A CUT-OUT KEY 



13 



was from c to d, in the second position from d to c, i.e. by moving 

 the rocker the direction of the current in the external part of the 

 circuit has been reversed. 



This key can also be used in a second way by removing the cross- 

 wires, when two circuits can be closed by it, either from c to D or from 

 e to f. Suppose, for instance, that the two ends of a muscle were 

 connected by wires to e and f, and the wires of a pair of electrodes 

 upon which the nerve is lying to c and d, then if the key be in the 

 position of the figure a current entering at A and leaving at b is sent 

 through the muscle, whilst if the rocker be rotated into the cups c and 

 d the current through the muscle is broken, and instead is sent through 

 the nerve. For the mode of connecting the key for such a purpose, 

 see fig. 80, p. 95. 



A KEY FOR CUTTING OUT EITHER THE MAKE OR 

 BREAK SHOCK 



This consists (tig. 19) of two spring keys, one between p 1 and p 2 , closed when 

 the spring b is brought into contact with the metal piece D, and the other one 

 between s 1 and s 2 . The contact is made in each case between two platinum 



Fig. 19. — A Form of Cut-out Key. 



surfaces, one under b and the other projecting 

 up from D, and similarly in the other key. 

 These keys are closed automatically by two 

 vulcanite sectors, v 1 and V s , which are carried 

 on an axis which can be rotated by hand or 

 driven by a running cord round the coned 

 pulley. These sectors can be rotated into any 

 position on the horizontal axis, and clamped by 

 screws. Fit up the key p 1 p 2 to make and 

 break a current through the primary, and s 1 s'-', so that it short-circuits 

 the secondary when it is closed. Thus the two terminals of the secondary 

 are comiected, one to s 1 and the other to s'-\ and the two electrode wires 



