210 



PHYSIOLOGY 



insulating handle carries two arcs of wire by which the pools at a and 6 may 

 be put into connection with either x and y, or the corresponding pools on 

 the opposite side.'.^It will be seen that with the cradle tipped to one side, as 

 in Fig. 45 A, the current from the battery enters the reverser at a ; this 

 proceeds up the wire of the cradle, down towards the right, then along the 

 cross-wire to the pool at x. x is therefore the anode, and y the cathode. 

 In Fig. 45 B the cradle has been swung over to the other side. Here the 

 cross-wires are not used at all by the current, which passes from a up the sides 

 and down the curved wire to y. In this case y is now the anode and x 

 the cathode, and the direction of the current through the circuit connected 

 with x and y is reversed. By taking out the cross- wires, Pohl's reverser 



FIG. 45. Diagram of Pohl's reverser. 



may be used as a simple switch, by which the current may be led into two 

 different circuits in turn. 



With this form of reverser difficulty is often experienced owing to dirt 

 accumulating on the mercury and forming an insulating layer between it and 

 the binding screw or copper wire. Several improved forms of reverser are 

 now made where the mercury poles are replaced by brass banks, and these are 

 generally to be preferred in practice. 



(2) INDUCED CURRENTS. In using these the muscle or nerve is stimulated 

 by the current of momentary duration produced in the secondary circuit of 

 an induction-coil by the make or break of a constant current in the primary. 



The construction -of the induction-coil or inductorium is founded on the fact 

 that if a coil of wire in connection with a galvanometer be placed close to (but 

 insulated from) another coil through which a current may be led from a battery, 

 it is found that on make and break of the current of the second coil a momentary 

 current is induced in the first. The induced current on make is in the reverse 

 direction, that on break in the same direction as the primary current. The 

 electromotive force of the induced current is proportional to the number of 

 turns of wire in the coils. The induction-coil consists of two coils, each con- 

 taining many turns of wire. The smaller coil (%, Fig. 46), consisting of a 



