PRACTICAL PHYSIOLOGY 17 



This is partly due to the fact that as the current of 

 the primary circuit is made and broken induced cur- 

 rents (extra currents) are formed in its own coils ; 

 the make extra current of closure being in the op- 

 posite direction to the battery current diminishes 

 the make induced current in the secondary circuit, 

 while the break extra current is cut off by the open- 

 ing of the primary circuit. The sharpness of the 

 break effect is also partly due to the fact that with 

 the keys generally used the opening of the primary 

 circuit is more sudden than the closure. 



Primary coil 



FIG. 20. EXPERIMENT TO SHOW BREAK "EXTRA" CURRENT. 



5. To show the existence of the " extra "cur- 

 rents remove the secondary coil altogether and 

 connect up the primary coil by T 1 and T* with a 

 battery and keys in the way shown in Fig. 20. 

 Place the electrodes on the tongue. Make and 

 break the battery circuit by closing and opening 

 the key, K\ If this is done when the pri- 

 mary coil is included in the circuit (i.e., with K* 

 open) the effect is greatly enhanced by the " extra " 

 current, but if the coil is shunted out by clos- 



