The finishing of the outside must be left to the maker"s taste. Varnishing, or polishing with warmed beeswax, will add to the general appearance, and keep out damp.
The total cost of a ten-drawer cabinet ought not to exceed eighteen pence.
A Tool Cabinet.
The wooden cabinet shown in Fig. 30 is constructed, as regards its case, in the same way as that just described, but the drawers are built up of several pieces. The over-all dimensions of the cabinet represented are as follows: Height, including plinth, 25 inches; width, 17-3/8 inches; depth, 10-1/2 inches. The drawers are 16 inches wide (outside), by 10-1/8 inches from back to front, and, reckoning from the bottom upwards, are 3-1/4, 3, 2-1/2, 2, 2, 2, 2, and 1-3/4 inches deep.
[Ill.u.s.tration: FIG. 30.--Large cabinet (a), details of drawer joints (b, c, d), and padlock fastening (e).]
The construction of the drawers is indicated by the diagrams, Fig. 30, b, c, d. The fronts are of 5/8-inch, the sides and backs of 3/8-inch, and the bottoms of (barely) 1/4-inch wood. The grooves should not come nearer than 1/8-inch to the bottom edge, or be more than 5/16 inch wide and deep. The possessor of a suitable "plough" plane will have no difficulty in cutting them out; in the absence or such a tool the cutting gauge and chisel must be used.
The back piece of a drawer has 1/4-inch less height than the front, to allow the bottom to be introduced. The ends or the bottom are bevelled off towards the top edge to fit the grooves, so that no part may be above the grooves.
Glue should be used to attach the sides of a drawer to the back and front in the first place, and nails be added when the glue has set. As an aid to obtaining perfect squareness, without which the drawers will fit badly, it is advisable to mark out on a board a rectangle having the exact inside dimensions of a drawer, and to nail strips of wood up to the lines on the inside. If the parts are put together round this template they will necessarily fit squarely.
Divisions.--If the drawers are to be subdivided in one direction only, the part.i.tions should run preferably from back to front, as this enables the contents of a compartment to be more easily seen. Where two-direction division is needed the part.i.tions are cut as shown in Fig. 31. All part.i.tions should touch the bottom, and be made immovable by gluing or nailing. It is a mistake to have so many divisions in a drawer that the fingers cannot get into them easily.
Wooden k.n.o.bs for the drawers can be bought very cheaply of any turner, or suitable bra.s.s k.n.o.bs at any ironmonger"s. Take care that the k.n.o.bs are in line with one another; otherwise the general appearance of the cabinet will suffer.
[Ill.u.s.tration: FIG. 31.--Divisions of drawer notched to cross each other.]
Lock and Key.--If a cabinet is intended for storage of articles of any value it should be provided with lock and key. One lock will secure all the drawers if attached to a flap hinged on one side to the cabinet, as shown in Fig. 30 a, to engage a catch projecting from one of the drawers. A special form of lock is sold for the purpose. If the single flap seems to give a lop-sided effect, place a fellow on the other side, and fit it with sunk bolts to shoot into the overhanging top and plinth. If you wish to avoid the expense and trouble of fitting a lock, subst.i.tute a padlock and a staple clinched through the front of a drawer and pa.s.sing through a slot in the flap (Fig. 30, e).
Alternative Method.--The fixing of the front bars can be avoided if the front of each drawer (except the lowest) be made to overhang the bottom by the depth of the runner. This method, of course, makes it impossible to stand a drawer level on a level surface.
X. TELEGRAPHIC APPARATUS.
The easily made but practical apparatus described in this chapter supplies an incentive for learning the Morse telegraphic code, which is used for sending sound signals, and for visible signals transmitted by means of flags, lamps, and heliograph mirrors. Signalling is so interesting, and on occasion can be so useful, that no apology is needed for introducing signalling apparatus into this book.
The apparatus in question is a double-instrument outfit, which enables an operator at either end of the line to cause a "buzzer" or "tapper" to work at the other end when he depresses a key and closes an electric circuit.
Each unit consists of three main parts--(1) the transmitting key; (2) the receiving buzzer or tapper; (3) the electric battery.
The principles of an installation are shown in Fig. 33. One unit only is ill.u.s.trated, but, as the other is an exact duplicate, the working of the system will be followed easily.
[Ill.u.s.tration: Fig. 32.--Morse alphabet]
A wooden lever, L, is pivoted on a support, A. Pa.s.sing through it at the forward end is a metal bar having at the top a k.n.o.b, K, which can be grasped conveniently in the fingers; at the other a bra.s.s screw, O, which is normally pulled down against the contact, N, by the spiral spring, S.
The contact M under K is in connection with the binding post T1 and N with binding post T3; K is joined up to T2, and O to T4.
T3 and T4 are connected with one of the line wires; T1 with the other wire through a battery, B; T3 with the other wire through the buzzer, R. [1]
[Footnote 1: For the buzzer may be subst.i.tuted the tapper, described on a later page.]
a.s.suming both keys to be at rest, as in Fig. 33, the two buzzers are evidently in circuit with the line wires, though no current is pa.s.sing. If the stem of K is depressed to make contact with M, the electric circuit of which the battery, B, forms part is completed, and the buzzer at the other end of the lines comes into action. Since the depression of K raises O off N, the "home" buzzer"s connection with the line wires is broken, to prevent the current being short-circuited. The fact that this buzzer is periodically in circuit, even when the key is being worked, makes it possible for the operator at the other end to attract attention by depressing his key, if he cannot read the signals sent.
[Ill.u.s.tration: Fig.33--Telegraphic apparatus; sending key, buzzer and battery]
Making the Keys.
