The electric motor to be treated in this chapter ill.u.s.trates very prettily the attractive force of a hollow, wire-wound bobbin on a movable core, when the electric current is pa.s.sed through the wire. If one inserts the end of an iron rod into the coil, the coil exerts a pull upon it, and this pull will cease only when the centre of the rod is opposite the centre of the coil. This principle is used in the "electric gun," which in its simplest form is merely a series of powerful coils arranged one behind another on a tube through which an iron or steel projectile can pa.s.s. The projectile closes automatically the circuit of each coil in turn just before reaching it, and breaks it before its centre is halfway through the coil, being thus pa.s.sed along from one coil to the other with increasing velocity.
Our motor is essentially a very inefficient one, its energy being small for the current used, as compared with a revolving motor of the usual kind. But it has the advantage of being very easy to make.
[Ill.u.s.tration: FIG. 37.--Electric reciprocating engine and battery.]
How it works.--The experimental engine, constructed in less than a couple of hours, which appears in Fig. 38, consists of a coil, C, strapped down by a piece of tin to a wooden bedplate; a moving plunger, P, mounted on a knitting-needle slide rod, SR; a wire connecting rod, SR; a wooden crank, K; and a piece of knitting-needle for crank shaft, on which are mounted a small eccentric bra.s.s wipe, W, and a copper collar, D. Against D presses a bra.s.s brush, B1 connected with the binding post, T1; while under W is a long strip of springy bra.s.s against which W presses during part of every revolution. T2 is connected to one end of the coil winding, and T1 through a 4-volt acc.u.mulator or three dry cells, with the other end of the coil.
When W touches B2 the circuit is completed, and the coil draws in the plunger, the contact being broken before the plunger gets home. The crank rotates at a very high speed if there is plenty of battery power, all the moving parts appearing mere blurs.
CONSTRUCTION.
The coil is made by winding 4 oz. of No. 32 cotton-covered wire (price 6d.
to 8d.) on a boxwood reel 2 inches long and 1-1/2 inches in diameter, with a 9/16-inch central hole. Before winding, bore a hole for the wire through one end of the reel, near the central part, and mount the reel on a lathe or an improvised spindle provided with a handle of some kind. The wire should be uncoiled and wound on some circular object, to ensure its paying out regularly without kinking; which makes neat winding almost impossible.
Draw a foot of the wire through the hole in the reel, and drive in a tiny peg--which must not protrude inwards--to prevent it slipping. Lay the turns on carefully, forcing them into close contact, so that the next layer may have a level bed. On reaching the end of the layer, be equally careful to finish it neatly before starting back again. When the wire is all on, bore a hole as near the edge of the finishing edge as possible, and draw the spare wire through. Then cut a strip of tough paper of the width of the coils, coat one side with paste, and wrap it tightly round the outside to keep the wire in place.
Note.--Insulation will be improved if every layer of wire is painted over with sh.e.l.lac dissolved in alcohol before the next layer is applied.
Flatten the reel slightly with a file at the points of contact with the baseboard, to prevent rolling.
The plunger is a tube of thin iron, 1/16 inch less in diameter than the hole in the reel, and 1/4 inch longer than the reel. If a ready-made tube is not available, construct one by twisting a piece of tin round a metal rod, and soldering the joint. As it is difficult to make a jointed tube cylindrical, and a close fit is needed to give good results, it is worth going to a little trouble to get a plunger of the right kind.
The ends of the plunger are plugged with wood and bored centrally for the slide rod, which should not be cut to its final length until the parts are a.s.sembled.
The crank shaft is 2-3/4 inches of a stout knitting needle mounted in a sheet bra.s.s bearing. The crank, a fragment of oak or other tough wood, is balanced, and has a throw of 5/8 inch. The crank-shaft hole should be a trifle small, so that the crank shall get a tight hold of the shaft without pinning. The collar, D, and wipe, W, are soldered to the shaft after this has been pa.s.sed through its bearings. The brush B1 should press firmly, but not unnecessarily so, against the collar. For B2 one must use very springy bra.s.s strip, a piece about 3 inches long and 1/4 inch wide being needed.
