094 



THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



ought to disappear, and in fact has disappeared. Sometimes 

 a battery and sometimes BuhmkorfFs induction coil is used for the 

 purpose, and sometimes again induced currents from magneto -electric 

 engines. 



In the beginning of this new application of electricity the battery 



Fro 448. Bidirmnate of potash battery for blasting mines. 



was always used. But a powerful battery was required, and metallic 

 conductors of a great diameter. A platinum coil embedded in the 

 powder is brought to incandescence when the circuit is closed, and 

 the explosion takes place ; a battery composed of bichromate of 

 potash elements is now used, inclosed in a box, and so arranged that 