Transmitting keys can be bought cheaply, but not so cheaply as they can be made. The only expense entailed in home manufacture is that of the screw terminals for connecting the keys with the lines and buzzers. These cost only a penny each, and, if strict economy is the order of the day, can be dispensed with should the apparatus not have to be disconnected frequently.
The size of the key is immaterial. The keys made by me have levers 1 inch wide and 5-1/2 inches long, oak being chosen as material, on account of its toughness. K is in each case a small wooden k.n.o.b on a piece of 3/16-inch bra.s.s rod; O a 1-1/2-inch bra.s.s screw; A a piece of sheet bra.s.s 3-1/2 inches long, marked off carefully, drilled 1/8 inch from the centre of each end for the pivot screws, and in four places for the holding-down screws, and bent up at the ends to form two standards. If you do not possess any bra.s.s strip, the lever may be supported on wooden uprights glued and screwed to the base.
[Ill.u.s.tration: Fig. 34--Telegraphic apparatus mounted on baseboard]
Contact M is a small piece of bra.s.s attached to the base by a screw at one end and by T1 at the other. K was drilled near the end to take the short coil of insulated wire joining it to T2, and O was similarly connected with T4.
The spring, S, should be fairly strong. A steel spiral with a loop at each end is most easily fitted. Drill holes in the lever and base large enough for the spring to pa.s.s through freely, make a small cross hole through the lever hole for a pin, and cut a slot across the base hole for a pin to hold the bottom of the spring. Adjust the lever by means of screw O so that there is a s.p.a.ce of about 1/4-inch between K and M when O and N are in contact, and after the spring has been put in position give the screw a turn or two to bring K down to within 1/16 inch of M. This will put the required tension on the spring.
The Buzzers.--For these I selected a couple of small electric bells, costing 2s. 6d. each. Their normal rate of vibration being much too slow for telegraphic purposes, I cut off the hammers to reduce the inertia, and so adjusted the contact screw that the armature had to move less than one hundredth of an inch to break the circuit. This gave so high a rate of vibration that the key could not make and break the circuit quickly enough to prevent the buzzer sounding.
A Morse Tapper or Sounder.
In postal telegraph offices a "sounder," and not a "buzzer," is generally used to communicate the signals. Instead of a continuous noise, lasting as long as the key at the transmitting station is held down, the operator at the receiving station hears only a series of taps made by an instrument called a "sounder." The principle of this simple device is ill.u.s.trated by the working diagrams in Fig. 35. M is a horseshoe magnet fixed to a base, A. Close to it is an armature, AR, of soft iron, attached to a lever, L, which works on a pivot and is held up against a regulating screw, P1, by the pull of the spring SP. When current pa.s.ses through the magnet the armature is attracted, and the point of the screw S2 strikes against P2; while the breaking of the circuit causes L to fly back against S1. The time intervening between the "down" and "up" clicks tells the operator whether a long or a short--dash or a dot--is being signalled.
[Ill.u.s.tration: FIG. 35.-Elevation and plan of telegraphic sounder.]
Materials.--A horseshoe magnet and armature taken from an electric bell provide the most essential parts of our home-made instrument in a cheap form. If these are available, expense will be limited to a few pence. Oak or walnut are the best woods to use for the lever, being more resonant than the softer woods, and for the standard B and stop V. Any common wood is good enough for the base A.
The lever L is 6 inches long, 1/2 inch deep, and 3/8-inch wide, and is pivoted at a point 4-1/4 inches from the stop end. The hole should be bored through it as squarely as possible, so that it may lie centrally without B being out of the square. A piece of metal is screwed to its top face under the adjusting screw S1.
The spring is attached to L and A in the manner already described on p. 89 in connection with the "buzzer."
The plate P2 should be stout enough not to spring under the impact of the lever. Fig. 36 is an end view of the standard B. The drilling of the pivot hole through this requires care. The screw S2 should be so adjusted as to prevent the armature actually touching the cores of the magnets when attracted. The ends of the magnet winding wire, after being sc.r.a.ped, are clipped tightly against the base by the binding posts T1 T2.
If sounders are used in place of buzzers they are connected up with the keys, batteries, and line wires in the manner shown in Fig. 33.
Batteries.
The dry cells used for electric bells are the most convenient batteries to use. They can now be purchased at all prices from a shilling upwards, and give about 1-1/2 volts when in good condition. One cell at each end will suffice for short distances, or for considerable distances if large conductors are used. If a single cell fails to work the buzzer strongly through the circuit, another cell must be added.
[Ill.u.s.tration: FIG. 36.--Standard for sounder.]
For ease in transport it will be found advisable to mount key, buzzer, and battery on a common baseboard, which should be provided with a cover and handle. The three parts are interconnected with one another, and the line wire terminals as sketched in Fig. 34. This arrangement makes the apparatus very compact and self-contained. As a finishing touch fit the lid inside with clips for holding a stiff-backed writing pad and pencil for the recording of messages.
Lines.--Fencing made of stout galvanized iron wires strung on wooden posts supplies excellent conductors for practice purposes, provided the posts be quite dry. In wet weather there will be leakage. (Fencing with metal posts is, of course, unsuitable, as every post short-circuits the current.) The two wires selected for land lines must be sc.r.a.ped quite bright at the points where the connections are to be made.
It is an easy matter to rig up a telegraph line of galvanized wire 1/12 to 1/8 inch in diameter, strung along insulators (the necks of bottles serve the purpose excellently) supported on trees, posts, or rough poles. The length of the line will be limited by the battery power available, but a 6-volt battery at each end will probably suffice for all experimental purposes. A second wire is not needed if one terminal at each end is connected with a copper plate sunk in the ground, or with a metal fence, drain-pipe, etc.
XI. A RECIPROCATING ELECTRIC MOTOR.