THE MAKE AND BREAK OF THE PRIMARY CIRCUIT 63 
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It is, of course, wholly undesirable to equip the pri- 
mary circuit with two keys, one of mercury to be 
used for making the circuit, and another of platinum for 
breaking it. I shall describe, therefore, an automatic 
make and break key with 
mercury contacts which has 
been proved by several years' 
experience to give uniform 
breaks and makes.* 
The key consists of a block 
of vulcanite 30 mm. long, 
20 mm. wide, and 25 mm. 
deep, having cut in it two 
vertical chambers (see Fig. FlG TI Diagram illustrating 
II), one (a) rectangular, 20 the principle of the vulcanite 
mm. long, 8 mm. wide, and 
20 mm. deep; the other (b) 
cylindrical, 6 mm. in diameter 
and 20 mm. deep. A hole, c 
(Fig. n), 3 mm. in diameter, 
is bored through from one of 
these cavities to the other at 
a depth of about 16 mm. Each of the chambers is in 
electrical communication with a binding post, and when 
filled with mercury they are in electrical communication 
with each other through the connecting- hole, c. 
* Martin: Am. Jour, of PhysioL, 1910, xxvi, p. 181. 
knife-blade key. The front of 
the block is broken away to 
show the relations of the parts 
within the chamber, a. a and 
b, mercury chambers; c, open- 
ing between a and b; d, vul- 
canite knife blade supported 
upon axis, o, which rotates 
within collar, e. 
