Chapter I

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Mount two binding-posts to the corners of the block, and connect the ends of the wire coil to them. Turn the block so that the needle points North and South and parallel to the coil of wire.

If a battery is connected to the binding-posts, the needle will fly around to a position at right angles to that which it first occupied.

An astatic galvanoscope is one having two needles with their poles in opposite directions. The word "astatic" means having no directive magnetic tendency. If the needles of an astatic pair are separated and pivoted separately, they will each point to North and South in the ordinary manner. But when connected together with the poles arranged in opposite directions they neutralize each other.

An astatic needle requires but very little current in order to turn it either one way or the other, and for this reason an astatic galvanoscope is usually very sensitive.

A simple instrument of this sort may be made by winding about fifty turns of No. 30-36 B. & S. gauge single-silk or cotton-insulated wire into a coil around a glass tumbler. After removing the coil from the glass, shape it into the form of an ellipse and fasten it to a small base-board.

Separate the strands of wire at the top of the coil so that they are divided into two groups.

[Illustration: Fig. 112.—Astatic Galvanoscope.]

Make a bridge or standard in the shape of an inverted U out of thin wooden strips and fasten it to the block.

The needles are ordinary sewing-needles which have been magnetized and shoved through a small carrier-bar, made from a strip of cardboard, with their poles opposite one another, as shown in the illustration.

[Illustration: Fig. 113.—Astatic Needles.]

They may be held in place in the cardboard strip by a small drop of sealing-wax.

A small hole is punched in the top of the carrier, through which to pass the end of a thread. The upper end of the thread passes through a hole in the bridge and is tied to a small screw-eye in the center of the upper side of the bridge.

The carrier-bar is passed through the space where the coil is split at the top. The lower needle should hang in the center of the coil. The upper needle should be above and outside the coil.

The terminals of the coil are connected to two binding-posts mounted on the base-block.

Owing to the fact that this galvanoscope is fitted with an astatic needle, the instrument does not have to be turned so that the coil may face North and South. The slightest current of electricity passing into the coil will instantly affect the needles.

An astatic galvanometer for the detection of exceedingly weak currents and for use in connection with a "Wheatstone bridge" for measuring resistance, as described farther on, will form a valuable addition to the laboratory of the boy electrician.

Make two small bobbins similar to those already described in connection with the volt and ammeter, but twice as long, as shown in Figure 114.

Wind each of the bobbins in the same direction with No. 36 silk-covered or cotton-covered wire, leaving about six inches free at the ends for connection to the binding-posts.

Fasten each of the bobbins to the base-board with glue. Do not nail or screw them in position, because the presence of nails or screws may impair the sensitiveness of the instrument. In mounting the bobbins, leave about one-sixteenth of an inch of space between the inside flanges, through which the needle may pass.

Connect the coils wound on the bobbins so that the end of the outside layer of the first coil is connected to the inside layer of the other coil. This arrangement is so that the current will travel through the windings in the same continuous direction, exactly the same as though the bobbin were one continuous spool.

[Illustration: Fig. 114.—Bobbin for Astatic Galvanometer.]

Magnetize two small sewing-needles and mount them in a paper stirrup made from good, strong paper, as shown in Figure 114. Take care that the poles are reversed so that the north pole of one magnet will be on the same side of the stirrup as the south pole of the other. They may be fastened securely by a drop of shellac or melted sealing-wax.

Cut out a cardboard disk and divide it into degrees as in Figure 115. Glue the disk to the top of the bobbins. A small slot should be cut in the disk so that it will pass the lower needle.

A wooden post should be glued to the back of the base. To the top of this post is fastened an arm from which are suspended the magnetic needles.

A fine fiber for suspending the needle may be secured by unraveling a piece of embroidery silk.

[Illustration: Fig. 115.—Completed Astatic Galvanometer.]

The upper end of the fiber is tied to a small hook in the end of the arm. The wire hook may be twisted so that the needles may be brought to zero on the scale. Zero should lie on a line parallel to the two coils.

The fiber used for suspending the needles should be as fine as possible. The finer the fiber is, the more sensitive will the instrument be.

The lower needle should swing inside of the two coils, and the upper needle above the disk.

How to Make a Wheatstone Bridge

The amateur experimenter will find many occasions when it is desirable to know the resistance of some of his electrical apparatus. Telephone receivers, telegraph relays, etc., are all graded according to their resistance in ohms. The measurement of resistance in any electrical instrument or circuit is usually accomplished by comparing its resistance with that of some known circuit, such as a coil of wire which has been previously tested.

The simplest method of measuring resistance is by means of a device known as the Wheatstone bridge. This instrument is very simple but at the same time is remarkably sensitive if properly made. A Wheatstone bridge is shown in Figure 116.

The base is a piece of well-seasoned hard wood, thirty inches long, six inches wide, and three-quarters of an inch thick.

Secure a long strip of No. 18 B. & S. gauge sheet-copper, one inch wide, and cut it into three pieces, making two of the pieces three inches long, and the other piece twenty-three and one-half inches long.