Bend it to the arc of a large circle, and screw one end down to the base by the binding screw T2. The other end, which should not touch the base, is confined by the heads of a couple of small screws, by means of which the strip is adjusted relatively to the wipe.
Fixing the Coil.--Cut a strip of tin 1-3/4 inches wide and 4 inches long.
Punch a couple of holes near one end, and nail this to the side of the base, with its forward end 4-1/4 inches from the crank shaft. Pa.s.s the strip over the coil, and bend it down towards the base. Drill a couple of screw holes, and screw the other end down so that the coil is gripped fairly tight.
Fixing the Plunger. Two small guides, G1 G2, are made for the plunger. The holes through which the slide rod moves should be a good fit, and their centres at the level of the centre of the coil. Screw holes are bored in the feet.
Pa.s.s the plunger through the coil, and place the guides on the rod. Then draw the plunger forward till 1/2 inch projects. Bring G1 close up to it, mark its position, and screw it to the base. The other guide, G2, should be 1-1/2 inches away from the rear of the coil.
[Ill.u.s.tration: Fig. 38.--Plan of electric reciprocating engine.]
The coil and guides must be adjusted so that the plunger does not touch the coil anywhere during a stroke, packings being placed, if necessary, under coil or guides. When the adjustment is satisfactory, screw the coil down tightly, and cut off any superfluous parts of the rod.
The Connecting Rod.--Bore a hole near the end of the plunger for a screw to hold the rear end of the connecting rod. Pull the plunger out till 1-3/4 inches project, turn the crank full forward, and measure off the distance between the centres of the plunger hole and the crank pin. Drive a couple of wire nails into a board, and twist the ends of a piece of 1/20-inch wire round them twice. This wire const.i.tutes a connecting rod amply strong enough to stand the pulls to which it will be subjected. Fix the rod in position.
Adjusting the Wipe.--Turn the wipe, W, round until it makes contact with B2, and, holding the crank shaft with a pair of pliers, twist the crank on it till it just begins the return stroke. Then turn the crank to find out how long the wipe remains in contact, and adjust the crank relatively to the wipe so that the crank is vertical when the period of contact is half finished. The length of this period is controlled by the set screws at the free end of B2.
OTHER DETAILS.
The fly wheel may be a disc of wood.
Oil all the rubbing parts slightly. Connect T1 to one terminal of the battery, T2 to the coil, and the other terminal of the battery to the coil.
Set the engine going. If it refuses to run, make sure that B1 is pressing against D. The speed of the engine may possibly be improved by careful adjustment of B2 and an alteration in the setting of the crank, and will certainly be accelerated by increasing the number of battery cells.
The cost of the engine described was about 1s, 3d., exclusive of the battery.
XII. AN ELECTRIC ALARM CLOCK.
Anybody who possesses an alarm clock with an external gong, an electric bell, and a battery, may easily make them combine to get the drowsiest of mortals out of bed on the chilliest of winter mornings. The arrangement has as its secondary advantages and capabilities--
(l) That the clock can be placed where its ticking will not disturb the person whom it has to arouse in due course (some of the cheaper clocks are very self-advertising);
(2) That one clock can be made to operate any number of bells in different parts of the house.
The main problem to be solved is, how to make the alarm mechanism of the clock complete an electric circuit when the alarm "goes off."
If you examine an alarm clock of the type described, you will find that the gong hammer lies against the gong when at rest, and that its shaft when in motion vibrates to and fro about a quarter of an inch.
[Ill.u.s.tration: FIG. 89.--Plan of release gear of electric alarm, as attached to clock.]
Fig. 39 shows a. method of utilizing the movement of the hammer. A piece of wood, 2 inches long, wide enough to fill the s.p.a.ce between the rear edge of the clock and the hammer slot, and 1/2 inch thick, has its under side hollowed out to the curvature of the clock barrel. This block serves as a base for two binding posts or terminals, T1 T2. A vertical slit is made in T1 and in this is soldered [to] one end of a little piece of spring bra.s.s strip, 1 inch long and 1/4 inch wide. To the back of the other end of the strip solder a piece of 1/20 inch wire, projecting l inch below the strip.
