Before mounting the ring on the base, the groove should be wound with 8 turns of No. 16 double cotton-covered magnet wire. The two ends may be tied together with a string to hold them temporarily.

Fasten two strips of wood 1/4-in. thick 5/8-in. wide and 11 in.

long across the sides of the ring with their upper edges pa.s.sing exactly through the center of the ring. An ordinary pocket compa.s.s, about 1-1/4 in. in diameter, is fitted in these strips so that the center of the needle or pointer will be exactly in the center of the ring and its zero point mark at the half-way point between the two strips. Put the ring in place on the base, as shown in the sketch, and connect the two ends of the wire to two binding-posts that are previously attached to the base. Coat the entire surface with brown sh.e.l.lac. Any deviation from the dimensions will cause errors in the results obtained by its use.

Remove all pieces of iron or steel and especially magnets in the near vicinity of the instrument when in use. Place the galvanometer on a level table and turn it until the needle, pointing north and south, and swinging freely, lies exactly in the plane of the coil, as shown in the cut. The needle then will point to zero if the directions have been followed closely. Connect one

[Ill.u.s.tration: Tangent Galvanometer]

cell of battery to the instrument and allow the current to flow through the coils. The needle of the compa.s.s will be deflected to one side or the other, and will finally come to rest at a certain angle-let us say 45 deg. The dimensions of the instrument are such that when the deflection is 45 deg. the current flowing through the coils upon the ring is 1/2 ampere. The ampere is the unit chosen to designate the strength of the electric current. For other angles the value of the current may be found from the following table:

Angles Degrees Current Amperes 10 .088 20 .182 30 .289 40 .420 45 .500 50 .600 55 .715 60 .865 70 1.375

As the magnetic force that acts upon a magnet needle varies in different places the values given for the current will not be true in all parts of the country. The table gives correct values for the immediate vicinity of Chicago and that part of the United States lying east of Chicago, and north of the Ohio river. The results given should be multiplied by 1.3 for places south of the Ohio river and east of the Mississippi.

** Home-Made X-Ray Instrument [151]

Two cylinders, AA, are mounted on a base, B, and mirrors, CC, are fitted at an angle of 45 deg. into these cylinders. Corresponding mirrors, EE, are put in the base parallel with those in those cylinders. An opening extends downward from D of each cylinder so that light entering at one end of the

[Ill.u.s.tration: Details of X-Ray Machine]

cylinder is reflected down at right angles by the first mirror to the second, from the second to the third, from the third to the fourth which reflects the light to the eye. Thus the light never pa.s.ses through the cylinders and the observer does not see through, but around any object inserted at X between the cylinders.

** How to Make a a Non-Polarizing Battery [151]

Bichromate batteries are very expensive to maintain and dry cells do not furnish enough amperage for some kinds of experimental work. A cell of a battery that will run 10 hours with an output of over 1 ampere can be made as follows: Secure a jar about 4 in. in diameter and 8 in. high and place in the bottom of this jar the lower half of a tin baking powder can, to which a wire has been soldered for connections. Place in the can a mixture of 2 oz.

black oxide of copper, 1 oz. black oxide of manganese and some iron filings.

Purchase a small crowfoot zinc and hang it about 1 in. above the half can. Prepare a 10 per cent solution of caustic soda and fill the jar within 1 in. of the top. Place on top the solution a thin layer of kerosene or paraffin. The cell will only cost about 50 cents to make and 25 cents for each renewal. When renewing, always remove the oil with a siphon.

--Contributed by Robert Canfield, University Park, Colo.

** A Home-Made Barometer [151]

Take 1/4 oz. of pulverized campor, 62 gr. of pulverized nitrate of pota.s.sium, 31 gr. nitrate of ammonia and dissolve in 2 oz.

alcohol. Put the solution in a long, slender bottle, closed at the top with a piece of bladder' containing a pinhole to admit air, says Metal Worker. When rain is coming the solid particles will tend gradually to mount, little crystals forming in the liquid, which otherwise remains clear; if high winds are approaching the liquid will become as if fermenting, while a film of solid particles forms on the surface; during fair weather the liquid will remain clear and the solid particles will rest at the bottom.

** Lock Lubricant [151]

A door lock may be lubricated by using some lead sc.r.a.ped from the lead in a pencil and put in the lock. This may be done by putting the sc.r.a.pings on a piece of paper and blowing them into the lock through the keyhole.

** Rust Proofing Bolts [151]

Where bolts are subject to rust, the threads should be painted with pure white lead; then they will not rust fast.

** Painting Yellow Pine [151]

When painting yellow pine exposed to the weather add a little pine tar with the priming coat.

** Revolving a Wheel with Boat Sails [152]

A novel windmill or revolving wheel can be made by placing a light wheel so it will turn freely on the end

[Ill.u.s.tration: An Unusual Type of Windmill]

of a post, and placing four small sailing boats at equal points on the rim of the wheel. It makes no difference which way the wind blows, the wheel will revolve in one direction. In Fig. 1 the direction of the wind is shown by the arrows, and how the sails catch the wind and cause the wheel to revolve. Figure 2 shows how the wheel will appear when complete. This device makes an attractive advertising sign.

** A Floating Electromagnet [152]

A piece of iron placed in a coil of wire carrying a current of electricity becomes an electromagnet. If such a coil and iron core be made small enough they can be attached to a cork and the cork, floating on a solution, will allow the magnet to point north and south. The sketch shows how to make such an instrument. A coil of insulated wire is wrapped around a small iron core, leaving a few inches of each end free for connections. The insulation is removed

[Ill.u.s.tration: Floating Electromagnet]

from these ends and they are run through a piece of cork. Attach to the wires, on the under side of the cork, a piece of zinc to one end and a piece of copper to the other. The cork is then floated on a solution of acid, with the zinc and copper hanging in the solution. If zinc and copper are used, the solution is made from water and blue vitriol. If zinc and carbon are used, the solution is made from sal ammoniac and water.

The float will move about on the solution until the magnet iron will point north and south. If two of them are floating on the same solution, they will move about and finally arrange themselves end to end with the coils and magnet cores pointing north and south.

--Contributed by C. Lloyd Enos.

** A Fish Bait [152]

A very effective fish bait is made by inclosing a live minnow in a short section of gla.s.s tube, which is filled with water and both ends closed with corks. This is used in place of the spoon.

** Homemade Air Thermometer [152]

The ill.u.s.tration shows the complete thermometer. The water in the gla.s.s tube is caused to rise and fall by the expansion and contraction of the air in the tin box. A paper-fastener box, about 1-1/4 in.

[Ill.u.s.tration: Air Thermometer]

deep and 2 in. in diameter will serve very well for the box A.

Solder in the side of the box 1-in. piece of 1/4-in. bra.s.s tubing, B, and then solder on the cover, C, so that the only escape for the air is through the bra.s.s tube. Secure a piece of 1/4-in. gla.s.s tubing - not shorter than 18 in.--and bend it as shown at D in the sketch. Hold the part of the tube to be bent in the broad side of a gas jet, and in a minute or two the tube will bend with its own weight. Any angle can be given gla.s.s tubing in this way. Connect the gla.s.s tube to B with a short piece of rubber hose, E. If the hose is not a tight fit, bind with a short piece of fine copper wire. The standard, F, is made from a piece of No. 10 wire about 10 in. long. To this standard solder the supporting wire, G--No.

14 wire will do. On one side bend the wire around the tube B, and on the other around the gla.s.s tube, D.