60 PHYSIOLOGICAL PHYSICS. [Chap. vi. 



down the arc to the wire at 3, which is, therefore, + , 

 through the instrument and back by the wire at 4, 

 which is, therefore, . From 4 the current passes up 

 the arc to b, down to 2, and so back to the battery. 

 Suppose now the bridge be reversed. The current 

 enters at 1, passes up a, but can no longer pass down 

 to 3, because the arc is raised out of the mercury. It 

 goes down the other side, therefore, which dips into 5. 

 But at 5 there is no wire to lead it off, except the limb 

 d of the cross. Along d, then, it proceeds to 4, and by 

 the wire at 4 passes to the instrument, from which it 

 returns by the wire to 3. The wire at 4 is therefore 

 4- instead of , as before ; and that at 3 is 

 instead of + , as before ; that is to say, the direction 

 of the current has been reversed. From 3 the current 

 passes across c to 6, up the arc to b, and back from 2 

 to the battery. Consequently, with the cross in, re- 

 versing the commutator reverses the direction of the 

 current through one and the same apparatus. Suppose 

 it were a nerve to which the current must be con- 

 veyed, by means of the commutator the current could 

 be sent up or down the nerve at pleasure. (2) Let the 

 cross be taken out ; then, when the bridge is inclined as 

 in the figure, the current would pass off by wires 

 attached to 3 and 4 ; when the bridge is reversed it 

 would pass off by wires at 5 and 6. So that one 

 apparatus might be connected with 3 and 4, and 

 another and entirely different apparatus with 5 and 6. 

 Hence, the cross being out, the current can be sent now 

 to one and now to another apparatus at pleasure, the 

 commutator acting thus as a double key. 



Electrodes. For convenience in the application 

 of electricity various forms of electrodes have been 

 devised. One form frequently used is that of Du 

 Bois-Reymond's platinum electrodes (Fig. 34). They 

 are formed of a stand with a projecting arm, movable 

 by a universal joint at c. The arm carries a glass 