The strip must be bent so that it presses naturally against T2. A little trigger, B, which you can cut out of sheet bra.s.s, is pivoted at a, where it must be raised off the base by a small washer. It projects 1/4 inch beyond the base on the gong support side. A square nick is cut in it at such a distance from a that, when the wire spike on C is in the nick, the strip is held clear of T2. The other end of the trigger, when the trigger is set, must be 1/8 inch from the shank of the alarm hammer--at any rate not so far away that the hammer, when it vibrates, cannot release C from the nick.
To fix the base on to the top of the clock, the works must be removed (quite an easy matter to accomplish) and holes bored for a couple of screws put through from the inside. If the underside of the base is not quite correctly curved, take care not to force in the screws far enough to distort the barrel. It is advisable to do the fitting of the parts of the release after the base has been fixed, and before the works are replaced.
The position of the hammer shaft can be gauged accurately enough from the slot in the case.
The tails of the terminals T1 T2 must be truncated sufficiently not to penetrate the base and make contact with the barrel, or a "short circuit"
will be evident as soon as the battery is connected up.
[Ill.u.s.tration: Fig. 40.--Electric alarm releaser, as attached to separate wooden clock casing.]
If the bell, battery, and clock are in the same room, a single dry cell will give sufficient current; but if the circuit is a long one, or several bells have to be operated, two or more cells will be required.
An Alternative Arrangement.--Should the reader prefer to have the clock quite free from the release--and this is certainly convenient for winding and setting the alarm--he should make a little wooden case for the clock to stand in, just wide enough to take the clock, and the back just as high as the top of the barrel. The release is then attached to a little platform projecting from the back, care being taken that the lever is arranged in the correct position relatively to the hammer when the clock is pushed back as far as it will go (Fig. 40).
If a self-contained outfit is desired, make the case two-storied: the upper division for the clock, the lower for the cell or cells. The bell may be attached to the front. A hinged fretwork front to the clock chamber, with an opening the size of the face; a door at the back of the cell chamber; and a general neat finish, staining and polishing, are refinements that some readers may like to undertake.
Setting the Alarm.--A good many alarm clocks are not to be relied upon to act within a quarter of an hour or so of the time to which they are set.
But absolute accuracy of working may be obtained if the clock hands are first set to the desired hour, and the alarm dial hand revolved slowly till the alarm is released. The hands are then set at the correct time, and the alarm fully wound.
XIII. A MODEL ELECTRIC RAILWAY.
The rapid increase in the number of electrically worked railways, and the subst.i.tution of the electric for the steam locomotive on many lines, give legitimate cause for wondering whether, twenty or so years hence, the descendants of the "Rocket" will not have disappeared from all the railways of the world, excepting perhaps those of transcontinental character.
[Ill.u.s.tration: Fig. 41.--Electric Locomotive.]
The change is already spreading to model plant, and not without good reason, as the miniature electric railway possesses decided advantages of its own. Instead of having to chase the locomotive to stop or reverse it, one merely has to press a b.u.t.ton or move a switch. The fascinations of a model steam locomotive, with its furnace, hissing of steam, business-like puffings, and a visible working of piston and connecting rods, are not to be denied, any more than that a full-sized steam locomotive is a more imposing object at rest or in motion than its electric rival. On the other hand, the ease of control already noticed, and the absence of burning fuel, water leakage, smoke and fumes, are strong points in favour of the electric track, which does no more harm to a carpet than to a front lawn, being essentially clean to handle. Under the head of cost the electric locomotive comes out well, as motors can be purchased cheaply; and connecting them up with driving wheels is a much less troublesome business than the construction of an equally efficient steamer. One may add that the electric motor is ready to start at a moment"s notice: there is no delay corresponding to that caused by the raising of steam.