The Boy Mechanic Part 57

--Contributed by W. C. Bliss, St. Louis, Mo.

** How to Make an Ammeter [203]

The outside case of this instrument is made of wood taken from old cigar boxes with the exception of the back. If carefully and neatly made, the finished instrument will be very satisfactory.

The measurements here given need not be strictly followed out, but can be governed by circ.u.mstances. The case should first be made and varnished and while this is drying, the mechanical parts can be put together.

[Ill.u.s.tration: Details of an Ammeter]

The back is a board 3/8 in. thick, 6-1/2 in. wide and 6-3/4 in.

long. The outer edges of this board are chamfered. The other parts of the case are made from the cigar box wood which should be well sandpapered to remove the labels. The sides are 3-1/4 in. wide and 5 in. long; the top and bottom, 3-1/4 in. wide and 4-1/2 in. long.

Glue a three cornered piece, A, Fig. 1, at each end on the surface that is to be the inside of the top and bottom pieces. After the glue, is set, fasten the sides to the pieces with glue, and take care that the pieces are all square. When the glue is set, this square box is well sandpapered, then centered, and fastened to the back with small screws turned into each three-cornered piece.

The front, which is a piece 5-1/4 in. wide and 6-1/2 in. long, has a circular opening cut near the top through which the graduated scale may be seen. This front is centered and fastened the same as the back, and the four outside edges, as well as the edges around the opening, are rounded. The whole case can now be cleaned and stained with a light mahogany stain and varnished. Cut another piece of board, B, Figs. 2 and 3, to just fit inside the case and rest on the ends of the three-cornered pieces, A, and glue to this board two smaller pieces, C, 3 in. square, with the grain of the wood in alternate directions to prevent warping. All of these pieces are made of the cigar box wood. Another piece, D, 3/8 in.

thick and 3 in. square, is placed on the other pieces and a U-shaped opening 1-3/4 in. wide and 2-1/2 in. high sawed out from all of the pieces as shown. The piece D is attached to the pieces C with four 1/2-in. pieces 2-5/8 in. long.

A magnet is made from a soft piece of iron, E, about 3/8 in.

thick, 1-1/4 in. wide and 2-3/4 in. long. Solder across each end of the iron a piece of bra.s.s wire, F, and make a turn in each end of the wires, forming an eye for a screw. These wires are about 2-1/2 in. long. Wind three layers of about No. 14 double cotton-covered copper wire on the soft iron and leave about 5 or 6 in. of each end unwound for connections.

The pointer is made as shown in Fig. 5 from 1/16-in. bra.s.s wire filed to make a point at both ends for a spindle. About 1/2 in.

from each end of this wire are soldered two smaller bra.s.s wires which in turn are soldered to a strip of light tin 1/4 in. wide and 2-5/8 in. long. The lower edge of this tin should be about 1/2 in. from the spindle. The pointer is soldered to the spindle 1/4 in. from one end. All of these parts should be bra.s.s with the exception of the strip of tin. Another strip of tin, the same size as the first, is soldered to two bra.s.s wires as shown in Fig. 4.

These wires should be about 1 in. long.

The spindle of the pointer swings freely between two bars of bra.s.s, G, 1/16 in. thick, 1/4 in. wide and 2-1/2 in. long. A small hole is countersunk in one of the bars to receive one end of the spindle and a hole 1/8 in. in diameter is drilled in the other and a thumb nut taken from the binding-post of an old battery soldered over the hole so the screw will pa.s.s through when turned into the nut. The end of the screw is countersunk to receive the other end of the spindle. A lock nut is necessary to fasten the screw when proper adjustment is secured. A hole is drilled in both ends of the bars for screws to fasten them in place. The bar with the adjusting screw is fastened on the back so it can be readily adjusted through the hole H, bored in the back. The pointer is bent so it will pa.s.s through the U-shaped cut-out and up back of the board B. A bra.s.s pin is driven in the board B to hold the pointer from dropping down too far to the left. Place the tin, Fig. 4, so it will just clear the tin, Fig. 5, and fasten in place. The magnet is next placed with the ends of the coil to the back and the top just clearing the tin strips. Two binding screws are fitted to the bottom of the back and connected to the extending wires from the coil.

The instrument is now ready for calibrating. This is done by connecting it in series with another standard ammeter which has the scale marked in known quant.i.ties. In this series is also connected a variable resistance and a battery or some other source of current supply. The resistance is now adjusted to show .5 ampere on the standard ammeter and the position of the pointer marked on the scale. Change your resistance to all points and make the numbers until the entire scale is complete.

When the current flows through the coil, the two tinned strips of metal are magnetized, and being magnetized by the same lines of force they are both of the same polarity. Like poles repel each other, and as the part Fig. 4 is not movable, the part carrying the pointer moves away. The stronger the current, the greater the magnetism of the metal strips, and the farther apart they will be forced, showing a greater defection of the pointer. --Contributed by George Heimroth, Richmond Hill, L. I.

