The Boy Mechanic Part 31

[Ill.u.s.tration: Shocking-Machine]

--Contributed by Geo. W. Fry, 903 Vine St., San Jose, Cal.

** An Automatic Lock [106]

The ill.u.s.tration shows an automatic lock operated by electricity, one cell being sufficient. When the circuit is broken a weight, A, attached to the end of the armature B, tends to push the other end of the armature into the screw eye or hook C, which is in the door, thus locking the door.

To unlock the door, merely push the b.u.t.ton E, The magnet then draws the armature out of the screw eye and the door is unlocked.

The dotted line at D shows the position of the armature when the circuit is complete and the door unlocked. The weight must be in proportion to the strength of the magnet. If it is not, the door will not

[Ill.u.s.tration: Automatic Electric Lock for Doors]

lock, or would remain locked. The b.u.t.ton can be hidden, as it is the key to the lock.

--Contributed by Claude B. Melchior, Hutchinson, Minn.

** Experiment with Two-Foot Rule and Hammer [106]

An example of unstable equilibrium is shown in the accompanying sketch. All that is needed is a 2-foot rule, a hammer, a piece of string, and a table or bench. The experiment works best

[Ill.u.s.tration: An Experiment in Equilibrium]

with a hammer having a light handle and a very heavy head.

Tie the ends of the string together, forming a loop, and pa.s.s this around the hammer handle and rule. Then place the apparatus on the edge of the table, where it will remain suspended as shown.

--Contributed by Geo. P. Schmidt, Culebra, Porto Rico, W. I.

** Simple Current Reverser [107]

On a block of hardwood draw a square (Fig. 1) and drill a hole in each corner of the square. Fill these holes with mercury and connect them to four binding posts (Fig. 1).

On another block of wood fasten two wires, as shown in Fig. 2, so that their ends can be placed in the holes in the first block.

Then connect up with the

[Ill.u.s.tration: Details of Reverser]

motor and battery as in Fig. 3. When the block is placed on with the big arrow A pointing in the direction indicated in Fig. 3, the current flows with the small arrows. To reverse turn through an angle of 90 degrees (Fig. 4). -- Contributed by F. Crawford Curry, Brockville, Ontario, Canada.

** Alarm Clock to Pull up Furnace Draft [107]

A stout cord, A, is attached to the draft B of the furnace, run through a pulley, C, in the ceiling and has a window weight, D, attached at the other end. A small stick is put through a loop in the cord at about the level of the table top on which the alarm clock F stands. The other end of stick E is placed under the key G of the alarm clock. When the alarm rings in the early morning, the key turns, the stick

[Ill.u.s.tration: Automatic Time Draft-Opener]

falls away, releasing the weight, which pulls the draft open.

--Contributed by Edward Whitney, 18 Gorham St., Madison, Wis.

** How to Transmit Phonograph Music to a Distance [107]

An interesting experiment, and one calculated to mystify anyone not in the secret, is to transmit the music or speech from a phonograph to another part of the house or even a greater distance. For an outdoor summer party the music can be made to come from a bush, or tree, or from a bed of flowers. The apparatus is not difficult to construct.

The cut shows the arrangement. Procure a long-distance telephone transmitter, D, including the mouthpiece, and fasten it to the reproducer of the phonograph. Also a watch case

[Ill.u.s.tration: The Long-Distance Phonograph]

receiver, R, which fasten to the horn. These parts may be purchased from any electrical-supply house. Connect two wires to the transmitter, running one direct to the receiver, and the other to the battery, thence to a switch, S, and then to the receiver.

The more batteries used the louder will be the sound produced by the horn, but avoid using too much battery or the receiver is apt to heat.

--Contributed by Wm. J. Farley, Jr., Camden, N. J.

** How to Make a Telescope [108]

With a telescope like the one here described, made with his own hands, a farmer boy not many years ago discovered a comet which had escaped the watchful eyes of many astronomers.

