This section is from the book "Cassell's Cyclopaedia Of Mechanics", by Paul N. Hasluck. Also available from Amazon: Cassell's Cyclopaedia Of Mechanics.
The essential parts are shown in the adjoining illustration. A push A is connected to a spring B. Underneath the push is a smaller spring contact C, and at the side opposite 13 is another contact D. B, C, and D each have terminals, not necessarily in the form sketched. By these terminals the connections required may be made. In the standing position B and D make contact, but by pressing A the circuit is made by way of B and C.
Construction of Double-contact Push.
 
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