This section is from the book "A Treatise On Architecture And Building Construction Vol3: Stair Building, Ornamental Ironwork, Roofing, Sheet-Metal Work, Electric-Light Wiring And Bellwork", by The Colliery Engineer Co.. Also available from Amazon: A Treatise On Architecture And Building Construction.
76. In nearly all electric appliances, particularly in those we are about to describe, the electromagnet is an all important feature. It was early discovered that when a current of electricity circulated through a coil of wire, magnetic poles, having the same properties of attraction and repulsion as a permanent steel magnet, were developed at each end of the coil. If a piece of iron be inserted in the coil, as in Fig. 54, the magnetic effects are much intensified; one end N of the iron core becomes a North pole, the other s a South pole, corresponding to the polarity of the coil before the introduction of the iron core. To prevent diversions of the current, the wire should be wrapped around with cotton or other insulating substance.
The intensity of magnetization depends on the amount of current flowing, also upon the number of turns of wire; the current, then, being measured in amperes, the magnetization is dependent upon the number of ampere-turns. Since the currents used in bell and in signal work are very small, it is necessary to wind a large number of turns around the magnet cores, thereby producing the same effect as would be obtained by a few turns of wire carrying a heavy current. The most usual form of electromagnet for bells is that shown in Fig. 55, and is made in three parts, namely, two iron cores, such as M, wound with the magnetizing coils, and a straight bar of iron b joining the two cores. The current passes through the coils in series, and should circulate around one core in the opposite direction to that taken around the other, producing a North pole at the free end of one core, and a South pole at the free end of the other, as this condition will give the greatest attractive power for the bar of iron, called the armature, which is generally used in connection with this form of electromagnet.
77. The principle involved in the action of the electric bell is an alternate attraction and release of a soft-iron armature placed in front of an electromagnet and carrying a hammer, the motion being due to intermittent currents through the wire of the electromagnet. In Fig. 56 is shown a type of skeleton bell, in which all the parts are visible. The battery wires are connected at the terminals t, t', and the course of the current is as follows: From the terminal t to the adjustment screw s, which is tipped with platinum in order to prevent oxidation of the contact surface, through the spring l and the end of the armature a to the coils of the magnets m, m', and out at the terminal t'. When no current is passing, the armature is held away from the poles of the electromagnets, as in the position shown, but as soon as a battery is connected up and a current sent through the coils, the magnets become energized and attract the armature a, which swings about the pivot p, causing the hammer h to strike the bell. This movement breaks the circuit at s and l, and the iron cores being thereby demagnetized, the spring c draws the armature away, when the spring l again touches the screw s, completing the circuit. As long, then, as the battery current is free to flow, this vibration of the armature and hammer will continue. The tension of the release spring c may be changed to suit the strength of the battery, by means of the regulating screw r, which is provided with nuts for this purpose on each side of the supporting pillar. The bell mechanism is usually enclosed, to prevent entrance of dust or insects, which may interfere with the working of the bell by lodging on the contact points, thereby preventing the current from passing through the magnets.
78. The bell just described is of the common vibrating class. When a bell is required to give a single stroke each time the circuit is closed, that is, for each pulsation of current, a slight difference in the connection of the ordinary bell is necessary. A wire is connected between the end of the magnet coil m and the terminal t, so that the circuit is simply from one terminal to the other through the coil. Hence, when a current passes through, the armature is attracted and held, a single stroke being given to the bell; on interrupting the current, the armature is drawn back to its normal position by the spring c.
79. The buzzer, Fig. 57, is used in places where an electric bell would be undesirable, as in small, quiet rooms or on desks, and is constructed on the same principle as the bell, except that the armature does not carry a hammer. In the illustration, the cover c is removed, showing the magnet coils m, m' and the armature a. An adjusting screw s is provided to regulate the stroke of the armature and the consequent intensity of sound. The wires from the push button and battery are secured at d and e, and, on closing the circuit, the rapid vibration of the armature causes a humming or buzzing sound, whence the name.
