PHYSIOLOGIC APPARATUS. 



677 



also to the cell. The amount of current passing through the nerve circuit 

 will be inversely proportional to the resistance of the nerve and directly 

 proportional to the difference of potential between A and S. If S is close 

 to A, the $ difference of potential is slight. If S is removed from A toward 

 B, the difference of potential is increased and the current sent through 

 the nerve circuit is increased. 



In many experiments it is necessary to reverse the direction of the cur- 

 rent, in other experiments to deflect it, without changing the position of 

 the electrodes. Both these results may be accomplished by the use of 



Ponl's commutator. This is a round block of wood with six cups, 

 each of which is in connection with a binding post (Fig. 341). In each of 

 the two cups marked i and 

 2, -J- and , is inserted one 

 end of a copper wire bent at 

 right angles. The other ends 

 of the wires are supported and 

 insulated by a hard-rubber 

 handle. To the top of each 

 wire is soldered a semicircular 

 copper wire. This arrange- 

 ment permits of a rocking 

 movement, whereby the oppo- 

 site ends of the semicircular 

 wires can be made to dip into 

 cups 3 and 4, and into cups 

 5 and 6 alternately. Two 

 wires crossed in the middle of 

 the block serve to connect- 

 opposite pairs of cups. When 

 in use, the cups are filled with 

 clean mercury. The method 

 of using the commutator is as 

 follows : 



i. Asa Current Reverser. The 

 positive and negative poles 



of the electric cell are connected by wires with binding posts i and 2 

 respectively. A key is interposed in the circuit. Wires are then carried 

 from binding posts 3 and 4 to the electrodes in connection with the 

 muscle or nerve. The rocker of the commutator is so turned that the 

 ends of the semicircular wires dip into cups 3 and 4. The direction of 

 the current will be on the closure of the circuit from i to 3, then from 

 3 along a wire to and through the tissue and back to 4, and thence 

 to the cell. If the position of the rocker be now reversed so that the 

 opposite ends of the semicircular wires dip into cups 5 and 6, the 

 direction of the current through the tissue will be reversed. The 

 positive current, after entering binding post i, will flow to 5; then 

 along one of the cross wires to 4; then along a wire to and through 

 the tissue and back to 3, along the opposite cross wire to 6, thence to 2, 

 and so back to the cell. 



FIG 340. RHEOCORD. 



