Track Construction.—­It is essential that the centre rail and at least one of the wheel rails shall have all joints bonded together to give a clear course to the electric current, and the centre rail must be insulated to prevent leakage and short-circuiting.  Where a track is laid down more or less permanently, the bonding is most positively effected by means of little fish-plates, screwed into the sides of the abutting rails; but in the case of a track which must be capable of quick coupling-up and uncoupling, some such arrangement as that shown in Fig. 42 is to be recommended.

Fig. 42 (a) is a cross vertical section of the track; Fig. 42 (c) a longitudinal view; while Fig. 42 (b) shows in plan a point of junction of two lengths of rail.

The wheel rails are made of carefully straightened brass strip 3/8 inch wide and 1/16 inch thick, sunk rather more than 1/8 inch into wooden sleepers (Fig. 42, a), 3-1/2 inches long and 3/4 inch wide (except at junctions).  The sleepers are prepared most quickly by cutting out a strip of wood 3-1/2 inches wide in the direction of the grain, and long enough to make half a dozen sleepers.  Two saw cuts are sunk into the top, 2 inches apart, reckoning from the inside edges, to the proper depth, and the wood is then subdivided along the grain.  The saw used should make a cut slightly narrower than the strip, to give the wood a good hold.  If the cut is unavoidably too large, packings of tin strip must be forced in with the rail on the outside.  To secure the rails further, holes are bored in them on each side of the sleeper (see Fig. 42, c), and fine iron or, brass wire is passed through these, round the bottom of the sleeper, and made fast.

[Illustration:  Fig. 43.—­Tin chair for centre rail of electric track.]

The centre rail is soldered to small tin chairs, the feet of which are pinned down to the sleepers.  The top of the rails must project slightly above the chairs, so that the current collector may not be fouled.

Junctions.—­At these points one 3/4-inch sleeper is reduced to 1/2-inch width, and the other increased to 1 inch, this sleeper being overlapped 3/8 inch by the rails of the other section.  To the outsides of the wheel rails are soldered the little angle plates, aa, bb, attached to the sleepers by brass tacks, which project sufficiently to take the brass wire hooks.  These hooks must be of the right length to pull upon the tacks in aa and make a good contact.  The centre rails are bonded by two strips of springy brass, riveted to one section, and forced apart at their free end by the interposed strip.  Two pins projecting from the narrower sleeper fit into holes in the wider to keep the sections in line at a junction.

General.—­The sleepers of straight sections are screwed down to 3/4 by 1/4 inch longitudinals, which help to keep the track straight and prevent the sleepers slipping.  Sections should be of the same length and be interchangeable.  Make straight sections of the greatest convenient length, to reduce the number of junctions.  Sleepers need not be less than 6 inches apart.  Fix the sleepers on the longitudinals before hammering the rails into the slots.