ELECTRICAL APPARATUS IN USE IN PHYSIOLOGICAL WORK 9 



as in the Morse key ; or by friction contact between two brass sur- 

 faces (friction key), as in that known as du Bois-Reymond's (fig. 12), 



FIG. 10. MERCURY KEY IN A BATTERY CIRCUIT. 

 and in the ordinary electric-light switches. They are used in two 



PIG. 11. CONTACT KEY IN A BATTERY CIRCUIT. 

 ways, viz. : either to simply close or open the circuit (direct method- 



PIG. 12. DIAGRAM OP SHORT-CIRCUITING KEY OP DU BOIS-REYMOND. 

 figs. 10, 11, 13) ; or by bridging across a part of the circuit a passage 



PIG. 13. FRICTION KEY IN A BATTERY CIRCUIT ; DIRECT METHOD OP USE. 



with very little resistance is offered through the key, and the current 

 is thus diverted from the main circuit and from the electrodes (short- 



