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.
(Contributed by H. Y. Margary)
Mechanical Bells. These bells are still used to a considerable extent, and for small houses are indeed most reliable. When properly constructed they require far less attention than either electric or pneumatic bells. The inferior fittings frequently used for mechanical bells have done much to bring them into bad repute, and if this point is attended to in the first place the bells will remain in good condition for years.
Another distinct point in favour of mechanical bells is that they rarely break down without giving warning by the wires becoming slack.
Fig. 246 shows a small system of bells of the mechanical type, illustrating the method of carrying the wires round angles by means of cranks.
Fig. 247 illustrates a few of the types of pulls or appliances fixed in apartments from which it is required to ring. The Bell-lever is that most frequently used for sitting-rooms. This consists of a bow, kept in a vertical position by means of a steel helical spring.
When the lever is pulled down it causes a drum to revolve, winding up a chain, and thus imparting a direct pull upon the wire connecting the chain to the bell. The Pendent, Sunk-bell, and Bell-slide pulls are of the type used for signalling from without an entrance door, their action being too simple to need explanation here; while the Rose-purchase-crank pull is frequently used in bedrooms. It is also known as the purchase-crank and tassel pull.
Drawn copper wire is used for mechanical bells from No. 16 to No. 19 B.W.G. It should be thoroughly stretched before use to prevent subsequent elongation, and should be so fixed that it may be quite taut between the cranks.
In cheap work the lengths of wire are passed through wire staples varying from 3/4 to 1 1/2 inches in length, driven into any convenient woodwork.
In good work all the wires should be carried in stout tubes of galvanised iron, brass, or copper concealed in the walls. Zinc tubing is sometimes used for the same purpose, but this material is not to be recommended, as both the wire and tube rapidly decay in the presence of any moisture, owing to the galvanic action that is set up between the two metals.
Great care should be taken to fix the tubes in the direction of the pull upon the wires they contain, and, when built up of several pieces, that the pieces are in true alignment, so that there may be no friction on the insides of the tubes. The ends of the tubes are frequently filed smooth or bossed out to a slightly larger diameter to prevent the wire from chafing upon them.
The direction of pull of the various wires is diverted by means of cranks, several types of which are shown in Fig. 248. When wires penetrate the walls the mortise crank is used, attached to a wrought board for appearance sake.
In good work it is very much better to use wheels and chains, as shown in Fig. 249, for diverting the direction of pull upon the wires. These should always be employed when the wires are carried in tubes, as the direction of the pull is always the same, thus preventing undue friction of the wires over the tube. For pulling round oblique or obtuse angles also, wheels and chains are more suitable than cranks.
The wheels and chains should be of good quality, with steel chains, brass wheels, and steel axles, otherwise they rapidly deteriorate and break.
This is a wrought board, 11 by 1 1/2
inches or 11 by 1 inches, usually moulded on the edges, painted, stained, or finished in any other way according to the quality of the work. This board is screwed to plugs driven into a wall in any convenient position where the bells are required to ring.
To this board all the bell wires are brought, as shown in Fig. 246. It will be found that in most houses many of the bells will have to pass round so many corners that a large number of cranks will have to be used, and a considerable effort will be required to make the bell ring. It will then usually be more convenient to carry the wires direct from every pull to the roof, and then to concentrate them by the use of cranks, or wheels and chains, fixed to any convenient woodwork, and to bring all the wires down close together against the wall upon which it is proposed to fix the bell board, where they are attached to cranks fixed on its face. Bell Carriages. - The bells are hung upon springs attached to brass bell carriages, as shown in Figs. 249 and 250.
The object of hanging the bells on to springs is to cause them to continue ringing for some time after the wire has been pulled. These springs are made of steel with an oxidised surface, bent to scrolls of one or two convolutions, and varying from 4 to 5 inches across.