The Boy Mechanic Part 29

** An Optical Illusion [99]

The engraving shows a perfectly straight boxwood rule laid over a number of turned bra.s.s rings of various sizes. Although the effect in the ill.u.s.tration

[Ill.u.s.tration: An Optical Illusion]

is less p.r.o.nounced than it was in reality, it will be noticed that the rule appears to be bent, but sighting along the rule from one end will show that it is perfectly straight.

The bra.s.s rings also appear distorted. The portions on one side of the rule do not appear to be a continuation of those on the other, but that they really are can be proved by sighting in the same manner as before.

--Contributed by Draughtsman, Chicago.

** New Method of Lifting a Table [99]

To perform this feat effectively the little device ill.u.s.trated will be required. To make it take a sheet-iron band, A, 3/4 in.

wide and attach a strap to fasten on the forearm between the wrist and elbow. Put a sharp needle point, B, through the sheet-iron so that it extends 3/4 in. outward. Make one of these pieces for each arm. In lifting the table first show the hands unprepared to the audience and also a tight table, removing the cover to show that the surface of the table is not prepared in any way. Then replace the table,

[Ill.u.s.tration: Table Lifting Device]

rest the hands upon it and at the same time press the needle points in the arm pieces into the wood of the table, which will be sufficient to hold it, says a correspondent of the Sphinx. Then walk down among the audience.

** How to Make a Paddle Boat [100]

A rowboat has several disadvantages. The operation of the oars is both tiresome and uninteresting, and the oarsman is obliged to travel, backward.

[Ill.u.s.tration: Paddle Boat]

By replacing the oars with paddles, as shown in the ill.u.s.tration, the operator can see where he is going and enjoy the exercise much better than with oars. He can easily steer the boat with his feet, by means of a pivoted stick in the bottom of the boat, connected by cords to the rudder.

At the blacksmith shop have a 5/8-in. shaft made, as shown at A, Fig. 2. It will be necessary to furnish a sketch giving all the dimensions of the shaft, which should be designed to suit the dimensions of the boat, taking care that sufficient clearance is allowed, so that the cranks in revolving will not strike the operator's knees. If desired, split-wood handles may be placed on the cranks, to prevent them from rubbing the hands.

The bearings, B, may be made of hardwood, but preferably of iron pipe filled with melted babbitt. If babbitt is used, either thoroughly smoke or chalk the shaft or wrap paper around it to prevent the babbitt sticking. The pieces of pipe may be then fastened to the boat by means of small pipe straps, such as may be obtained at any plumber's at a very small cost.

The hubs, C, should be made of wood, drilled to fit the shaft and mortised out to hold the paddles, D. The covers, E, may be constructed of thin wood or galvanized iron and should be braced by triangular boards, as shown in Fig. 1. If galvanized iron is used, it should be exposed to the weather two or three months before painting, or the paint will come off, spoiling its appearance.

[Ill.u.s.tration: Detail of Paddle Boat]

** Peculiar Properties of Ice [100]

Of all the boys who make s...o...b..a.l.l.s probably few know what occurs during the process. Under ordinary conditions water turns to ice when the temperature falls to 32 degrees, but when in motion, or under pressure, much lower temperatures are required to make it a solid. In the same way, ice which is somewhat below the freezing point can be made liquid by applying pressure, and will remain liquid until the pressure is removed, when it will again return to its original state. Snow, being simply finely divided ice, becomes liquid in places when compressed by the hands, and when the pressure is removed the liquid portions solidify and unite all the particles in one ma.s.s. In extremely cold weather it is almost impossible to make a s...o...b..ll, because a greater amount of pressure is then required to make the snow liquid.

This process of melting and freezing under different pressures and a constant temperature is well ill.u.s.trated by the experiment shown in Figs. 1, 2 and 3. A block of ice, A, Fig. 1, is

[Ill.u.s.tration: Experiment with a Block of Ice]

supported at each end by boxes BB, and a weight, W, is hung on a wire loop which pa.s.ses around the ice as shown. The pressure of the wire will then melt the ice and allow the wire to sink down through the ice as shown in Fig. 2. The wire will continue to cut its way through the ice until it pa.s.ses all the way through the piece, as shown in Fig. 3. This experiment not only ill.u.s.trates how ice melts under pressure, but also how it solidifies when the pressure is removed, for the block will still be left in one piece after the wire has pa.s.sed through.

