H. Fontaine (translated by 

 Dr. Higgs) may help to explain the nature of the voltaic arc : 

 ' In truth, the voltaic arc is a portion of the electric circuit 

 possessing the properties of all other parts of the same 

 circuit. The molecules swept away from point to point 

 (that is, from one carbon end to the other) ' constitute 



