CHAP, vii.] ELECTRICITY AND MAGNETISM. 333 



knife, as, for example, in the operation of amputating the 

 tongue for cancer. Platinum is chosen on account of its 

 infusibility, but even platinum wires are fused by the 

 current if too strong. Carbon alone, of conductors, resists 

 fusion. 



37O. Blasting by Electricity. In consequence of 

 these heating effects, electricity can be applied to fire 

 blasts and mines, stout conducting wires being carried 

 from an appropriate battery at a distance to a special 

 fuze^ in which a very thin platinum wire is joined in the 

 circuit. This wire gets hot when the current flows, and 

 being laid amidst an easily combustible substance to 

 serve as a priming, ignites this and sets fire to the charge 

 of gunpowder. Torpedoes can thus be exploded beneath 

 the water, and at any desired distance from the battery. 



The special case of heat developed or abstracted by a 

 current passing through a junction of dissimilar metals, 

 known as Peltier's effect, is mentioned in Art. 380. 



LESSON XXXII. The Electric Light. 



371. The Voltaic Arc. If two pointed pieces of 

 carbon are joined by wires to the terminals of a power- 

 ful voltaic battery or other generator of electric currents, 

 and are brought into contact for a moment and then 

 drawn apart to a short distance, a kind of electric flame 

 called thd voltaic arc is produced between the points 

 of carbon, and a brilliant light is emitted by the white 

 hot points of the carbon electrodes. This phenomenon 

 was discovered in 1 809 by Sir H. Davy, and its explana- 

 tion appears to be the following : Before contact the 

 difference of potential between the points is insufficient 

 to permit a spark to leap across even yirroir of an mcn f 

 air-space, but when the carbons are made to touch, a 

 current is established. On separating the carbons the 

 momentary extra -current due to self-induction of the 



