The single-needle key (or commutator) is illustrated in Fig. 100 and the circuit is shown in Fig. 101
and is also redrawn for clarity. When neither key is pressed the down line is joined to the up line via the galvanometer
and contacts a2, b2, and the local battery is out of circuit. The local set is in the receive condition and
the coil instrument will respond to incoming signals.
When the left key (b) is pressed, the positive terminal of the battery is connected to the Down line via b3,
while the negative terminal of the battery is joined to the Up line via b1 and a3. This sends a dot.
When the right key is pressed the positive terminal of the battery is connected to the Up line via a3,
while the negative terminal of the battery is joined to the Down line a1 and b3. This sends a dash.
If both keys are pressed (not a valid condition) the set is in the same state as when neither
are pressed - Up is joined to Down via a3 and b3, and the battery is out of circuit.
Note that the needle responds to both transmitted and received signals.
