142 EXPERIMENTAL PHYSIOLOGY. 



Contractions occur, but they gradually get feebler. The 

 contractions are due to polarisation of the electrodes. 



LESSON XXV. 



SINGLE INDUCTION SHOCKS INTER- 

 RUPTED CURRENT BREAK EXTRA- 

 CURRENT HELMHOLTZ'S 

 MODIFICATION. 



1. Single Induction Shocks Apparatus. Grove's cell charged, 

 induction machine, five wires, two du Bois keys, and ordinary 

 electrodes. 



(a.) Connect one wire from the battery to the binding screw, 

 s", of the induction machine (Fig. 46). Join the other wire 

 from the battery to a du Bois key, and use the third wire to 

 connect the key with the binding screw, s". The key is 

 used according to the first method, i.e., for make and 

 break, so that the primary current can be made or broken at 

 will. To the binding screws of the secondary coil attach two 

 wires, and connect them to the short-circuiting du Bois key, 

 and to the latter the electrodes as in Fig. 54. 



(b.) Open the short-circuiting key, push the secondary 

 coil pretty near to the primary, and place the points of the 

 electrodes on the tip of the tongue, or hold them between 

 the forefinger and thumb moistened with water. Close the 

 key in the primary circuit, i.e., make the circuit, and instan- 

 taneously at the moment of making, a shock is induced in 

 the secondary coil, R", and is felt on the tip of the tongue 

 or finger. It is called the closing or make induction shock. 

 All the time the key is closed the galvanic current is cir- 

 culating in the primary spiral, but it is only when the 

 primary current is made or broken that a shock is induced 

 in the secondary spiral. 



(c.) Break the primary current by raising the key, and 



