This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol2", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
The bells are cast of an alloy of one part of pure tin and three or four parts of copper. Good quality bells are turned in a lathe after casting, and are lacquered to prevent corrosion. They should vary in their sound, so as to indicate from which apartment they are rung. Bells are usually specified by weight, and vary from 14 oz. to 2 lbs. each.
When there are a great number of bells, pendulums are attached to the bell carriages, and these continue vibrating some time after the bell has ceased to ring, so as to indicate which bell has rung, the name of the apartments being painted upon the bell board near the bells or upon the pendulum balls.
Plate VIII. Mosaic Decoration, St. Albans Church, Streatham Park, S.W.
Designed and Executed by Messrs. Jas. Powell & Sons, Whitefriars Glass Works, F.C.
Spiral springs of copper - covered steel are sometimes used, attached to one arm of some of the cranks, as shown at A, Fig. 246, to keep them in position when the wires have stretched, and to give elasticity to the pull.
Electric Bells. This type of bell has now come into almost universal use, owing to the convenience with which the wires which conduct the currents can be carried behind skirtings, or under floors in any direction, and round any angles however acute or obtuse.
Fig. 251 shows a bell and indicator system suitable for ordinary domestic purposes, consisting of pushes, batteries, bells, indicator, and wires.
The construction of an ordinary bell-push is shown in Fig. 252. Its use is to complete an electric circuit, in order that the bell may ring. A gap is made in the circuit on the push, each of the two ends of the wire being connected to one of the two metal springs which are secured to the base of the push. When the button at the centre of the push is pressed it brings the two springs into contact, then forming a passage for the current. Pushes are made in various patterns and forms. Some are made to place under a dinner-table near the carver's chair, so that a bell may be rung by pressure of the foot. Fig. 253 shows a pull suitable for a front door or gate post, in which the contact is made by drawing a piece of metal, attached to the handle of the pull, between two flat springs. Pushes are fixed to plugs in the wall in convenient positions.
These are contrivances for generating the necessary current for working the bells and indicators. The battery most suited for this purpose is composed of a number of cells, known as Leclanche cells, which will remain in good order for three or four years with very little attention. These cells are composed of two poles, one of zinc and one of carbon, immersed in an exciting fluid consisting of a solution of ammonium chloride in water. The carbon pole is surrounded by small particles of manganese dioxide, for oxidising the bubbles of hydrogen which accumulate round the carbon pole when the current is flowing and checks the action of the cell. These cells are connected up in series, i.e. carbon to zinc; the remaining four poles at either end of the battery being connected one to the bell and one to the indicator, as shown in Fig. 251.
In large systems of bells secondary batteries are used, i.e. so-called accumulators, or cells which have to be recharged from time to time from some external electric supply. These cannot therefore be conveniently used unless the building is fitted with electric light, when two sets of batteries must be employed, one being for use while the other is recharging.
The bell shown in Fig. 251, and known as the Trembling Bell, is the type of bell most suitable for ordinary domestic purposes. When any push is pressed the current flows from the carbon pole of the battery, through the right-hand terminal of the bell, to the contact screw through a spring to the electromagnets, which immediately become magnetised, and attract the soft-iron armature to which the hammer is attached, thus imparting a blow to the gong. The armature, being drawn towards the magnet, draws the spring away from the contact screw, thus breaking the electric circuit, when the magnet immediately becomes demagnetised, and the armature flies back to its original position. This whole process, being repeated, causes the hammer to vibrate against the gong so long as the push is being pressed.
Single-stroke Bells are sometimes used in which the contact screw is dispensed with, the carbon pole being connected directly to one end of the wire forming the electromagnet, which attracts the armature when the push is pressed, causing the hammer to impart a single blow to the gong.
A type of bell known as the Continuous Ringing Bell is shown in the upper part of Fig. 255, in which case the armature of the upper electromagnet is attracted as soon as a push is pressed. This releases the lever L, which flies into contact with the contact point P, thus causing the current to pass round the lower electromagnets when the bell rings, until it is stopped by pulling the cord K, so as to reset the lever L in its original position.
