THE MAKE AND BREAK OF THE PRIMARY CIRCUIT 67 
edge of / by a stout spring, in such fashion that when 
either slit is engaged/ is prevented from moving. Each 
of the levers, k and k' bears at its tip an arm, r, r' (Fig. 
13), which presses upon the releasing device, and when 
the lever is depressed to a certain point disengages it, 
allowing the blade to rotate. The amount of motion 
of the blade is limited by setting two posts, m and m', 
Sit such positions that the lower apices of / strike them 
when sufficient movement has occurred. 
After experimenting with various operating devices the 
one described above has been adopted as combining the 
greatest number of desirable features with the fewest 
defects. The two levers, k and k 1 ', which are depressed 
alternately for making and breaking the circuit, are so 
placed as to lie naturally under the first and second 
fingers of either the right or the left hand. The springs, 
/ and l f , need not be stiff, hence little pressure need be 
exerted upon the levers, and there is correspondingly 
little fatigue from continuous operation of the key. 
The springs are brought under tension only during the 
use of the instrument; when it is not in use, they hang 
free. Thus their stiffness does not vary with the lapse 
of time, as would be the case were one or the other under 
constant tension. 
The Short-circuiting Device. A desideratum in any 
key which is to be used for stimulating tissues with single 
induction shocks is a device for short-circuiting auto- 
