and
to the coherer an injurious shock; and, second, to prevent the accidental
operation of the transmitter when the aerial conductor is connected to
receiver. The arm of the key is prolonged beyond its pivot, and carries
an insulated contact which is permanently connected to the aerial conductor.
Below this contact on the base of the instrument is the terminal of the
receiver. The arm is so arranged that immediately after its release
by the operator, subsequent to the sending of a message, it turns about
upon its pivot, bringing the above mentioned contact and terminal together,
so connecting the receiver with the aerial conductor.
In the drawing b' and b4 indicate the contacts of an ordinary Morse key and a high insulating handle. The extension arm b has an insulated contact b2. When the key is released by the operator its longer arm falls by its own weigh, the contact b2 descending the contact b3.
[The above was taken from pages 188 and 189 of Sewall's book which was the first wireless telegraphy book published in the United States]