THE MAKE AND BREAK OF THE PRIMARY CIRCUIT 65 
When the vulcanite strip is so rotated as to uncover the 
hole, c, the mercury in the two cavities reunites and thus 
makes the circuit. The reunion of the separated mer- 
cury masses should take place as smoothly as possible. 
To bring this about, the vulcanite knife blade is tapered 
at the edges so that it may plough through the mercury 
with as little disturbance as possible. 
FIG. 12. Diagram of the operating device for the knife-blade key; 
vertical view. /, triangle of brass bearing slits, g, g', and wings, w, w', 
rotating about axis, 0; /, /', actuating springs; &, &', levers for bringing 
tension upon springs, and at the same time operating release, i; m, m', 
stops for limiting motion of knife blade. 
The Operating Device. To secure uniformity of ac- 
tion the vulcanite blade must be operated automati- 
cally, hand operation being liable to wide variations in 
the speed with which the contact is made or broken. 
The method adopted in this instrument is illustrated by 
the diagrams (Figs. 12 and 13). The axis of rotation of 
the blade, o (Figs, n and 12), after passing through the 