80. The circuit-closing devices used for electric bells are quite different from those used on lighting circuits, as the current and E. M. F. in the former case are very small, and there is no burning of the contacts. The ordinary switch for closing the circuit of an electric bell is called a push button, and is illustrated in Fig. 58. The ends of the line wire are brought up through a hole in the wooden base a, and held under the screws on the brass contact springs b, c. The cap d, in the lower view, when screwed in place, holds the ivory button e which, on being pressed down, forces the two springs together and completes the circuit, causing the bell to ring.
81. An electric-bell switch may easily be arranged in connection with the ordinary bell pull of a front door, in the manner shown in Fig. 59. The wires from the bell and battery are made fast by the screws s, s' to the tongues t, t', which are insulated from each other by the block r to which they are secured. The free ends press upon a rubber bushing b, and a brass washer w is drawn between the brass tongues, thereby closing the electric circuit, when the knob k is pulled.
82. The battery used for electric bells is illustrated in Fig. 60. This is the Leclanche cell, and consists of a porous cup P, containing the carbon electrode C, to which a binding post B is attached; also a zinc rod Z, both being enclosed in a glass jar with a contracted top. The zinc rod is provided with a binding screw B1 which serves as the negative terminal of the cell, B being the positive terminal. Before the battery will furnish current, the jar must be filled to the point shown in the cut with a saturated solution of sal ammoniac, and, in connecting up, the zinc of one jar is joined by a short piece of wire to the carbon of the next. This gives a series of grouping, which is usually required in bellwork. When, after considerable use, the current from the cells becomes feeble, it will be necessary to replace the liquid, but it frequently happens that the power may be restored by the addition of a little water to make up for evaporation. The zinc will, in course of time, be consumed, and must be replaced, and the sal ammoniac may also be renewed at the same time, although it should last longer than a single zinc rod of the usual size (3/8 in. diameter). After five or six rods have been used, the material inside the porous cup will no longer act effectively, and this means, practically, a new carbon and cup, since they may be procured very cheaply. In the case of renewal of a large number of cups, they may be returned to the manufacturer, and recharged by him.
83. It is sometimes desirable to use current from a dynamo or from lighting mains to operate the bells. Of course, it will not be allowable to connect the bells directly across the line, because they would immediately be burnt out, and it is necessary to interpose sufficient resistance to cut down the current to the required amount. The resistance of a bell is usually about 20 ohms, and since it may be rung from one cell of battery, or two cells if there is a long line wire, the E. M. F. across the bell may be taken as 2 volts.
The usual working current is then 2/20 = .1 ampere. If we denote by x the resistance to be added, the total resistance
= x + 20, which, by Ohm's law, is equal to
E = 110 = 1,100
C .1 ohms, and x = 1,100 - 20 = 1,080 ohms. The resistance of a 16-candlepowcr 110-volt lamp is, approximately, 220 ohms, and five lamps in series will be 5 X220 = 1,100 ohms.
The current will then be
110 = .098 ampere, which
1,100 + 20 is near enough to the required amount. The connections are shown in Fig. 61, where a, b are the 110-volt lighting mains, l the five lamps in series, and c, d the wires to the bells. If this current is found to be insufficient, one of the lamps may be cut out, when the total resistance will be (4x220) + 20 + y, the line resistance being denoted by y. If this is 2 ohms, the total resistance will be 902 ohms, and the current = 1/9 1/0 0/2 = .22 ampere, corresponding to an E. M. F. across the bell of 20/902 x 110 = 2.44 volts. An intermediate reduction of current may be obtained by substituting a 32-candlepower lamp for one of those of 16 candlepower.
84. For special alarm purposes, it is sometimes desirable that the bell should continue to ring after the push is released. This is accomplished by the use of an automatic drop, which closes an extra, or shunt, circuit as soon as a
B current passes along the main circuit. Fig. 62 shows two views of an automatic drop, A being a side elevation, and B a front view with the cover removed. There are three terminals on the baseboard; those marked a and b are connected to the ends of the magnet coil, the end at b being also connected to the frame f ; terminal c makes connection to the spring contact d. The bell circuit is closed first through a-b by means of the push button; the armature c is at once attracted, thereby releasing the gravity springy, which makes contact with the spring d, establishing a circuit between b and c which cuts out the push button.