The Track
We will consider this first, as its design must govern, within certain limits, the design of the locomotive. There are three systems of electrical transmission available.
1. The trolley system, with overhead cable attached to insulators on posts, to carry the current one way, the rails being used as the "return." This system has the disadvantages a.s.sociated with a wire over which the human foot may easily trip with disastrous effect.
2. That in which one of the wheel rails is used for taking the current to the motor, and the other as the return. The objection to the system is that the wheels must be insulated, to prevent short circuiting; and this, besides causing trouble in construction, makes it impossible to use the ordinary model rolling stock. To its credit one may place the fact that only two rails are needed.
3. The third and, we think, best system, which has an insulated third rail as one half of the circuit, and both wheel rails as the return, the motor being kept in connection with the third rail by means of a collector projecting from the frame and pressing against the top of the third rail.
The last, for reasons of convenience, is placed between the wheel rails. We will a.s.sume that this system is to be employed.
[Ill.u.s.tration: FIG. 42.--Details of rails for electric track.]
Gauge.--For indoor and short tracks generally it is advisable to keep the gauge narrow, so that sharp curves may be employed without causing undue friction between rails and wheels. In the present instance we specify a 2-inch gauge, for which, as also for 1-1/2 and 1-1/4 inch, standard rolling stock is supplied by the manufacturers.
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 ab.u.t.ting 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 bra.s.s 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, bra.s.s wire is pa.s.sed through these, round the bottom of the sleeper, and made fast.
[Ill.u.s.tration: 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 bra.s.s tacks, which project sufficiently to take the bra.s.s 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 bra.s.s, 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.
[Ill.u.s.tration: FIG. 44.--Laying out a curve for electric track.]
Curves.--A simple method of laying out a semi-circular curve is shown in Fig. 44. Sleepers and longitudinals are replaced by 1/2-inch boards, 8 inches wide. Three pieces, about 32 inches long each, have their ends bevelled off at an angle of 60 degrees, and are laid with their ends touching. Two semi-circles of 24 and 22 inch radius are drawn on the boards to indicate the positions of the rails, and short decapitated bra.s.s nails are driven in on each side of a rail, about an inch apart, as it is laid along one of these lines. (See Fig. 44. A.) The inside nails must not project sufficiently to catch the wheel f.l.a.n.g.es. The spring of the bra.s.s will prevent the rail falling out of place, but to make sure, it should be tied in with wire at a few points. The centre rail should on the curves also be 3/8 inch deep, and raised slightly above the bed so as to project above the wheel rails. The method already described of bonding at joints will serve equally well on curves. If the outer rail is super-elevated slightly, there will be less tendency for the rolling stock to jump the track when rounding the curve.
When the rails are in place the boards may be cut with a pad-saw to curves corresponding with the breadth of the track on the straight. If the boards incline to warp, screw some pieces of 1/8-inch strip iron to the under side across the grain, sinking the iron in flush with the wood.
The bra.s.s strip for the rails costs about one penny per foot run. Iron strip is much cheaper, but if it rusts, as it is very likely to do, the contact places will need constant brightening.
Points.--Fig. 45 shows the manner of laying out a set of points, and connecting up the rails. The outside wheel rails, it will be seen, are continuous, and switching is effected by altering the position of the moving tongues, pivoted at PP, by means of the rod R, which pa.s.ses through a hole in the continuous rail to a lever or motor of the same reversible type as is used for the locomotive. If a motor is employed, R should be joined to a crank pin on the large driven cog--corresponding to that affixed to the driving wheel (Fig. 47)--by a short rod. The pin is situated at such a distance from the axle of the cog wheel that a quarter of a revolution suffices to move the points over. The points motor must, of course, have its separate connections with the "central station." To show how the points lie, the rod R also operates a semaph.o.r.e with a double arm (Fig. 46), one end of which is depressed--indicating that the track on that side is open--when the other is horizontal, indicating "blocked." The arms point across the track.
[Ill.u.s.tration: FIG. 45.--Points for electric railway.]
Details.--The tongues must be bevelled off to a point on the sides respectively nearest to the continuous rails. The parts AA are bent out at the ends to make guides, which, in combination with the safety rails, will prevent the wheels jumping the track. Care should be taken to insulate centre rail connecting wires where they pa.s.s through or under the wheel rails.
It is advisable to lay out a set of points, together with motor and signals, on a separate board.
[Ill.u.s.tration: Fig. 46.--Double-armed signal, operated by points.]
Preservation of Track.--All the wooden parts of an outdoor track should be well creosoted before use.
The Electric Locomotive.
An elevation and a plan of this are given in Fig. 47. The two pairs of wheels are set close together, so that they may pa.s.s easily round curves.
[Ill.u.s.tration: Fig. 47.--Plan and elevation of electric locomotive.]
The Motor.--A motor of ordinary type, with electro field magnets, is unsuitable for traction, as it cannot be reversed by changing the direction of the current, unless a special and rather expensive type of automatic switch be used. While a motor of this kind is, in conjunction with such a switch, the most efficient, the motor with permanent field magnets is preferable as regards cost and ease of fixing. It can be reversed through the rails. The armature or revolving part must be tripolar to be self-starting in all positions.
