ELECTRIC BELLS. Bells or gongs struck by hammers vibrated by an electro-magnet placed in an electric circuit. The. apparatus and accessories consist of the bell, the push-button or circuit-closer, the conducting wire, and the battery. The battery may be composed of any of the constant elements described under ELECTRO-GALVANIC BATTERIES. For household purposes the Leclanche cell will be found excellently well suited, the number of elements used being regulated by the extent and consequent resistance of the circuit. The button or circuit-closer consists of two metallic strips p and g, Fig. 1171, placed one above the other. In its normal state the upper strip is separated from the lower one by a spring. To the strips p and g the conducting wires a and c are secured, and as the strips are separated the circuit remains open. It is closed when desired by pressing the knob p'. The button is inclosed in a wooden or rubber ease.

Fig. 1172 is a convenient device for combining a number of keys within a small compass, Eight push-buttons, corresponding to as many distinct circuits, are arranged at equal distances around a cylindrical case, within which the connections between the metallic strips and wires are made. Each wire is separately insulated by a silk covering, and all are made into a single strand as they leave the case.

The ordinary, form of bell used for giving single taps is shown in Fig. 1173. It consists of an electro-magnet M M, opposite the poles n s of which is placed the armature with its clapper k. The latter in its normal position is held back from the bell G by a spiral spring attached to the movable upright d, which serves to regulate its tension. The stroke of the armature is limited by the setscrew r.

By a slight modification of the connections in the bell instrument, the apparatus can be used both as a vibrator and as an instrument to give single taps.



The general plan is shown in Fig. 1174, in which Hand e are the electromagnet and armature. S is a switch which can be turned on B or E at pleasure. When it is on E, the apparatus becomes a vibrating instrument; when turned on B,. there is no interruption of the current with the attraction of the armature, and the instrument simply responds by single taps to each closing of the circuit by the push-button. The path of the current, when the switch is on B and E respectively, is sufficiently evident from the figure without further description.

When it is desirable to produce a very loud sound, the double bells and double electromagnets are usually employed in the vibrating apparatus. In general, the principle of all vibrating bells is that of the self-acting make and break; but, when the contacts are rigid points, the vibrations of the armature take place only within narrow limits, and the arrangement cannot very well be utilized for ringing a bell. Siemens has devised a plan, in his dial instruments, which answers the purpose much better, by giving the armature a greater range of movement; but the adaptation of this device to the ringing of bells for simple calls is a little troublesome, and in fact, for general use, would be altogether too complicated. By far the most preferable way of obtaining the desired range of stroke is that already described, in which a spring of some kind forms part of the path for the current, and which, with the attraction of the armature, follows the latter for such a distance as may be required.

When one battery is to serve for operating several of the bells above described, the vibrators can not all be placed in one circuit, as each one interrupts the circuit independently of the others; and it is impossible, or rather impracticable, to make the armatures of the various instruments so that they will all vibrate in exactly the same time, or always be in unison. The plan generally adopted for such cases is shown in Fig. 1175, where each bell, I, II, III, has a separate conducting wire of its own, as represented by the numerals 1, 2, 3, and a return wire, L L, serves for all. If, now, one of the bells is operated by the pressure of a push-button in 1, 2, or 3, as the case may be, it acts without in any way interfering with the others, as they are all quite independent of the circuit thus interrupted.

The fault just noticed in connection with the vibrating armature, causing a break at each vibration, may be remedied in a very easy manner simply by causing the armature to cut its own magnet out of circuit after each attraction. The principle works very satisfactorily, and will be readily understood by reference to Fig. 1176. m m are the coils of the electromagnet; a, the armature, to which the clapper k is attached by means of a rather stiff spring; and f , an elastic steel spring, which readily follows the to-and-fro movement of the armature for a short distance. As will be seen, a current arriving at C passes through the wire 1, coils m m, and wire 2, to the line L ; the armature is thus attracted to the spring f. The forward movement of the armature brings the spring f against a contact c , and forms the shunt quite independent of the armature. As the resistance of this route is exceedingly small compared to that of the helices, almost the entire current passes by the new path, and the cores become demagnetized. The retractile force of the spring now preponderates, and the armature falls against the back stop, breaking the shunt circuit on its way. As this arrangement does not break the main circuit, any desired number can be placed in the same line and worked without interfering with each other.

When the bell system is to be used for long distances, or when a very loud ringing is desired, for which purpose the main line current, as a rule, is not sufficient, a relay and local battery are generally used; and with the heaviest apparatus, requiring still more power, the ringing is done by means of weights. Fig. 1177 represents an arrangement devised by Aubine, in which a single set of electro-magnets, M M, serve both for the relay and the call. A small projection on the upper end of the armature a, when the latter is in its normal position, supports the lever 3, keeping it from making contact with spring 4, and at the same time holding it firmly against spring 2. When now a current is sent into the line, it passes along the connection 1 to spring 2, thence to lever 3 and its connecting wire to spring f and armature a, and from there on through the coils to earth. This causes an attraction of the armature; lever 3 falls down on spring 4 and closes the local circuit, which again results in a magnetization of the core. The armature is thus made to vibrate in the manner already described, and a violent ringing is set up, which continues until, by pressure on the knob b , lever 3 is again raised and supported by the armature projection. (See "Talking Phonograph, and other Novelties," Prescott, New York, 1879, from which the foregoing is abridged.)