PRACTICAL PHYSIOLOGY O 



from the electrodes (short-circuit met hod Fig. 12). 

 For this purpose du Bois-Reymond's key is espe- 

 cially well suited. A key which is constructed so 

 as to cause a current to flow either in one direc- 

 tion or in the reverse direction in a circuit is called 

 a reverser or commutator. One of the most con- 



FIG. 12. FRICTION KEY ; SHORT-CIRCUIT METHOD OF USE. 



venient is Pohl's commutator (Fig. 13), which con- 

 sists of a square plate of vulcanite or other non- 

 conducting material in which are six cups of 

 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. 



FIG. 13. POHL'S COMMUTATOR. 



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. 14). 



Rheochords. A rheochord is an apparatus for 

 dividing a constant current by offering a circuit of 

 relatively small resistance which is capable of being 

 varied so that a variable part only of the current 