Another peculiar property of ice is its tendency to flow. It may seem strange that ice should flow like water, but the glaciers of Switzerland and other countries are literally rivers of ice. The snow which acc.u.mulates on the mountains in vast quant.i.ties is turned to ice as a result of the enormous pressure caused by its own weight, and flows through the natural channels it has made in the rock until it reaches the valley below. In flowing through these channels it frequently pa.s.ses around bends, and when two branches come together the bodies of ice unite the same as water would under the same conditions. The rate of flow is often very slow; sometimes only one or two feet a day, but, no matter how slow the motion may be, the large body of ice has to bend in moving.

This property of ice is hard to ill.u.s.trate with the substance itself, but may be clearly shown by sealing-wax, which resembles ice in this respect. Any attempt to bend a piece of cold sealing-wax with the hands results in breaking it, but by placing it between books, as shown on page 65, or supporting it in some similar way, it will gradually change from the original shape A, and a.s.sume the shape shown at B.

** Return-Call Bell With One Wire [101]

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

To use only one wire for a return call bell connect up as shown in the diagram, using a closed circuit or gravity battery, B. The current is flowing through both bells all the time, the same as the coils of a telegraph sounder, but is not strong enough to ring both connected in series. Pressing either push b.u.t.ton, P, makes a short circuit of that bell and rings the one at the other end of the line.

--Contributed by Gordon T. Lane, Crafton, Pa.

** Circuit Breaker for Induction Coils [101]

Amateurs building induction coils are generally bothered by the vibrator contacts blackening, thus giving a high resistance contact, whenever there is any connection made at all. This trouble may be done away with by departing from the old single-contact vibrator and using one with self-cleaning contacts as shown. An old bell magnet is rewound full of No. 26 double cotton-covered wire and is mounted

[Ill.u.s.tration: Interrupter for Induction Coil]

upon one end of a piece of thin sheet iron 1 in. by 5 in. as per sketch. To the other end of the strip of iron is soldered a piece of bra.s.s 1/64 in. by 1/4, in. by 2 in., on each end of which has been soldered a patch of platinum foil 1/4 in. square.

The whole is connected up and mounted on a baseboard as per sketch, the contact posts being of 1/4 in. by 1/2 in. bra.s.s, bent into shape and provided with platinum tipped thumb screws. The advantage of this style of an interrupter is that at each stroke there is a wiping effect at the heavy current contact which automatically cleans off any carbon deposit.

In the wiring diagram, A is the circuit breaker; B, the induction coil, and C, the battery.

--Contributed by A. G. Ward, Wilkinsburg, Pa.

** Spit Turned by Water Power [102]

Many of the Bulgarian peasants do their cooking in the open air over bonfires. The ill.u.s.tration shows a laborsaving machine in use which enables the cook to go away and leave meat roasting for an hour at a time. The

[Ill.u.s.tration: For a Summer Camp]

ill.u.s.tration shows how the spit to which the meat is fastened is constantly turned by means of a slowly moving water wheel. Some of our readers may wish to try the scheme when camping out. The success depends upon a slow current, for a fast-turning wheel will burn the meat.

** A Short-Distance Wireless Telegraph [102]

The accompanying diagrams show a wireless-telegraph system that I have used successfully for signaling a distance of 3,000 ft. The transmitter consists of an induction coil, about the size used for automobiles, a key or push-b.u.t.ton for completing the circuit, and five dry batteries. The small single-point switch is left open as shown when sending a message, but when receiving it should be closed in order that the electric waves from the antenna may pa.s.s through the coherer. The coherer in this case is simply two electric-light carbons sharpened to a wedge at one end with a needle

[Ill.u.s.tration: Wiring Diagram for Wireless Telegraph]

connecting the two, as shown. An ordinary telephone receiver is connected in series with the coherer, as shown. To receive messages hold the receiver to the ear and close the switch, and answer by opening the switch and operating the key. --Contributed by Coulson Glick, Indianapolis.