ELEMENTARY EXPERIMENTAL PHYSIOLOGY 85 



do this connect wires to the terminals at its base. One of these 

 is insulated and the drum consequently acts as an interrupter of 

 the primary circuit except at the instant at which the striker 

 makes contact with the knife edge projecting from the insulated 

 terminal. 



By this means one induced shock (make and break) is given 

 at a fixed point in each revolution of the drum. 



III. The Use of the Commutator. 



The commutator, one form of which is shown in Fig. 92, 

 consists of six mercury pools arranged in three pairs and connected 

 with terminals. The centre pair can be connected at will by 

 means of a movable bridge with either the left-hand pair or the 

 right-hand pair (Fig. 93). 



FIG. 93. 



A . It is used to reverse the direction of the current in the circuit. 

 Connecting the diagonally opposite pools at either end are cross 

 wires insulated from each other. 



The current is led from the electric supply to the middle 

 pools. 



(i) When the bridge is placed as in Fig. 93, i, the current from 

 the positive terminal follows the metallic circuit and reaches 

 electrode A direct. If now the gap between A and C is bridged 

 the current flows into the left lower pool, across the bridge to 

 the lower middle pool and back to the negative terminal on 

 the bench. 



(ii) When the bridge is placed as in Fig. 93, n, the current 

 from the positive terminal passes to the right upper pool, 

 then along the diagonal wire to the left lower pool and 

 reaches electrode a. On bridging the gap the current flows to the 

 left upper pool along the wire to the right lower pool, across the 

 bridge to the lower middle pool and so back to the negative 

 terminal on the bench. 



