ELECTRICAL APPARATUS IN USE IN PHYSIOLOGICAL WORK I I 



circuit a passage with very little resistance is offered through the key, 

 and the current is thus diverted from the main circuit and from the 

 electrodes (short-circuit method Fig. 14). For this purpose du Bois- 

 Reymond's key is especially well suited. 



A key which is constructed so as to cause a current to now either 

 in one direction or in the reverse direction in part of a circuit is called 

 a reverser or commutator. One of the most frequently used is Pohl's 

 commutator (Fig. 15), which consists of a plate of vulcanite or other 



FIG. 17. Waller's commutator. 



non conducting material in which are six cups filled with mercury 

 connected with terminals. Four of the cups are joined diagonally, 

 two and two, by crossed wires. A rocking double bridge of copper 

 serves, on being moved to one side or the other, to effect the reversal. 



If the crossed wires are removed the Pohl can be used as a switch 

 for diverting a current into one or other of two circuits (Fig. 16). 



Other commutators have friction-contacts in place of mercury : of 

 these the simplest are Waller's (Fig. 17), which has a rotating action ; 

 and Malcolm's (Fig. 18), which has a sliding action. The principle is 

 the same as the mercury commutator, and it is often convenient to 

 dispense with mercury. 



