10 



EXPERIMENTAL PHYSIOLOGY 



circuit method fig. 14). For this purpose du Bois-Reymond's key is 

 especially well suited. 



PIG. 14. FRICTION KEY; SHORT-CIRCUIT METHOD OF USE. 



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

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



FIG. 15. DIAGRAM OP POHL'S COMMUTATOR. 



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

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



FIG. 16. POHL'S COMMUTATOR USED AS SWITCH (CROSS WIRES REMOVED). 



other non-conducting material in which are six cups of mercury con- 

 nected with terminals. Four of the cups are joined diagonally, two 



