Fig. 42.
The great point of departure is the discarding of the unsightly magnet box, and the hemispherical bell (see [Fig. 32]), and substituting a bell of the Church type (see [Fig. 42]), and placing inside it an electro-magnet specially arranged. The inventors use a single solenoidal magnet of a peculiar construction, by which the armature is attracted by both poles simultaneously. By this means less than half the usual quantity of wire is required, thus reducing the external resistance of the circuit one half. Moreover the armature, besides being magnetised by induction, as acted on in the ordinary method of making electric bells, is by Messrs. Jensen's plan directly polarised by being in actual magnetic contact by the connection of the gimbal (which is one piece with the armature) with the core iron of their magnet. It is thus induced to perform the largest amount of work with the smallest electro-motive force. Instead of the armature and clapper being in a
straight line attached to a rigid spring, which necessitates a considerable attractive power to primarily give it momentum, in the Jensen Bell the armature and hammer are in the form of an inverted U, and being perfectly balanced from the point of suspension, the lines of force from a comparatively small magnetic field suffice to set this improved form of armature into instant regular vibration. By using a flexible break and make arrangement instead of the usual armature spring and set screw (at best of most uncertain action), it is found that a much better result is attained, and by this device the armature can be set much nearer the poles of the magnet with sufficient traverse of the hammer. This is in strict accordance with the law of inverse squares, which holds that the force exerted between two magnetic poles is inversely proportionate to the square of the distance between them, or, in other words, that magnets increase proportionately in their power of attraction as they decrease in the square of the distance. It will now be seen why these bells require so little battery power to ring them: firstly, the armature and hammer are so perfectly balanced as to offer but little resistance; secondly, the external resistance to the current is reduced; and thirdly, the best possible use is made of the electro-magnetic force at disposal.
§ 50. The next modification which demands attention is the so-called "Circular bell." This differs from the ordinary form only in having the action entirely covered by the dome. Except, perhaps, in point of appearance,
this presents no advantages to that. The bells known as "Mining bells" resemble somewhat in outward appearance the circular bell; but in these mining bells the action is all enclosed in strong, square teak cases, to protect the movement, as far as possible, from the effects of the damp. All the parts are, for the same reason, made very large and strong; the armature is pivoted instead of being supported on a spring, the hammer shank being long, and furnished with a heavy bob. The domes or bells are from 6 inches to 12 inches in diameter, and are generally fitted with single stroke movement, so as to enable them to be used for signalling. The hammer shank, with its bob, and the dome, which stands in the centre of the case, are the only parts left uncovered, as may be seen on reference to Figs. [43 A] and [B], where the exterior and interior of such a bell are shown.
Fig. 43 A.