An ordinary electric bell is merely a vibrating contact-breaker carrying a small hammer on its spring, which hammer strikes a bell placed within its reach as long as the vibration of the spring continues. The necessary apparatus comprises a battery to supply the force, wires to conduct it, circuit-closers to apply it, and bells to give it expression.
The Leclanche battery (see p. 89) is the best for all electric bell systems. On short circuits, 2 cells may suffice, increasing up to 4 or 6 as required. It is false economy to use a battery too weak to do its work properly. The battery should be placed where it will not be subject to changes of temperature, e.g., in an underground cellar.
The circuit wire used in England for indoor situations is " No. 20 " copper wire, covered with guttapercha and cotton. In America, "No. 18, first-class, braided, cotton-covered, office wire " is recommended, though smaller and cheaper kinds are often used. The wire should be laid with great regard, to keeping it from damp, and ensuring its perfect insulation. Out of doors, for carrying long distances overhead, ordinary galvanized iron wire is well adapted, the gauge running from " No. 4" to "No. 14," according to conditions. Proper insulators on poles must be provided, avoiding all contact with foreign bodies; or a rubber-covered wire encased in lead may be run underground.
The circuit-closer, or means of instantaneously completing and interrupting the circuit, is generally a simple press-button. This consists of a little cylindrical box, provided in the centre with an ivory button, which is either (1) attached to a brass spring that is brought into contact with a brass plate at the back of the box on pressing the button, or (2) is capable of pressing together 2 springs in the box. A wire from the battery is attached to the spring of the press-button, and another from the bell is secured to the brass plate. Platinum points should be provided on the spring and plate when the contact takes place. While the button is at rest, or out, the electric circuit is broken; but on being pressed in, it completes the circuit, and the bell rings.
The relative arrangement and connection of the several parts is shown in Fig. 40: a, Leclanche cell; b, wire; c, press-button; d, bell. When the distance traversed is great, say £ mile, the return wire e may be dispensed with, and replaced by what is known as the "earth circuit," established by attaching the terminals at f and g to copper plates sunk in the ground.
The bells used are generally vibrating ones, and those intended for internal house use need not have a higher resistance than 2 or 3 ohms. At other times, single-stroke and continuous-ringer bells have to be provided, the latter being arranged to continue ringing until specially stopped. The bell may or may not be fitted with an annunciator system; the latter is almost a necessity when many bells have to ring to the same place, as then 1 bell only is requisite. A single-stroke bell is simply a gong fixed to a board or frame, an electro-magnet, and an armature with a hammer at the end, arranged to strike the gong when the armature is attracted by the magnet. A vibrating bell has its armature fixed to a spring which presses against a contact-screw; the wire forming the circuit, entering at one binding-screw, goes to the magnet, which in turn is connected with the armature; thence the circuit continues through the contact-screw to the other binding-screw, and out. When set in motion by electricity, the magnet attracts the armature, and the hammer strikes the bell; but in its forward motion, the spring leaves the contact-screw, and thus the circuit is broken; the hammer then falls back, closing the circuit again, and so the action is continued ad libitum, and a rapid vibratory motion is produced, which makes a ringing by the action of the successive blows of the hammer on the gong.
The following useful hints on electric bell systems are condensed from Lock-wood's handy little volume on telephones.
With regard to the battery, he advises to keep the sal-ammoniac solution strong, yet not to put so much in that it cannot dissolve. Be extremely careful to have all battery connections clean, bright, and mechanically tight, and to have no leak or short circuit. The batteries should last a year without further attention, and the glass jars never ought to be filled more than | full.
The simplest system is 1 bell operated by 1 press-button. The arrangement of this is the same whether the line be long or short. Set up the bell in the required place, with the gong down or up as may be chosen; fix press-button where wanted, taking all advantages offered by the plan of the house; e.g. a wall behind which is a closet is an excellent place to attach electrical fixtures, because then it is easy to run all the wires in the closets, and out of sight. Set up the battery in a convenient place, and, if possible, in an airtight box. Calculate how much wire will be requisite, and measure it off, giving a liberal supply; joints in inside work are very objectionable, and only admissible where absolutely necessary. Cut off insulation from ends of wire where contact is to be made to a screw. Only 3 wires are necessary, i.e. (1) from 1 spring of the press-button to 1 pole of the battery, say the carbon, (2) from the other spring of the button to 1 binding-screw of the bell, (3) from the other pole of the battery to the other binding-screw of the bell. In stripping wires, leave no ragged threads hanging; they get caught in the binding-screw, and. interfere with the connection of the parts.
After stripping the wire sufficiently, make the ends not only clean but bright. Never run 2 wires under 1 staple. A button-switch should be placed in the battery-circuit, and close to the battery, so that, to avoid leakage and accidental short circuiting when the bells are not used for some time, it may be opened.