The Boy Mechanic Part 92

of paper is first folded, Fig. 1, then the corners on one end are doubled over, Fig. 2, and the whole piece finished up and held together with a paper clip as in Fig. 3. The paper clip to be used should be like the one shown in Fig. 4. If one of these clips is not at hand, form a piece of wire in the same shape, as it will be needed for balancing purposes as well as for holding the paper together. Grasp the aeroplane between the thumb and forefinger at the place marked A in Fig. 3, keeping the paper as level as possible and throwing it as you would a dart. The aeroplane will make an easy and graceful flight in a room where no air will strike it.

--Contributed by J.H. Crawford, Schenectady, N. Y.

** Bronze Liquid [329]

Banana oil or amyl acetate is a good bronze liquid.

** A Wrestling Mat [330]

The cost of a wrestling mat is so great that few small clubs can afford to own one. As we did not see our way

[Ill.u.s.tration: Made of Bed Mattresses]

clear to purchase such a mat, I made one of six used bed mattresses (Fig. 1) purchased from a second-hand dealer. I ordered a canvas bag, 12 ft. 3 in. by 12 ft. 9 in., from a tent company, to cover the mattresses. The bag consisted of two pieces with the seam along each edge. The mattresses were laid side by side and end to end and the bag placed on and laced up as shown in Fig. 2.

--Contributed by Walter W. White, Denver, Colo.

** A Pocket Voltammeter [330]

Remove the works and stem from a discarded dollar watch, drill two 3/16 in. holes in the edge, 3/4 in. apart, and insert two binding-posts, Fig. 1, insulating them from the case with cardboard. Fold two strips of light cardboard, 1/2 in. wide, so as to form two oblong boxes, 1/2 in. long and 3/16 in. thick, open on the edges. On one of these forms wind evenly the wire taken from a bell magnet to the depth of 1/8 in. and on the other wind some 20 gauge wire to the same depth. Fasten the wire with gummed label, to keep it from unwinding.

Glue the coils to the back of the case and connect one wire from each binding-post as shown in Fig. 2, while the other two wires are connected to an induction coil lead which is inserted in the hole from which the stem was removed. Fasten a bra.s.s-headed tack to the case at the point F with sealing wax or solder and bend a wire in the shape shown in Fig. 3 to swing freely on the tack.

Attach a piece of steel rod, 3/4 in. long, in the center coil, C, Fig. 2.

A rubber band, D, connects the steel rod C with the top of the watch case. The ends of the rubber are fastened with sealing wax.

The rubber keeps the pointer at zero or in the middle of the scale. Do not use too strong a rubber. A dial may be made by cutting a piece of stiff white paper so it will fit under the crystal of the watch. An arc is cut in the paper, as shown in Fig.

1, through which the indicator works.

To calibrate the instrument, first mark the binding-post A, which is connected to the coil of heavy wire, for amperes and the other post, V, to the coil of small wire for volts. Connect the lead and the post marked A to one, two and three cells and each time mark the place of the pointer on the dial. Take corresponding readings on a standard ammeter and mark the figures on the dial. The volt side of the dial may be calibrated in the same manner, using a voltmeter instead of the ammeter. The place where the

[Ill.u.s.tration: Voltammeter in a Watch Case]

indicator comes to rest after disconnecting the current is marked zero.

--Contributed by Edward M. Teasdale, Warren, Pa.

** A Film Washing Trough [331]

[Ill.u.s.tration: Washing a Negative Film]

The washing of films without scratching them after they are developed and fixed is very difficult in hot weather. A convenient washing trough for washing full length films is shown in the accompanying sketch. The trough must be made for the size of the film to be washed. Cut a 1/4-in. board as long as the film and a trifle wider than the film's width. Attach strips to the edges of the board to keep the water from spilling over the sides.

Cut a hole in one side of a baking powder can about half way between the top and bottom, large enough to admit a fair-sized stream of water from a faucet. Then solder the cover to the can and punch a number of holes about 1/4 in. apart along the opposite side from where the large hole was cut. Place this can on one end of the trough, as shown, with the large hole up.

Some heavy wire bent in the shape of a U and fastened to the under side of the trough at the can end will furnish supports to keep that end of the trough the highest and place the opening in the can close beneath the water faucet. A common pin stuck through one end of the film and then in the trough close to the can will hold it in position for washing. Five minutes' washing with this device is sufficient to remove all traces of the hypo from the film.

--Contributed by M. M. Hunting, Dayton, O.

** Wood Burning [331]

[Ill.u.s.tration: Burnt Wood]

Burnt wood work done with an ordinary reading gla.s.s and the sun's rays.

** The Diving Bottle [331]

This is a very interesting and easily performed experiment ill.u.s.trating the transmission of pressure by liquids. Take a wide-mouthed bottle and fill almost full of water; then into this bottle place, mouth downward, a small vial or bottle having just enough air in the bottle to keep it barely afloat. Put a sheet of rubber over the mouth of the large bottle, draw the edge down over the neck and wrap securely with a piece of string thus forming a tightly stretched diaphragm over the top. When a finger is pressed on the rubber the small bottle will slowly descend until the pressure is released when the

[Ill.u.s.tration: Pressure Experiments]

small bottle wilt ascend. The moving of the small bottle is caused by the pressure transmitted through the water, thus causing the volume of air in the small tube to decrease and the bottle to descend and ascend when released as the air increases to the original volume.

This experiment can be performed with a narrow-necked bottle, provided the bottle is wide, but not very thick. Place the small bottle in as before, taking care not to have too much air in the bottom. If the cork is adjusted properly, the bottle may be held in the hand and the sides pressed with the fingers, thus causing the small bottle to descend and ascend at will. If the small bottle used is opaque, or an opaque tube such as the cap of a fountain pen, many puzzling effects may be obtained. --Contributed by John Shahan, Auburn, Ala.

** How to Make an Inexpensive Wooden Fan [332]

Select a nice straight-grained piece of white pine about 1/4 in.

thick, 3/4 in. wide and 4 in. long. Lay out the design desired and cut as shown in Fig. 1, and then soak the wood in hot water to make it soft and easy to split. Cut the divisions very thin with a sharp knife down to the point A, as shown in the sketch, taking care not to split the wood through the part left for the handle.

The fan is then finished by placing each piece over the other as in Fig. 2. This will make a very pretty ornament.

--Contributed by Fred W. Whitehouse, Upper Troy, N.Y.

[Ill.u.s.tration: Cutting the Wood and Complete Fan]

** Combination Telegraph and Telephone Line [332]

The accompanying diagrams show connections for a short line system

[Ill.u.s.tration: Wiring Diagram]

(metallic circuit) of telegraph where a telephone may be used in combination on the line. The telephone receivers can be used both as receivers and transmitters, or ordinary telephone transmitters, induction coils and battery may be used in the circuit with a receiver. If a transmitter is used, its batteries may be connected in circuit with a common push b.u.t.ton which is held down when using the telephone. On a 1000-ft. line, four dry cells will be sufficient for the telegraph instruments and two cells for the telephone.

--Contributed by D. W. Milter.

** How to Make a Miniature Windmill [333]