IO 



EXPERIMENTAL PHYSIOLOGY 



11), as in the Morse key; or by friction contact between two brass 

 surfaces (friction key\ as in that known as du Bois-Reymond's 



Fin. 14. Friction key ; short-circuit method of use. 



FIG. 15. Diagram of Pohl's commutator. 



Fiu. 16. Pohl's commutator used as switch (cross wires removed). 



(Figs. 12, 13, 14), and in the ordinary electric-light switches. They 

 are used in two ways, viz. : either to simply close or open the circuit 

 (direct method Figs. 10, 11, 12) ; or by bridging across a part of the 



