180 PHYSIOLOGICAL PHYSICS. [Chap. xvn. 



the space between the end of the cylinder and 

 the upright which supports it. By the revolution 

 of the cylinder the copper wire is dipped into the 

 dish or trough containing mercury, carried through 

 it and out at the other side. To make the circuit, 

 therefore, dip one end of a wire into the trough, 

 the other end being connected with a screw of the 

 primary spiral of the inductorium, from the induc- 

 torium lead one wire to the battery, and carry another 

 wire direct from the battery to the screw connected 

 with the pivot on which the cylinder moves (a, Fig. 88). 

 Thus the current from the battery reaches the screw 

 supporting the axis of the cylinder, passes across to 

 ike cylinder, and from it, when the copper wire which 

 it carries dips into the mercury trough, the current 

 passes to the mercury, and so gains the wire that 

 takes it to the inductorium round the primary coil, 

 and back to the battery. Thus, when the copper wire 

 carried round by the cylinder dips into the mercury 

 trouafh the circuit is closed, when the wire leaves the 



O ' 



trough the circuit is opened, and with each opening 

 and closing an induced current is produced in the 

 secondary coil which stimulates the muscle. 



The pendulum myograpliion of Pick is 

 shown in Fig. 89, in the improved form given to it by 

 Helmholtz. The lower end of the pendulum carries 

 a smoked glass plate A, which can be moved up or 

 down in its frame by turning the wheel shown above 

 it. It can also be moved to either side. By the up or 

 down movement the period of oscillation of the pen- 

 dulum would be altered, and to prevent this a similar 

 plate is placed at the back which moves up when the 

 front one moves down, and vice versa. [The second 

 plate is not shown in the figure.] When the pen- 

 dulum is pulled to one side, it is caught and held there 

 by a piece of brass a projecting downward, being 

 fixed by the catch b. When this catch is pulled 