Mount the copper strips on the base, as shown, being very careful to make the distance between the inside edges of the end-pieces just twenty-five inches. The strips should be fastened to the base with small round-headed brass screws. Mount two binding-posts on each of the short strips in the positions shown in the illustration, and three on the long strip. These binding-posts should pass through the base and make firm contact with the strips.

[Illustration: Fig. 116.—Wheatstone Bridge.]

Then make a paper scale twenty-five inches long, and divide it into one hundred equal divisions one-quarter of an inch long. Mark every fifth division with a slightly longer line, and every tenth division with a double-length line.

Start at one end and number every ten divisions, then start at the other end and number them back, so that the scale reads 0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, from right to left at the top and 0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, from left to right at the bottom.

Solder a piece of No. 30 B. & S. gauge German-silver wire to one of the short copper strips opposite the end of the scale, and then stretch it tightly across the scale and solder it to the strip at the other end.

Make a knife-contact by flattening a piece of heavy copper wire as shown in Figure 117. Solder a piece of flexible wire, such as "lamp cord," at the other end. It is well to fit the contact with a small wooden handle, made by boring out a piece of dowel.

The instrument is now practically complete.

[Illustration: Fig. 117.—Knife-Contact.]

In order to use the Wheatstone bridge, it is necessary to have a set of resistances of known value. The resistance of any unknown circuit or piece of apparatus is found by comparing it with one of the known coils. It is just like going to a store and buying a pound of sugar. The grocer weighs out the sugar by balancing it on the scales with an iron weight of known value, and taking it for granted that the weight is correct, we would say that we have one, five, or ten pounds of sugar, as the case may be.

The Wheatstone bridge might be called a pair of "electrical scales" for weighing resistance by comparing an unknown coil with one which we know has a certain value.

The next step is to make up some standard resistance coils. Secure some No. 32 B. & S. gauge single-cotton-covered wire from an electrical dealer and cut into the following lengths, laying it straight on the floor but using care not to pull or stretch it.

1/2 ohm coil—3 feet 1/2 inch 1 ohm coil—6 feet 1 1/4 inches 2 ohm coil—12 feet 2 1/2 inches 5 ohm coil—30 feet 6 1/4 inches 10 ohm coil—61 feet 20 ohm coil—122 feet 30 ohm coil—183 feet 50 ohm coil—305 feet

These lengths of wire are then wrapped on the spools in the following manner.

[Illustration: Fig. 118.—Resistance-Coil. _A_ shows how the Wire is doubled and wound on the Spool. _B_ is the completed Coil.]

This method of winding is known as the non-inductive method, because the windings do not generate a magnetic field, which might affect the galvanometer needle used in connection with the Wheatstone bridge as described later on.

Each length of wire should be doubled exactly in the middle, then wrapped on the spools like a single wire, the two ends being left free for soldering to the terminals as shown in Figure 118, B.

The spools may be the ordinary reels upon which cotton and sewing-silk are wrapped.

The terminals of the spools are pieces of stout copper wire, No. 12 or No. 14 B. & S. gauge. Two pieces of wire about three inches long are driven into holes bored in the ends of each spool. A small drop of solder is used permanently to secure the ends of the coil to each of the heavy wire terminals.

The spools are then dipped into a pan of molten paraffin and boiled until the air bubbles cease to rise.

The spools should be marked 1, 2, 10, 20, 30, and 50, according to the amount of wire each one contains as indicated in the table above.

How to Use a Wheatstone Bridge for Measuring Resistance

The instrument is connected as in Figure 116.

The unknown resistance or device to be measured is connected across the gap at _B_. One of the standard known coils is connected across the gap at _A_. A sensitive galvanometer or a telephone receiver and two cells of battery are also connected as shown.

If a telephone receiver is used, place it to the ear. If a galvanometer is used instead, watch the needle carefully. Then move the sharp edge of the knife-contact over the scale along the German-silver "slide wire" until a point is reached when there is no deflection of the needle or no sound in the telephone receiver.

If this point lies very far on one side or the other of the center division on the scale, substitute the next higher or lower known resistance spool until the point falls as near as possible to the center of the scale.

When this point is found, note the reading on the scale carefully. Now comes the hardest part. Almost all my readers have no doubt progressed far enough in arithmetic to be able to carry on the following simple calculation in proportion which must be made in order to find out the resistance of the unknown coil.

The unknown resistance, connected to _B_, bears the same ratio to the known coil, at _A_, that the number of divisions between the knife-contact and the right-hand end of the scale (lower row of figures) bears to the number of divisions between the knife-edge and the left-hand end of the scale (upper row of figures).

We will suppose that a 5-ohm coil was used at _A_ in a test, and the needle of the galvanometer stopped swinging when the knife-contact rested on the 60th division from the left-hand end, or on the 40th from the right. Then, in order to find the value of the unknown resistance at _B_, it is simply necessary to multiply the standard resistance at _A_ by the number of left-hand divisions and divide the product by the number of right-hand divisions. The answer will be the resistance of _B_ in ohms.

The calculation in this case would be as follows:

5 X 40 = 200

200/60 = 3.33 ohms

3.33 ohms is the resistance of _B_.

This explanation may seem very long and complex, but if you will study it carefully you will find it to be very simple. When once you master it, you will be enabled to make many measurements of resistance which will add greatly to the interest and value of your experiments.

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