** How to Make an Equatorial [204] Condensed from article contributed by J. R. Chapman, F.R.A.S. Austwick Hall. W.

Yorks.h.i.+re. England

This star finder can easily be made by anyone who can use a few tools as the parts are all wood and the only lathe work necessary is the turned shoulder on the polar axis and this could be dressed and sandpapered true enough for the purpose. The base is a board 5 in. wide and 9 in. long which is fitted with an ordinary wood screw in each corner for leveling. Two side pieces cut with an angle equal to the colat.i.tude of the place are nailed to the base and on top of them is fastened another board on which is marked the hour circle as shown. The end of the polar axis B, that has the end turned with a shoulder, is fitted in a hole bored in the center of the hour circle. The polar axis B is secured to the board with a wooden collar and a pin underneath. The upper end of the polar axis is fitted with a 1/4-in. board, C, 5-1/2 in. in diameter. A thin compa.s.s card divided into degrees is fitted on the edge of this disk for the declination circle.

The hour circle A is half of a similar card with the hour marks divided into 20 minutes. An index pointer is fastened to the base of the polar axis. A pointer 12 in. long is fastened with a small bolt to the center of the declination circle. A small opening is made in the pointer into which an ordinary needle is inserted.

This needle is adjusted to the degree to set the pointer in declination and when set, the pointer is clamped with the bolt at the center. A bra.s.s tube having a 1/4-in. hole is fastened to the pointer.

The first thing to do is to get a true N and S meridian mark. This can be approximately obtained by a good compa.s.s, and allowance made for the magnetic declination at your own place. Secure a slab of stone or some other solid flat surface, level this and have it firmly fixed facing due south with a line drawn through the center and put the equatorial on the surface with XII on the south end of the line. Then set the pointer D to the declination of the object, say Venus at the date of observation. You now want to know if this planet is east or west of your meridian at the time of observation. The following formula will show how this may be found. To find a celestial object by equatorial: Find the planet Venus May 21, 1881, at 9 hr. 10 min. A. M. Subtract right ascension of planet from the time shown by the clock, thus:

hour minute second 9 hr. 10 min. shows mean siderial. 1 0 0 Add 12 hrs 12 --- --- --- 13

Right ascension of Venus 2 10 --- --- --- Set hour circle to before meridian 10 50 0 Again------------------ At 1 hr. 30 min. mean clock shows 5 20 0 Right ascension of Venus 2 10 0 --- --- --- Set hour circle to 3 10 0

Books may be found in libraries that will give the right ascension and declination of most of the heavenly bodies.

The foregoing tables a.s.sume that you have a clock rated to siderial time,

[Ill.u.s.tration: Home-Made Equatorial]

but this is not absolutely necessary. If you can obtain the planet's declination on the day of observation and ascertain when it is due south, all you have to do is to set the pointer D by the needle point and note whether Venus has pa.s.sed your meridian or not and set your hour index. There will be no difficulty in picking up Venus even in bright sunlight when the plant is visible to the naked eye.

** Electric Light Turned On and Off from Different Places [205]

How nice it would be to have an electric light at the turn in a stairway, or at the top that could be turned on before starting up the stair and on reaching the top turned out, and vice

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

versa when coming down. The wiring diagram as shown in the ill.u.s.tration will make this a pleasant reality. This wiring may be applied in numerous like instances. The electric globe may be located at any desired place and the two point switches are connected in series with the source of current as shown in the sketch. The light may be turned on or off at either one of the switches.

--Contributed by Robert W. Hall, New Haven, Conn.

** How to Make a Bunsen Cell [206]

This kind of a cell produces a high e.m.f. owing to the low internal resistance. Procure a gla.s.s jar such as used for a gravity battery, or, if one of these cannot be had, get a glazed vessel of similar construction. Take a piece of sheet zinc large enough so that when it is rolled up in the shape of a cylinder it will clear the edge of the jar by about 1/2 in. Solder a wire or binding-post to the edge of the cylinder for a connection.

[Ill.u.s.tration: Cross Section and Completed Cell]

Secure a small unglazed vessel to fit inside of the zinc, or such a receptacle as used in a sal ammoniac cell, and fill it with a strong solution of nitric acid. Fill the outer jar with a solution of 16 parts water and 5 parts sulphuric acid. The connections are made from the zinc and carbon.

** Optical Illusion [206]

Can you tell which of these three figures is the tallest? Make a guess, and then verify its correctness by measurement.

[Ill.u.s.tration: Who is tallest?]

** One Way to Cook Fish [206]

One of the best and easiest ways of cooking fish while out camping is told by a correspondent of Forest and Stream. A fire is built the size for the amount of food to be cooked and the wood allowed to burn down to a glowing ma.s.s of coals and ashes. Wash and season your fish well and then wrap them up in clean, fresh gra.s.s, leaves or bark. Then, after sc.r.a.ping away the greater part of the coals, put the fish among the ashes, cover up with the same, and heap the glowing coals on top. The fish cooks quickly--15 or 20 minutes--according to their size.

If you eat fish or game cooked after this fas.h.i.+on you will agree that it cannot be beaten by any method known to camp culinary savants. Clay also answers the purpose of protecting. the fish or game from the fire if no other material is at hand, and for anything that requires more time for cooking it makes the best covering. Wet paper will answer, especially for cooking fish.

** Hardening Copper [206]

A successful method of hardening copper is to add 1 lb. of alum and 4 oz. a.r.s.enic to every 20 lb. of melted copper and stir for 10 minutes.