described, there are meter windings in the line during times of
calling, but none in the line during times of conversation. The balance
of the line, therefore, is undisturbed at all times wherein balance is
of any importance.

In both systems just described, the meters of all lines are in their
respective central offices. Meters for use at subscribers' stations have
been devised and there is no fundamental reason why the record might not
be made at the subscriber's station instead of, or in addition to, a
central-office record. Experience has shown that confidence in a meter
system can be secured if the meters be positive, accurate, and reliable.
The labor of reading the meters is much less when they are kept in
central offices. Subscribers may have access to them if they wish.

_Prepayment Method._ Prepayment measured-service mechanisms permit a
coin or token to be dropped into a machine at the subscriber's telephone
at the time the conversation is held. A variety of forms of telephone
coin collectors are in use, their operations being fundamentally either
electrical or mechanical.

Electrically operated coin collectors require either that the coin be
dropped into the machine in order to enable the central office to be
signaled in manual systems, or the switches to be operated in automatic
systems, or they require that the coin be dropped into the machine after
calling, but before the conversation is permitted.

Western Electric Company coin collectors, shown in Fig. 459, may be
operated in either way in connection with manual systems. The usual way
is to require the coin to be dropped before the central-office line lamp
can glow. The operator then rings the called subscriber and upon his
answering places a sufficient potential upon the calling line to operate
the polarized relay and to drop the coin into the cash box. If the
called subscriber does not answer or his line is busy, potential is
placed on the calling line, moving the polarized relay in the other
direction and dropping the coin into a return chute so that the
subscriber may take it. If it is preferred that the coin be paid only on
the request of the operator, the return feature need not be provided.

In both forms of operation, the Western Electric coin collector is
adapted to bridge its polarized relay between one limb of the line and
ground during the time a coin rests on the pins, as shown in Fig. 459.
When no coin is on the pins--_i. e._, before ca