Transmitter Switch (Microphone Switch)
Operating the handset switch closes the circuit consisting of the
switch, battery, transmitter, and 2-3 section of the induction coil. Since
this circuit has a relatively low impedance, large current changes are
produced by the transmitter when it is activated by speaking into the
transmitter. These current changes induce relatively high voltages in
the entire 1-4 winding of the induction coil and across terminals L1 and
L2 to which the line is connected.