§ 68. Whenever possible, the wiring of a house, etc., for bell work, should be done as soon as the walls are up and the roof is on. The shortest and straightest convenient route from bell to battery, etc., should always be chosen where practicable to facilitate drawing the wire through and to avoid the loss of current which the resistance of long lengths of wire inevitably entails. The wires should be run in light zinc tubes nailed to the wall.
Fig. 74.
In joining up several lengths of tubing, the end of one piece of tube should be opened out considerably of a trumpet shape for the other piece to slip in; and the end of this latter should also be slightly opened out, so as not to catch in the covering of any wire drawn through it. The greatest care must be exercised in drawing the wires through the tubes or otherwise, that the covering be not abraded, or else leakage at this point may take place. In cases where tubes already exist, as in replacing old crank bells by the electric bells, the new wires can be drawn through the tubes, by tying the ends of the new wire to the old wire, and carefully pulling this out, when it brings the new wire with it. Or if the tubes are already empty, some straight stout wire may be run through the tubes, to which the new
wires may be attached, and then drawn through, using, of course, every possible precaution to avoid the abrasion of the insulating covering of the wire, which would surely entail leakage and loss of current. All the old fittings, cranks, levers, etc., must be removed, and the holes left, carefully filled with dowels or plaster. In those cases where it is quite impossible to lay the wires in zinc or wooden tubes (as in putting up wires in furnished rooms already papered, etc.), the wires may be run along the walls, and suspended by staples driven in the least noticeable places; but in no case should the two wires (go and return) lie under the same staple, for fear of a short circuit. It must be borne in mind that each complete circuit will require at least two wires, viz., the one leading from the battery to the bell, and the other back from the bell to the battery; and these until connection is made between them by means of the "contact" (pull, push, or key) must be perfectly insulated from each other. In these cases, as far as possible, the wires should be laid in slots cut in the joists under the floor boards, or, better still, as tending to weaken the joists less, small holes may be bored in the joists and the wires passed through them; or again, the wires may be led along the skirting board, along the side of the doorpost, etc., and when the sight of the wires is objectionable, covered with a light ornamental wood casing. When the wires have been laid and the position of the "pushes," etc., decided upon, the blocks to which these are to be fastened must be bedded in the plaster. These blocks may be either square or circular
pieces of elm, about 3 inches across, and 1 inch thick, bevelled off smaller above, so as to be easily and firmly set in the plaster. They may be fastened to the brickwork by two or three brads, at such a height to lie level with the finished plaster. There must of course be a hole in the centre of the block, through which the wires can pass to the push. When the block has been fixed in place, the zinc tube, if it does not come quite up to the block, should have its orifice stopped with a little paper, to prevent any plaster, etc., getting into the tube. A little care in setting the block will avoid the necessity of this makeshift. A long nail or screw driven into the block will serve to mark its place, and save time in hunting for it after the plastering has been done. When the blocks have been put in their places, and the plastering, papering, etc., done, the wires are drawn through the bottom hole of the push (after the lid or cover has been taken off), [Fig. 74], and a very small piece of the covering of the wire having been removed from each wire, and brightened by sand papering, one piece is passed round the shank of the screw connected with the lower spring, shown to the right in [Fig. 74], and the other round the shank of the screw connected to the upper spring, shown to the left in the Fig. The screws must be loosened to enable the operator to pass the wire under their heads. The screws must then be tightened up to clench the wire quite firmly. In doing this, we must guard against three
things. Firstly, in pulling the wire through the block, not to pull so tightly as to cut the covering against the edge of the zinc tube. Secondly, not to uncover too much of the wire, so as to make contact between the wires themselves either at the back of the push, or at any other part of the push itself. Thirdly, to secure good contact under the screws, by having the ends of the wires quite clean, and tightly screwed down.
§ 69. In all cases where the wires have to be taken out of doors, such as is necessitated by communication from house to outhouses, stables, greenhouses, etc., over head lines (No. 18 gauge, gutta-percha tape and tar covering) should be used. Where overhead lines are not admissible, either as being eyesores, or otherwise, the wires may be laid in square wooden casings of this section ┗━┛, the open part of which must be covered by a strip of wood laid over it. The wood must have been previously creosoted, in the same manner as railway sleepers. This mode admits of easy examination. Iron pipes must, however, be used if the lines have to pass under roads, etc., where there is any heavy traffic. And it must be borne in mind that however carefully the iron pipes, etc., be cemented at the joints, to make them watertight, there will always be more electrical leakage in underground lines than in overhead ones. In certain rare cases it may be needful to use iron wires for this purpose instead of copper; in this case, as iron is six or seven times a worse conductor than copper, a much heavier wire must be employed to get the same effect. In other
words, where iron wire is used, its section must be not less than seven times that of the copper wire which it replaces.