is, of course, necessary that provision be
made whereby the magnets which furnish the energy for stepping the
wheels will not be energized by the ringing current. This is
accomplished in one of several ways, the most common of which is to
have the stepping magnets polarized or biased in one direction and the
bells at the various stations oppositely biased, so that the ringing
current will not affect the stepping magnet and the stepping current
will not affect the ringer magnets.

After a conversation is finished, the line may be restored to its
normal position in one of several ways. Usually so-called release
magnets are employed, for operating on the releasing device at each
station. These, when energized, will withdraw the holding pawls from
the ratchets and allow them all to return to their normal positions.
Sometimes these release magnets are operated by a long impulse of
current, being made too sluggish in their action to respond to the
quick-stepping impulses; sometimes the release magnets are tapped from
one limb of the line to ground, so as not to be affected by the
stepping or ringing currents sent over the metallic circuit; and
sometimes other expedients are used for obtaining the release of the
ratchets at the proper time, a large amount of ingenuity having been
spent to this end.

As practically all step-by-step party-line systems in commercial use
have also certain other features intended to assure privacy of
conversation to the users, and, therefore, come under the general
heading of lock-out party-line systems, the discussion of commercial
examples of these systems will be left for the next chapter, which is
devoted to such lock-out systems.

Broken-Line Method. The broken-line system, like the step-by-step
system, is also essentially a lock-out system and for that reason only
its general features, by which the selective ringing is accomplished,
will be dealt with here.

_Principles_. In this system there are no tuned bells, no positively
and negatively polarized bells bridged to ground on each side of the
line, and no step-by-step devices in the ordinary sense, by which
selective signaling has ordinarily been accomplished on party lines.
Instead of this, each instrument on the line is exclusively brought
into operative relation with the line, and then removed from such
operative relation until the subscriber wanted is connected, at which
time all of the other instruments are locked out and the line