METHODS OF ELECTRICAL STIMULATION 51 



The central binding posts are prolonged upwards 

 and each is slotted to receive a brass bar, which is 

 pivoted in the slot by a horizontal pin. The brass 

 bars are held parallel by two rubber rods which serve 

 as handles. When the bars are depressed to one side 

 or the other, they engage between plates of spring 

 brass set into brass blocks, each of which carries a 

 binding screw. Cross-wires enter these blocks, as 

 shown in the figure. At one end the cross- wires are 

 soldered into the blocks, thus making an electrical 

 contact. The two blocks at the other end are per- 

 forated by rubber cores or " bushings " through which 

 the cross-wires pass. The cross-wires, therefore, make 

 no electrical contact with these blocks. When a con- 

 tact is desired, the nut borne on the head of each 

 cross- wire is turned until its face presses against the 

 brass block outside the bushing. In this position the 

 key serves as a pole-changer, or commutator. When the 

 nut on the cross-bar between the central posts is turned 

 until its face presses against the post, it will short- 

 circuit the central posts. 



Polarization Current. Place two pieces of 

 platinum foil in a solution of copper sulphate, 

 and connect tbem to a pole-changer (without 

 cross-wires). Connect the remaining pairs of 

 posts with two dry cells in series (carbon of one 

 cell connected with zinc of other), and with the 



