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.
This is the most satisfactory method of wiring a building, and consists in providing a complete network of tubing, laid beneath the plaster of the walls, between partitions and under floors, to receive the wires at any time and allow of their removal if necessary. Such a system of tubing is known as interior conduit. The tubing is made of papier mache, which, by treatment with a bituminous solution, is rendered waterproof and insulating. In appearance it is like Fig. 44, and is made in all sizes from 1/4 inch to 1 1/4- inches inside diameter, in lengths of about 10 feet. Joints are made by carefully smoothing the ends of two pieces so that they will make a close butt joint with no rough edges, and threading them into a metal sleeve, which is then compressed into the tube by means of a special tool. The joint then appears as shown in Fig. 45. The conduit is also made with a sheet-brass covering, for protection against rats and mice. Another form is that of an iron tube with an insulating coat on the inside, and screwed joints between different lengths. Wires of opposite polarity may be placed in these iron-armored conduits, but the papier-mache tubes may contain only one wire each. The tubes are secured in place by metal staples or brass clips. A staple driver should be used for the former, to prevent damage to the tubing.
57. To insure the easy passage of the conductor, all corners are rounded, an elbow, Fig. 4G, being employed for flexible wires, and a junction box, as shown in Fig. 47, for rigid conductors. The ends of the tubes should project well into the box through the openings o, o, etc., and a thoroughly strong soldered joint should be made between the wires, and it should be wrapped with insulating tape as a further precaution. When the tubes are all in position, a fishing wire is pushed through from one outlet to another, and the electric-light wire is connected to the end and pulled through.
58. At the points where branch circuits start, junction boxes, such as that shown in Fig. 48, are used. At each side are openings m, m, m', m' to take the main conduit tubes, and smaller ones at b, b, b', b' for the branch tubes. A porcelain branch block is introduced to carry the connections and cut-outs, as in similar cases already considered.