These are used to indicate the room from which the signal was given. In Fig. 251 it is assumed that the push No. 2 has been pressed, when the current has flowed from the battery through the bell and push to the second electromagnet from the left on the indicator, and back to the battery; and in doing so it has energised that electromagnet, which has attracted the armature and allowed an enamelled disc bearing the number or name of the room which contains push No. 2 to fall forward and appear behind a small glass window in the cover of the indicator board. The indicator signal is replaced by pressing a knob at the right-hand side of the indicator. It should be noted that an electric circuit is only complete when there is a way for the current to flow out from the carbon pole back to the zinc pole of the battery.
The wires used for conduction of the current used for electric bells are of copper, not less than No. 20 S.W.G., well tinned, and covered with gutta-percha or indiarubber, over which a double layer of cotton or silk is wound or plaited; and they should be well soaked in paraffin wax before use. When joints have to be made in wires, a short length of each end to be joined should be stripped of its insulating material, and the bare ends should be twisted and thoroughly soldered together, and bound round with strips of gutta-percha, which should then be warmed and passed tightly round the wire.
The wires are often fixed to convenient woodwork behind skirtings and along floor joints by means of wire staples. In better class work the wires are encased in thin iron tubes or run along walls behind picture nails or special wood casing, as shown in Fig. 254.
A complete Hotel System of Bells is shown in Fig. 255, where it will be noticed that when any push is pressed not only is a continuous ringing bell and the room indicator on the same floor set in action, but a bell and an indicator in the hotel bureau is set in action at the same time.
The upper bell with its battery, indicator, resetting battery and push are all fixed in the attendant's room on the first floor, while the lower bell battery and indicator are fixed in the hotel bureau, which is usually on the ground floor.
The bell on the floor continues to ring until it is stopped by pulling the cord K, while the bell in the bureau rings only when the push is pressed.
The indicators used in this system are of a different type to those used in a domestic system. Thus when push No. 1 is pressed the current passes from the lowest battery through the indicator coil, and the armature attached to indicator No. 1 is attracted towards the coil, where it remains whether a current is passing or not, owing to the pivot upon which it is supported being a little below the centre of gravity of the armature and its disc. In this position the number on the indicator signal appears before an aperture in the indicator casing. The attendant, before attending to the call, presses the push connected with the resetting battery, which passes a current through all the coils R, thus drawing any displaced indicators back to their original position, indicating to the bureau attendant that the call has been attended to.
It should be noted that all the coils marked C are used for moving the signals, so that its number appears before the aperture in the indicator case, while the coils marked R are all used to reset the signals.
All the ends of the wires forming the electromagnet should be connected to terminals such as T, or to screws in strips of copper D, as these make more reliable joints, offering the least possible resistance to the passage of an electric current. Should there be a number of floors it is only necessary to repeat the arrangement as shown for the first floor.
It will be noted that there are two separate wires from each floor to the bureau, one for the main circuit and one for the resetting circuit, while a wire * called the neutral wire connects all the floors to the bureau.
Pneumatic Bells. These are used to a considerable extent at the present day, and indeed they possess many advantages under certain circumstances over either mechanical or electric bells. The power used for working these bells is compressed air, but no more labour or inconvenience is required for forcing this compressed air than that of the pressure of the finger upon a push, fixed to the wall or in any other convenient position. These pushes consist of a casing similar to that of an electric push, an indiarubber ball taking the place of the spring. A slight pressure on the central button compresses the air within the india-rubber ball, causing it to enter a small metal tube to which the ball is connected. At the other end of the tube a bell is fixed, which is actuated by a small bellows being inflated by the air which issues from this end of the tube. This kind of bell remains in good order for a considerable length of time, and only needs the rubber balls of the pusher replaced about once in five or six years.