armature winding shown diagrammatically in that figure, is terminated
in the pin _5_, while the other terminates in the pin _7_. When the
armature is assembled in the frame of the generator it is evident that
the frame itself is in metallic connection with one end of the
armature winding, since the pin _5_ is in metallic contact with the
armature casting and this is in contact with the frame of the
generator through the bearings. The frame of the machine is,
therefore, one terminal of the generator. When the generator is
assembled a spring of one form or another always rests against the
terminal pin _7_ of the armature so as to form a terminal for the
armature winding of such a nature as to permit the armature to rotate
freely. Such spring, therefore, forms the other terminal of the
generator.

Automatic Shunt. Under nearly all conditions of practice it is
desirable to have the generator automatically perform some switching
function when it is operated. As an example, when the generator is
connected so that its armature is in series in a telephone line, it is
quite obvious that the presence of the resistance and the impedance of
the armature winding would be objectionable if left in the circuit
through which the voice currents had to pass. For this reason, what is
termed an _automatic shunt_ is employed on generators designed for
series work; this shunt is so arranged that it will automatically
shunt or short-circuit the armature winding when it is at rest and
also break this shunt when the generator is operated, so as to allow
the current to pass to line.

[Illustration: Fig 74. Generator Shunt Switch]

A simple and much-used arrangement for this purpose is shown in Fig.