168 H. J. Denham 
engraved on the head, as in the figure, or filled in with red and black. 
Before use all contacts should be amalgamated. The use of iron or 
steel wire should be avoided, particularly when oil is used, as the 
system may act as a Lodge coherer. 
Modifications. Where intermittent contact only is required, a 
spring may be fitted over the distance piece or sleeve ( D ), to lift the 
contacts out of the mercury when pressure on the actuating head is 
released. 
When the instrument is required only for “dry” work, the long 
stem and actuating head may be dispensed with, and the numbers 
of the circuits engraved on the selector disk, which should be fitted 
with a central knob, and then serves as a temperature shield. 
By fitting the selector disk with bridge-pieces other than those 
shown, the switch may be used for a very large number of com¬ 
binations and complex circuits. The number of mercury cups is 
limited only by the size of the base. 
Shirley Institute, 
Didsbury, Manchester. 
November 30, 1921. 
