THE MAKE AND BREAK OF THE PRIMARY CIRCUIT 69 
at one margin of the base. From p a wire leads through 
the body of the vulcanite block to the rod, /, to which it 
is soldered near the axis of rotation of the rod. From 
p' two wires are carried through the block, one to the 
axis of rotation of the lever, k, to which it is soldered, the 
other to the axis of k', where it is soldered likewise. 
Thus both levers are in electrical connection with the 
post, p', and the rod, t, in similar connection with the 
post, p. Soldered to the levers, k and k', at the points, z 
and z', are pins of platinum projecting downward. These 
pins are so placed that the mercury cup, 0, can be 
brought directly below one or the other of them according 
as t is rotated. Their length is so adjusted that the pin 
dips into the mercury when the lever is depressed enough 
to release the mechanism, but is clear of the mercury at 
all other times. 
If the binding posts, p and p 1 ', are connected in parallel 
into the secondary circuit of the inductorium and the 
rod, /, is rotated so as to bring the mercury cup below the 
lever which is pressed when the primary circuit is made 
(the left-hand one in this instrument) the make shocks 
are all short-circuited. Bringing the mercury cup be- 
low the other lever short-circuits all the break shocks. 
When the rod is placed in an intermediate position, 
neither makes nor breaks are affected. To prevent all 
possibility of accidental diversion of the secondary cur- 
rent into the hand of the operator, vulcanite shields are 
