The Boy Mechanic Part 36

[Ill.u.s.tration: Lightning Arrester]

between the pieces A and B allowing a s.p.a.ce of 1/16-in. all around the edge. One binding-post and a small screw will hold the piece of bra.s.s, C, in place on the wood. The connections are made from the line wires to the two upper binding-posts and parallel from the lower binding-posts to the instrument. The third binding-post on C is connected to the ground wire. Any heavy charge from lightning will jump the saw teeth part of the bra.s.s and is grounded without doing harm to the instruments used. --Contributed by Edwin Walker, Chicago, Ill.

** A Home-Made Punt [123]

A flat bottom boat is easy to make and is one of the safest boats, as it is not readily overturned. It has the advantage of being rowed from either end, and has plenty of good seating capacity.

This punt, as shown in Fig. 1, is built 15 ft. long, about 20 in.

deep and 4 ft. wide. The ends are cut sloping for about 20 in.

back and under. The sides are each made up from boards held together with battens on the inside of the boat near the ends and in the middle. One wide board should be used for the bottom piece.

Two pins are driven in the top board of each side to serve as oarlocks.

The bottom is covered with matched boards not over 5 in. wide.

These pieces are placed together as closely as possible, using white lead between the joints and nailing them to the edges of the side boards and to a keel strip that runs the length of the punt, as shown in Fig. 2. Before nailing the boards place lamp wicking between them and the edges of the side boards. Only galvanized nails should be used. In order to make the punt perfectly watertight it is best to use the driest lumber obtainable. At one end of the punt a skag and a rudder can be attached as shown in Fig. 3.

[Ill.u.s.tration: Easy to Build and Safe to Use]

** Photographers' Printing Frame Stand [123]

When using developing papers it is always bothersome to build up books or

[Ill.u.s.tration: Adjustable to Any Height]

small boxes to make a place to set the printing frame in front of the light. Details for making a small stand that is adjustable to any desired height are shown in the sketch. In Fig. 1 is shown the construction of the sliding holder. A piece of 1/4-in. gas pipe, A, is cut 1 in. long and fitted with a thumbscrew, B. The piece of pipe is soldered to the middle on the back side of a piece of metal that is about 4 by 4-1/2 in. with its lower edge turned up to form a small shelf as shown at C. The main part of the stand is made by inserting a 5/16-in. rod tightly into a block of hard maple wood that is 1 in. thick and 3-1/2 in. square (Fig 2). The pipe that is soldered to the metal support will slide up and down the rod and the thumbscrew can be set to hold it at the desired point.

** Heat and Expansion [124]

Take an electric light bulb from which the air has not been exhausted and immerse it in water and then break off the point. As there is a vacuum in the bulb it will quickly fill with water.

Shake the bulb gently until a part of the water is out and then screw the bulb into a socket with the point always downward. Apply the current and the heated air inside will soon expand and force the water out with great rapidity. Sometimes this experiment can be done several times by using the same bulb.

--Contributed by Curtiss Hill, Tacoma, Wash.

** Photographing a Streak of Lightning [124]

The accompanying ill.u.s.tration is a remarkable photograph of a streak of lightning. Many interesting pictures of this kind can be made during a storm at night. The camera is set in a place where it will not get wet and left standing with the shutter open and the plate ready for the exposure. Should a lightning streak appear within the range of the lens it will be made on the plate, which can be developed in the usual manner. It will require some attention to that part of the sky within the range of the lens so as to not make a double exposure by letting a second flash enter the open lens.

--Contributed by Charles H. Wagner.

Borax may be used as a solvent for sh.e.l.lac gum.

** How to Make a Small Single-Phase Induction Motor [124]

By C. H. Bell

The following notes on a small single-phase induction motor, without auxiliary phase, which the writer has made, may be of interest to some of our readers, says the Model Engineer. The problem to be solved was the construction of a motor large enough to drive a sewing machine or very light lathe, to be supplied with 110-volt alternating current from a lighting circuit, and to consume, if possible, no more current than a 16-cp. lamp. In designing, it had to be borne in mind that, with the exception of insulated wire, no special materials could be obtained.

[Ill.u.s.tration: Motor]

The principle of an induction motor is quite different from that of the commutator motor. The winding of the armature, or "rotor,"

has no connection with the outside circuit, but the current is induced in it by the action of the alternating current supplied to the winding of the field-magnet, or "stator." Neither commutator nor slip rings are required, and all sparking is avoided.

