46 THE SIGNS OF LIFE [LECT. 



the induction shock (and first part of the response) occurring 

 while the galvanometer is plugged. 



It will be most convenient if we make a first pair of trials to 

 witness the after-effects of two single break induction shocks in + 

 and directions. 



It is necessary, in first place, to get rid of the make induction 

 shock. You will readily see by reference to the diagram how 

 this is done. The primary circuit of the inductorium is closed 

 by a spring key, and while this is done, the secondary coil is 

 short-circuited at the keyboard (see Fig. 19, p. 42). Then this 

 plug is removed and the spring-key released so that a break 

 induction shock is passed through the eyeball. And finally the 

 galvanometer plug is removed, so that the current (if any) 

 aroused in the eyeball, traverses the galvanometer and deflects 

 its spot. These steps are quite automatically made after a little 

 practise, and at a quite sufficiently uniform rate ; the electro- 

 motive response of the eyeball is so prolonged that it is not 

 necessary to hurry ; although obviously for comparative trial it 

 is preferable, as well as more convenient, to use a special key 

 that breaks short-circuit of the galvanometer at a regular interval 

 (?-> iVth sec.) after breaking the primary circuit.* It is evident 

 that in order to plug and unplug the galvanometer without dis- 

 turbing its spot, all current in circuit must be compensated. 

 This compensation has been adjusted at the outset, and must 

 be exactly re-adjusted before each trial. Then we shall be 

 assured that a given deflection is in reality due to current 

 aroused by excitation, and not to any accidental current in 

 circuit. 



I may now proceed with the experiment. I begin by testing 

 the compensation, and, if necessary, adjusting it. I then send a 

 break induction shock through the eyeball in the negative 

 direction, and afterwards unplug the galvanometer. The spot 

 flies off scale to your right. That has been a blaze-current, of 

 positive direction in response to an induction shock of negative 

 direction ; it is not a polarisation current ; although at first 

 sight by reason of its direction it might have been set down as 



* See Appendix, Fig. 68, p. 166. 



