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 micrometer-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 automatically 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