Unfortunately, this little machine is not self-starting, but a slight pull on the belt just as the current is turned on is all that is needed, and the motor rapidly gathers speed provided no load is put on until it is in step with the alternations of the supply. It then runs at constant speed whether given much or little current, but stops if overloaded for more than a few seconds.

The stator has four poles and is built up of pieces of sheet iron used for stove pipes, which runs about 35 sheets to the inch. All the pieces are alike and cut on the lines with the dimensions as shown in Fig. 1, with the dotted line, C, to be filed out after they are placed together. Each layer of four is placed with the pointed ends of the pieces alternately to the right and left so as to break joints as shown in Fig. 2. The laminations were carefully built up on a board into which heavy wires had been driven to keep them in place until all were in position and the whole could be clamped down. In the middle of the pieces 1/4-in. holes, B, were then drilled and 1/4-in. bolts put in and tightened up, large holes being cut through the wood to enable this to be done. The armature tunnel was then carefully filed out and all taken apart again so that the rough edges could be sc.r.a.ped off and the laminations given a thin coat of sh.e.l.lac varnish on one side.

After a.s.sembling a second time, the bolts were coated with sh.e.l.lac and put into place for good. Holes 5-32 in. in diameter were drilled in the corners, A, and filled with rivets, also varnished before they were put in. When put together they should make a piece 2 in. thick.

This peculiar construction was adopted because proper stampings were not available, and as every bit of sheet iron had to be cut with a small pair of tinners' snips, it was important to have a very simple outline for the pieces. They are not particularly accurate as it is, and when some of them got out of their proper order while being varnished, an awkward job occurred in the magnet which was never entirely corrected. No doubt some energy is lost through the large number of joints, all representing breaks in the magnetic circuit, but as the laminations are tightly held together and the circuit is about as compact as it could possibly be, probably the loss is not as great as it would appear at first sight.

The rotor is made of laminations cut from sheet iron, as shown in Fig. 3, which were varnished lightly on one side and clamped on the shaft between two nuts in the usual way. A very slight cut was taken in the lathe afterwards to true the circ.u.mference. The shaft was turned from 1/2-in. wrought iron, no steel being obtainable, and is shown with dimensions in Fig. 4. The bearings were cast of babbitt metal, as shown in Fig. 5, in a wooden mold and bored to size with a twist drill in the lathe. They are fitted with ordinary wick lubricators. Figures 6 and 7 are sections showing the general arrangement of the machine.

The stator is wound full with No. 22 double cotton-covered copper wire,

[Ill.u.s.tration: Motor]

about 2-1/2 lb. being used, and the connections are such as to produce alternate poles--that is, the end of the first coil is joined to the end of the second the beginning of the second to the beginning of the third, and the end of the third to the end of the fourth, while the beginnings of the first and fourth coils connect to the supply.

The rotor is wound with No. 24 double cotton-covered copper wire, each limb being filled with about 200 turns, and all wound in the same direction. The four commencing ends are connected together on one side of the rotor and the four finis.h.i.+ng ends are soldered together on the other. All winding s.p.a.ces are carefully covered with two layers of cambric soaked in sh.e.l.lac, and as each layer of wire was wound, it was well saturated with varnish before the next was put on.

This type of motor has drawbacks, as before stated, but if regular stampings are used for the laminations, it would be very simple to build, having no commutator or brushes, and would not easily get out of order. No starting resistance is needed, and as the motor runs at constant speed, depending upon the number of alternations of the supply, a regulating resistance is not needed.

** Carbolic Acid Burns [126]

The pain of carbolic acid burns can be relieved promptly by was.h.i.+ng with alcohol, if applied immediately. If too late for alcohol to be of use, brush with water containing saturated solution of picric acid.

** How to Make a Paper Book Cover [126]

Book covers become soiled in handling and especially school books.

Various methods are applied for making a temporary cover that will protect the book cover. A paper cover can be quickly made by using a piece of paper larger than both covers on the book when they are open. Fold the paper on the long dotted line, as shown in Fig. 1.

When the folds are made the paper should then be just as wide as the book cover is high. The ends are then folded on the short dotted lines, which will make it appear as shown in Fig. 2. The paper thus folded is placed on the book cover as shown in Fig. 3.

--Contributed by C. E. McKinney, Jr., Newark, N. J.

[Ill.u.s.tration: To Protect Book Covers]