260 VOLTAIC ELECTRICITY. 







the screw, closing the circuit and again magnetizing the core. The 

 plate is thus made to vibrate with great rapidity, each oscillation 

 making or breaking the circuit of the inducing current, and thus 

 creating a series of induced currents in the secondary coil ( 404), 

 which produce effects greater than can be produced by any electric 

 machine. Fig. 206 represents an induction coil made by E. S. 

 Ritchie, of Boston, for the U. S. Military Academy at West Point. 

 In this instrument there is no automatic interrupter, the break-piece 

 being operated by a ratchet-wheel and crank. 



411. Spark from Induction Coil. If the ends 

 of the secondary coil be connected, opposite currents alter- 

 nately traverse the connecting wire. When the ends are dis- 

 connected, as shown in Fig. 206, the inverse current cannot 

 overcome the resistance of the intervening air because of 

 its low electromotive power. The direct current, pro 

 duced by breaking the primary circuit, is alone able 

 to force its way in the form of a spark. The sparks 

 vary with the power of the instrument. An induction coil 

 has been made that gives a spark over 40 inches in length 

 a result incomparably greater than that obtainable from 

 any electric machine. The induction coil may be used to 

 produce any of the effects of Motional electricity, it being 

 at the same time nearly free from the limitations which 

 atmospheric moisture places upon all electric machines. 



Note. For an ordinary Ruhmkorff' s coil, one to three Bunsen or 

 potassium bi chromate elements will suffice. The effect of the coil 

 is generally increased by placing, in the base of the instrument, a 

 condenser made of many sheets of tinfoil separated by layers of 

 oiled silk. Alternate layers of the tinfoil are connected, t. e., the 

 first, third, fifth, seventh, etc., layers are connected, as also are the 

 second, fourth, sixth, eighth, etc. The odd numbered layers are 

 connected with one end of the primary coil ; the even numbered 

 layers with the other end. One object of this is to prevent the 

 spark otherwise produced at the break-piece of the primary circuit. 



412. Thermo-electricity. // a circuit be made 



