176 



ELECTROTONUS. 



[ca. xvi. 



pairs of pools. Fig. 185 shows how, by altering the position of 

 the cradle, the direction of the current from one electrode to the 



Fig. 185. Pohl's commutator, with cross wires. (After Waller.) 



other is reversed. The numbers i, 2, 3, fec. indicate the path of 

 the current in the two cases. 



Sometimes the reverser is used without the cross wires for a different 

 purpose. The battery wires are connected as before with the middle 

 mercury pools. Each lateral pair of pools is connected by wires to a pair 

 of electrodes. The two pairs of electrodes may be applied to two portions 

 of a nerve, or to two different nerves, and by tilting the cradle to right or 

 left the current can be sent through one or the other pair of electrodes. 



Fig. 186. Simple rheochord. 



The rheochord is an instrument by means of which the strength 

 of a constant current passed through a nerve may be varied. It 

 consists of a long wire (r, r, r) stretched on a board. This is 

 placed as a bridge on the course of the battery current. (See 

 fig. 1 86.) The current is thus divided into two parts: one part 

 through the bridge, the other through the nerve which is laid 

 across the two non-polarisable electrodes at the ends of the wires. 

 The resistance through the bridge is varied by the position of the 

 slider (s s). The farther the slider is from the battery end of the 

 instrument the longer is the bridge, and the higher its resistance, 

 so that less current goes that way and more to the nerve. 



The next figure shows the more complicated form of rheochordi 



