ELEMENTARY EXPERIMENTAL PHYSIOLOGY 79 



JT IG- 77. The rheocord arranged to vary the strength of a current passing 



through a nerve. 



It consists of two parallel wires connected by a movable metal slider S. By moving- the slider 8 

 to the right the resistance of the rheocord in circuit and therefore the amount of battery 

 current passing through the nerve would be increased. 



If an electrode is to be applied to the skin, it should have a large 

 surface covered with chamois leather saturated in a strong solution 



FIG. 78. Two forms of electrodes. 



of common salt in order that the resistance of the skin may be 

 overcome. 

 'In certain physiological investigation non-polarisable electrodes 



FIG. 79. A mercury key. 



FIG. 80. A spring key. 



FIG. 81. A friction or Du 

 Bois key. 



