8 



PRACTICAL PHYSIOLOGY 



the circuit (make and break the current) by con- 

 necting the wires together either through a pool of 

 mercury (inercury key Fig. 9) ; or by contact be- 



FIG. 9. MERCURY KEY IN A BATTERY CIRCUIT. 



tween a platinum plate and platinum point (contact 

 key Fig. 10), as in the Morse key ; or by friction 

 contact between two brass surfaces {friction 



FIG. 10. CONTACT KEY IN A BATTERY CIRCUJT. 



as in that known as du Bois-Reymond's (Fig. n), 

 and in the ordinary electric-light switches. They 

 are used in two ways, viz. : either to simply close 



FIG. ii. FRICTION KEY IN A BATTERY CIRCUIT; DIRECT METHOD OF USE. 



or open the circuit (direct method} ; or by bridg- 

 ing across a part of the circuit a passage with very 

 little resistance is offered through the key, and the 

 current is thus diverted from the main circuit and 



