XXVIII.] 



SHOCKS AND CURRENTS. 



173 



(b.) Effect on Tongue.— AVhile Ncof's hammer is vibrating, 

 apply the electrodes to the tongue as before, noting the effect pro- 

 duced and how it varies on altering the distance between the 

 secondary and primary coils. 



Fia. too. -Induction Coil arranged for interrupted or repeated shocks, with 

 Neef's Hammer in the Primary Circuit. 



(c.) Note also how the strength of the induced shocks varies with the 

 angular deviation of the secondary spiral, the distance between the two 

 spirals being kept constant (p. 172). 



3. The Break Extra-Current of Faraday. — When a galvanic 

 current traversing tlie primary coil of an induction macbine is 

 made or broken, each turn of the wire exerts an inductive influence 

 on the others. When the current is ma'fe, the direction of the 

 extra-current is arjainst that of the battery current, but at break it 

 is in the same direction as the 

 battery current. Apparatus. — 

 Daniell's cell, two Du Bois keys, 

 five wires, primary coil of in- 

 duction coil, electrodes (or nerve- 

 muscle preparation). 



(a.) Arrange the apparatus 

 accoi'ding to the scheme (fig. 

 1 01). Notice that both keys 

 and the primary coil of the 



induction machine are in the Fio.ioi.— Scheme of the Break Extra-Cuneut 

 .. ,, , , . B. Battery; K. and K". Keys; P. Pnmar> 



primary circuit, the keys being coil ; N. Nerve ; M. Muscle. 



so arranged that either the 

 primary coil, P, 

 short-circuited. 



or the electrodes attached to key K', can be 



