12 PRACTICAL PHYSIOLOGY 



parts of the wire (Fig. 16). The battery circuit 

 and the experimental circuit are both connected 

 with the ends of the wire. When the rider is 

 brought in contact with these ends the battery 

 'current is completely short-circuited, but when the 

 rider is moved away from the ends a gradually in- 

 creasing resistance is inserted into the short circuit 

 formed by the rheochord and its rider, and propor- 

 tionally more of the battery current passes into the 

 experimental circuit. 



Induction coil. If the wires of two separate 

 circuits are at any point near to and parallel with 

 one another and, in the one circuit, the current of 

 a battery is either made or broken by the closing 

 or opening of a key, an induced current is set up 

 in the other or secondary circuit at the instant of 

 such closing or opening, but not during the passage 

 of the primary current. The induced or secondary 

 current is always of very short duration, but has a 

 much higher electromotive force than the primary 

 or inducing current. 



In order to multiply the inductive effect the two 

 circuits always take the form of closely coiled 

 wires (Fig. 1 7) (that of the secondary circuit 

 being very fine and having very numerous coils), 

 and to still further increase the effect the primary 

 coil is wrapped round a core formed of a bundle 

 of soft iron wires which are magnetized and de- 

 magnetized on the closing and opening of the 

 primary circuit, thus enhancing the induction 

 effects. 



For physiological purposes the induction coil 



