DETERMINATION OF THE LATENT PERIOD 113 



a pin attached to the spring-handle K, when a complete 

 turn has been made : the recording-plate is thus held 

 arrested at its highest position. When desired, a pressure 

 on the handle k releases the disc, the axis of the motor 

 begins to unwind, and the plate is allowed to fall. The 

 motor is fully wound at the beginning, and the partial 

 unwinding during one revolution is exactly compensated 

 before the next observation by the winding necessary to 

 pull up the plate. Owing to the constancy of this winding, 

 the rate of fall in successive experiments is kept the same. 

 The pressure of the handle K, which releases the plate, 

 also causes ' make ' of the current in the primary coil. This 

 circuit of the primary coil is completed in addition through 

 a contact-breaking device. 



This consists of a long strip of ebonite, fixed along 

 one edge of the recording-plate carrier. On the lower 

 end of the ebonite a conducting-strip of platinum is sunk 

 in and provided with a binding-screw. In front of this 

 slides a rod with contact-point tipped with platinum. 

 This can be adjusted up or down by means of a fine micro- 

 meter-screw, A. When the recording-plate is released, 

 carrying with it the conducting-strip, the primary circuit 

 is broken as soon as the line of junction between platinum 

 and ebonite is reached. This sudden interruption of the 

 primary current gives rise in the secondary coil to an 

 instantaneous break-shock, which passes through the plant. 

 In order that shocks in successive experiments shall always 

 be given at the same definite predetermined position in 

 the fall of the plate, the following device is adopted : The 

 recording-plate, as we have seen in a previous chapter, 

 slides up and down a vertical support of triangular section. 

 A movable peg fixed in the support holds it temporarily 

 at a certain selected point chosen as that at which, during 

 the descent of the plate, the shock is to be given auto- 

 matically to the plant. For the purpose of adjustment a 

 galvanometer is interposed in the primary circuit. So 

 long as the point of contact-rod is in touch with the 