A motor of sufficient power can be bought for half a crown or less--in any case more cheaply than it can be made by the average amateur.
The motor used for the locomotive ill.u.s.trated was taken to pieces, and the magnet M screwed to a strip of wood 1-5/8 inches wide; and for the original armature bearings were subst.i.tuted a couple of pieces of bra.s.s strip, HH, screwed to two wooden supports, SS, on the base, E (Fig. 47, a). It was found necessary to push the armature along the spindle close to the commutator piece, C, and to shorten the spindle at the armature end and turn it down to the size of the original bearing, in order to bring the motor within the s.p.a.ce between the wheels.
The place of the small pulley was taken by an 8-toothed pinion wheel, engaging with a pinion soldered to the near driving wheel, the diameter of which it exceeded by about 3/16 inch. The pair, originally parts of an old clock purchased for a few pence, gave a gearing-down of about 9 times.
The position of the driven wheels relatively to the armature must be found experimentally. There is plenty of scope for adjustment, as the wheels can be shifted in either direction longitudinally, while the distance between wheel and armature centres may be further modified in the length of the bearings, BE. These last are pieces of bra.s.s strip turned up at the ends, and bored for axles, and screwed to the under side of the base. To prevent the axles sliding sideways and the wheels rubbing the frame, solder small collars to them in contact with the inner side of the bearings.
The Frame.--Having got the motor wheels adjusted, shorten E so that it projects 2 inches beyond the centres of the axles at each end. Two cross bars, GG, 3-1/2 inches long, are then glued to the under side of E, projecting 1/8 inch. To these are glued two 3/8-inch strips, FF, of the same length as E. A buffer beam, K, is screwed to G. A removable cover, abedfg, is made out of cigar-box wood or tin. The ends rest on GG; the sides on FF. Doors and windows are cut out, and handrails, etc., added to make the locomotive suggest the real thing--except for the proportionate size and arrangement of the wheels.
Electrical Connections.--The current collector, CR, should be well turned up at the end, so as not to catch on the centre rail joints, and not press hard enough on the rail to cause noticeable resistance. The fixed end of CR is connected through T2 with one brush, B, and both wheel bearings with T1.
[Ill.u.s.tration: FIG. 48.--Reversing switch.]
Electrical Fittings.--The best source of power to use is dry cells giving 1-1/2 to 2 volts each. These can be bought at 1s. apiece in fairly large sizes. Four or five connected in series will work quite a long line if the contacts are in good condition.
A reversing switch is needed to alter the direction of the current flow.
The construction of one is an exceedingly simple matter. Fig. 48 gives a plan of switch and connection, from which the principle of the apparatus will be gathered. The two links, LL, are thin springy bra.s.s strips slightly curved, and at the rear end pivoted on the binding posts T1 T2. Underneath the other ends solder the heads of a couple of bra.s.s nails. The links are held parallel to one another by a wooden yoke, from the centre of which projects a handle. The three contacts C1 C2 C3 must be the same distance apart as the centres of the link heads, and so situated as to lie on the arcs of circles described by the links. The binding post T3 is connected with the two outside contacts--which may be flat-headed bra.s.s nails driven in almost flush with the top of the wooden base--by wires lying in grooves under the base, and T4 with the central contact. As shown, the switch is in the neutral position and the circuit broken.
[Ill.u.s.tration: Fig. 49.--Multiple battery switch.]
Multiple Battery Switch.--To control the speed of the train and economize current a multiple battery switch is useful. Fig. 49 explains how to make and connect up such a switch. The contacts, C1 to C5, lie in the path of the switch lever, and are connected through binding posts T1 to T6 with one terminal of their respective cells. The cells are coupled up in series to one another, and one terminal of the series with binding posts T0 and T6.
By moving the lever, any number of the cells can be put in circuit with T7.
The b.u.t.ton under the head of the lever should not be wide enough to bridge the s.p.a.ce between any two contacts. Change the order of the cells occasionally to equalize the exhaustion.
[Ill.u.s.tration: FIG. 50.--Adjustable resistance for controlling current.]
Resistance.--With acc.u.mulators, a "resistance" should be included in the circuit to regulate the flow of current. The resistance shown in Fig. 50 consists of a spiral of fine German silver wire lying in the grooved circ.u.mference of a wood disc. One of the binding posts is in connection with the regulating lever pivot, the other with one end of the coil. By moving the lever along the coil the amount of German silver wire, which offers resistance to the current, is altered. When starting the motor use as little current as possible, and open the resistance as it gets up speed, choking down again when the necessary speed is attained.
General.--All the three fittings described should for convenience be mounted on the same board, which itself may form the cover of the box holding the dry cells or acc.u.mulators.
SOME SUGGESTIONS.
Instead of dry cells or acc.u.mulators a small foot or hand operated dynamo generating direct, not alternating current, might be used. Its life is indefinitely long, whereas dry cells become exhausted with use, and acc.u.mulators need recharging from time to time. On occasion such a dynamo might prove very convenient.
Anyone who possesses a fair-sized stationary engine and boiler might increase the realism of the outdoor track by setting up a generating station, which will give a good deal of extra fun.
XIV. A SIMPLE RECIPROCATING ENGINE.