First, get two pieces of plate gla.s.s, 6 in. square and 1 in.

thick, and break the corners off to make them round, grinding the rough edges on a grindstone. Use a barrel to work on, and

[Ill.u.s.tration: Homemade Telescope]

fasten one gla.s.s on the top of it in the center by driving three small nails at the sides to hold it in place. Fasten, with pitch, a round 4-in. block of wood in the center on one side of the other gla.s.s to serve as a handle.

Use wet grain emery for coa.r.s.e grinding. Take a pinch and spread it evenly on the gla.s.s which is on the barrel, then take the gla.s.s with the handle and move it back and forth across the lower gla.s.s, while walking around the barrel; also rotate the gla.s.s, which is necessary to make it grind evenly. The upper gla.s.s or speculum always becomes concave, and the under gla.s.s or tool convex.

Work with straight strokes 5 or 6 in. in length; after working 5 hours hold the speculum in the sunshine and throw the rays of the sun onto a paper; where the rays come to a point gives the focal length. If the gla.s.s is not ground enough to bring the rays to a point within 5 ft., the coa.r.s.e grinding must be continued, unless a longer focal length is wanted.

Have ready six large dishes, then take 2 lb. flour emery and mix in 12 qt. of water; immediately turn the water into a clean dish and let settle 30 seconds; then turn it into another dish and let settle 2 minutes, then 8 minutes, 30 minutes and 90 minutes, being careful not to turn off the coa.r.s.er emery which has settled. When dry, turn the emery from the 5 jars into 5 separate bottles, and label. Then take a little of the coa.r.s.est powder, wetting it to the consistency of cream, and spread on the gla.s.s, work as before (using short straight strokes 1-1/2 or 2 in.) until the holes in the gla.s.s left by the grain emery are ground out; next use the finer grades until the pits left by each coa.r.s.er grade are ground out. When the two last grades are used shorten the strokes to less than 2 in. When done the gla.s.s should be semi-transparent, and is ready for polishing.

When polishing the speculum, paste a strip of paper 1-1/3 in. wide around the convex gla.s.s or tool, melt 1 lb. of pitch and turn on to it and press with the wet speculum. Mold the pitch while hot into squares of 1 in., with 1/4-in. s.p.a.ces, as in Fig. 1. Then warm and press again with the speculum, being careful to have all the squares touch the speculum, or it will not polish evenly. Trim the paper from the edge with a sharp knife, and paint the squares separately with jeweler's rouge, wet till soft like paint. Use a binger to spread it on with. Work the speculum over the tool the same as when grinding, using straight strokes 2 in. or less.

When the gla.s.s is polished enough to reflect some light, it should be tested with the knife-edge test. In a dark room, set the speculum against the wall, and a large lamp, L, Fig. 2, twice the focal length away. Place a large sheet of pasteboard, A, Fig. 2, with a small needle hole opposite the blaze, by the side of the lamp, so the light

[Ill.u.s.tration: Detail of Telescope Construction]

from the blaze will shine onto the gla.s.s. Place the speculum S, Fig. 2, so the rays from the needle hole will be thrown to the left side of the lamp (facing the speculum), with the knife mounted in a block of wood and edgeways to the lamp, as in K, Fig.

2. The knife should not be more than 6 in. from the lamp. Now move the knife across the rays from left to right, and look at the speculum with the eye on the right side of the blade. When the focus is found, if the speculum is ground and polished evenly it will darken evenly over the surface as the knife shuts off the light from the needle hole. If not, the speculum will show some dark rings, or hills. If the gla.s.s seems to have a deep hollow in the center, shorter strokes should be used in polishing; if a hill in the center, longer strokes. The polishing and testing done, the speculum is ready to be silvered. Two gla.s.s or earthenware dishes, large enough to hold the speculum and 2 in. deep, must be procured. With pitch, cement a strip of board 8 in. long to the back of the speculum, and lay the speculum face down in one of the dishes; fill the dish with distilled water, and clean the face of the speculum with nitric acid, until the water will stick to it in an unbroken film.

The recipe for silvering the speculum is:

Solution A: Distilled water.............................4 oz.