PHYSIOLOGIC APPARATUS 763 



ment, whereby the opposite 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 com- 

 mutator is as follows: 



i. As a 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 semicir- 

 cular 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 



FIG. 339. POHL'S COMMUTATOR. A, Arranged as a current reverser; B, as a cur- 

 rent deflector. 



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. 



2. As a Current Deflector. When it is desirable to deflect the current to two 

 pairs of electrodes differently situated, wires are carried from binding 

 posts 3 and 4 to one pair, and from 5 and 6 to the other pair. The cross 

 wires are then removed. According to the position of the rocker the 

 current will be deflected to one or the other. 



The Inductorium. This is an apparatus designed for the purpose of 

 obtaining single or rapidly succeeding electric currents by induction. Its 

 construction is based on facts discovered by Faraday, some of which are the 

 following: 



If two circuits, a primary and a secondary, are placed parallel to each 

 other, the former connected with a galvanic cell, the latter with a galvan- 

 ometer, it is found that, at the moment the primary circuit is made, and at the 

 moment it is broken, a current is induced in the secondary circuit, as shown 

 by a momentary deflection of the galvanometer needle. During the con- 



