[Illustration:  Fig. 79.  Polarized Bell]

Kellogg Ringer. Another typical ringer is that of the Kellogg Switchboard and Supply Company, shown in Fig. 80.  This differs from that of the Western Electric Company mainly in the details by which the armature adjustment is obtained.  The armature supporting yoke 1 is attached directly to the cores of the magnets, no supporting side rods being employed.  Instead of providing means whereby the armature may be adjusted toward or from the poles, the reverse practice is employed, that is, of making the 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 indicates that the bell is to be operated only by negative impulses.