Marine or waterproof bells have an iron cover fitting tight over a rubber gasket; they are for marine, or mining, work.
Polarized, or magneto, bells are used in telephone work, and are rarely operated by a battery, but have a miniature dynamo generator operated by hand, or power, to supply the actuating current.
Most bells are classed for size by the diameter of the gong, a four-inch bell being one with a gong four inches in diameter; a six-inch bell one with a six-inch gong, and so on.
According to the use for which they are intended, bells may be vibrating, as before described, single-stroke, shunt or short-circuiting, differential, continuous-ringing, or adapted for circuits of high voltage.
The Single-stroke Bell.
The bell before described, and again shown in [Fig. 5], is a vibrating, or trembling, bell. It is often desired to have the hammer give only one stroke for each pressure of the push, as in signaling with a code of taps; in this case a single-stroke bell is used. The circuit from the binding posts is then directly through the magnet coils without any break at the contact screw, as in [Fig. 6].
Fig. 5 Fig. 6 Fig. 7
In adjusting such a bell to give a clear sound, press the armature up against the iron magnet cores and then bend back the hammer until it just clears the gong. The spring of the hammer wire will carry the hammer sufficiently forward to hit the gong. The tone will be clearer than if the hammer dampered the gong by pressing against it when the armature was nearest the core.
By bringing out a third connection, a vibrating bell may be made both single stroke and vibrating.