poles themselves extensible. This is
done by means of the iron screws _2_ which form extensions of the
cores and which may be made to approach or recede from the armature by
turning them in such direction as to screw them in or out of the core
ends.

[Illustration: Fig. 80. Polarized Bell]

[Illustration: Fig. 81. Biased Bell]

_Biased Bell._ The pulsating-current generator has already been
discussed and its principle of operation pointed out in connection
with Fig. 77. The companion piece to this generator is the so-called
biased ringer. This is really nothing but a common alternating-current
polarized ringer with a light spring so arranged as to hold the
armature normally in one of its extreme positions so that the tapper
will rest against one of the gongs. Such a ringer is shown in Fig. 81
and needs no further explanation. It is obvious that if a current
flows in the coils of such a ringer in a direction tending to move the
tapper toward the left, then no sound will result because the tapper
is already moved as far as it can be in that direction. If, however,
currents in the opposite direction are caused to flow through the
windings, then the electromagnetic attraction on the armature will
overcome the pull of the spring and the tapper will move over and
strike the right-hand gong. A cessation of the current will allow the
spring to exert itself and throw the tapper back into engagement with
the left-hand gong. A series of such pulsations in the proper
direction will, therefore, cause the tapper to play between the two
gongs and ring the bell as usual. A series of currents in a wrong
direction will, however, produce no effect.

Conventional Symbols. In Fig. 82 are shown six conventional symbols
of polarized bells. The three at the top, consisting merely of two
circles representing the magnets in plan view, are perhaps to be
preferred as they are well standardized, easy to draw, and rather
suggestive. The three at the bottom, showing the ringer as a whole in
side elevation, are somewhat more specific, but are objectionable in
that they take more space and are not so easily drawn.

[Illustration: Fig. 82. Ringer Symbols]

Symbols _A_ or _B_ may be used for designating any ordinary polarized
ringer. Symbols _C_ and _D_ are interchangeably used to indicate a
biased ringer. If the bell is designed to operate only on positive
impulses, then the plus sign is placed opposite the symbol, while a
minus sign so placed