would carry the generator currents much more
readily and thus defeat the purpose for which it was intended.

In order that the requisite impedance may be given to the ringers
employed for bridging party lines, it is customary to make the cores
rather long and of somewhat larger diameter than in series ringers and
at the same time to wind the coils with rather fine wire so as to
secure the requisite number of turns. Bridging bells are ordinarily
wound to a resistance of 1,000 or 1,600 ohms, these two figures having
become standard practice. It is not, however, the high resistance so
much as the high impedance that is striven for in bridging bells; it
is the number of turns that is of principal importance.

As has already been stated, the generators used for bridging lines are
made capable of giving a greater current output than is necessary in
series instruments, and for this purpose they are usually provided with
at least four, and usually five, bar magnets. The armature is made
correspondingly long and is wound, as a rule, with about No. 33 wire.

Sometimes where a bridged party line terminates in a central-office
switchboard it is desired to so operate the line that the subscribers
shall not be able to call up each other, but shall, instead, be able
to signal only the central-office operator, who, in turn, will be
enabled to call the party desired, designating his station by a
suitable code ring. One common way to do this is to use biased bells
instead of the ordinary polarized bells. In order that the bells may
not be rung by the subscribers' generators, these generators are made
of the direct-current type and these are so associated with the line
that the currents which they send out will be in the wrong direction
to actuate the bells. On the other hand, the central-office generator
is of direct-current type and is associated with the line in the right
direction to energize the bells. Thus any subscriber on the line may
call the central office by merely turning his generator crank, which
action will not ring the bells of the subscribers on the line. The
operator will then be able to receive the call and in turn send out
currents of the proper direction to ring all the bells and, by code,
call the desired party to the telephone.

[Illustration: ONE WING OF OPERATING ROOM, BERLIN, GERMANY Ultimate
Capacity 24,000 Subscribers' Lines and 2,100 Trunk Lines.
Siemens-Halske Equipment. Note Horizontal Disposal of Multip