258 



VOLTAIC ELECTRICITY. 



FIG. 204. 



to produce exactly similar vibrations in b. "It is as if the 

 close approach and quick oscillation of the piece of soft 

 iron fretted or tantalized the magnet and sent a series of 

 electrical shudders through the iron nerve." 



(a.) We have here the principle of the telephone, so far as elec- 

 tric action is involved. Further consideration of this instrument 

 must be deferred until we have learned more concerning sound. 

 (See 445.) 



41O. Ruhmkorff's Coil. The induction coil, often 

 called, from the name of its inventor, Ruhmkorff's coil, 

 is a contrivance for producing induced currents in a 

 secondary coil by closing and opening, in rapid suc- 

 cession, the circuit of a current in the primary coil. 



The axis of the 

 coils is a bundle 

 of soft iron wires. 

 These wires usu- 

 ally terminate in 

 two small plates 

 of soft iron which 

 thus form the 



ends of the wire 

 FIG. 205 



bundle. Around 



this bundle is wound the primary coil of insulated copper 



