Chapter VI

CURRENT-DETECTORS AND GALVANOMETERS

A current-detector is a necessary part of the amateur electrician’s equipment; technically, this piece of apparatus is called a galvanoscope.

When a wire or a number of them are brought near a magnetic needle or a small compass, the needle will be deflected from its north and south line and will point east and west, or west and east, according to the direction in which the current is passing through the wires. All wires that are conducting electricity have a magnetic field, and when brought near the magnetized needle of a compass they have the power to act on it the same as another and stronger magnet would.

The action of detectors depends upon two things--first, the magnetized needle that, when properly balanced, will point north and south; and, secondly, a current of electricity passing through a wire or wires held above the needle, or both above and below it. This is more clearly shown in Fig. 1, where a compass is resting on a wire connected to a battery. The wire also passes over the top of the compass, which doubles the electro-magnetic field.

When the compass (with the needle pointing north) is resting on the wire that is attached to the zinc pole of a battery, and when the end of the wire that passes back over the top of the compass is attached to the carbon pole, the needle will fly around and point to the east. When the wires are reversed, the needle will point to the west. Thus the combination of a battery or other source of electric current, a magnetic needle, and a coil of wire properly arranged, make an instrument that will detect electric currents and may be correctly called a current-detector. The pressure of more or less current is determined by instruments wound with wire of different sizes; the finer the wire the more sensitive the instrument, and consequently the more delicate. A very weak current can only be detected with a delicate and sensitive instrument. The coarser the wire and the larger the instrument, the better it will be for testing strong currents that would perhaps burn out the fine wire of the more delicate apparatus.

This instrument, when placed between a source of electricity and a piece of apparatus, such as a bell, a motor, or lamp, does not weaken the current, for it requires no waste of electricity to operate the magnetic needle. Consequently, when a very weak current is being used for any tests, it is well to place a detector between the battery and the apparatus to show that the current is actually passing through the wire.

A simple detector is made by winding fifteen or twenty feet of cotton-insulated copper wire No. 26 or 28 around the lower end of a drinking-glass. Leave six inches of each end loose; then after slipping the coil from the glass, tie the wires with thread at least four times around the circle, so as to bind the wires together. Press two sides of the hoop together so as to flatten it; then with paraffine attach the coil to a square block of wood, as shown in Fig. 2.

From a thin clock-spring, not more than three-eighths of an inch wide, cut a piece two inches and a half long, and with a stout pair of tin-shears cut the ends so as to point them, as shown in Fig. 3 A. With two pair of pliers bend a hump at the middle of the strip on the dotted lines shown in A, so that a side-view will appear like B in Fig. 3. Turn this strip over on a hard-wood block or a piece of lead, and with a stout steel-wire nail and a hammer dent the inverted [V] at the middle so that it will rest on the top of a needle-point without falling off.

With three little pieces of wood make a bridge and attach it to the wooden base over the paraffine that holds the wire-coil, and drive a needle down in the middle of it, taking care that it does not go through the back and touch the wires underneath. On this needle hang the strip of steel spring, and, if it does not properly balance, trim it with the shears or a hard file until it is adjusted properly. Rub this piece of steel over the pole ends of a large horseshoe magnet, or place it within the helix of a large coil and turn a powerful current through the coil. This will magnetize the strip of steel, which will then become a magnetic needle and hold the magnetism. Attach two binding-posts to corners of the block, and make the loose ends of the coil-wires fast to them. You now have a current-detector, or galvanoscope, as shown in Fig. 4. Turn the block so that the needle points to north and south and parallel to the strands of wire.

When the conductors from the poles of a battery or dynamo are made fast to the binding-posts, the needle will fly around to a position at right angles to that which it first occupied, as shown by the dotted line A A in Fig. 4. When the connection is broken the needle will turn around again and point to north and south, since the magnetic field about the wire ceases and disappears the instant the circuit is broken.

This is one of the strange and unknown phenomena of electricity, for while the current exerts a force that deflects the needle, it does not destroy its magnetism. On the breaking of the contact, no matter how long it may have held the needle away from its true course, it again points to north, and its magnetism is not affected.

Another simple current-detector is shown in Fig. 5. A piece of broomstick is sawed in half and both pieces are made fast to a block which is mounted on a base of wood three-quarters of an inch in thickness. The vertical block should measure five inches long, three inches high, and five-eighths of an inch thick. The half-circular pieces of wood are mounted so that the flat surfaces are three inches apart and the lower edges are one inch above the base-block. These may be held in place with glue and screws driven through the back of the vertical block and into the ends of the projecting half-circular pieces. The base-block is six inches long and four inches wide, and the vertical block is mounted on it one inch from an edge. The pieces of broomstick are two inches long, and at the front ends a thin bar of brass or copper is screwed fast to hold them apart and in proper position, as shown at A in Fig. 5. To improve the appearance of this mounting, all the wood-work may be stained and shellacked before the metal parts are attached.

With No. 26, 28, or 30 cotton-insulated wire make from fifteen to twenty wraps about the middle of the half-circular pieces of wood and carry the ends down through small holes in the base-block and thence through grooves cut at the under side of the block to the front corners, where they are to be made fast to binding-posts. A needle is to be set in the base-block midway between the two pieces of half-circular wood and through the strands of wire. Great care must be taken that the needle does not touch any bare wires, and as a precautionary measure it would be well to wrap the needle with a piece of insulating-tape where it passes through the strands of wire. Now place on the top of it a magnetized piece of steel, as described for the detector shown in Fig. 4. As it may not always be convenient to turn the instrument so that the needle points north, a small bar of magnetized steel or a stout needle that has been magnetized with a horseshoe magnet or a helix, may be laid across the half-circular wood pieces, so that it is parallel with the top layer of wires--in fact, it should rest on top of them.

By means of this needle, or bar, the magnetic piece of steel balanced on the vertical needle between the upper and lower strands of insulated wire may be held in one position no matter which way the block is turned. When the current passes in through one binding-post and out through the other (having thus travelled through the coil on the half-circular blocks) the needle is deflected and points out at the brass bar and back at the upright block.

When making any of these pieces of apparatus, where delicately balanced magnetic needles are employed, all parts of the mounting blocks or other sections must be put together with glue and brass nails or screws. It will not do to use steel or iron nails, screw-eyes, or washers, nor pieces of sheet-iron, tin, or steel, for they will exert their influence on the vital parts of the apparatus and so destroy their usefulness. This is not so important when making buzzers, bells, motor-induction coils, or similar things, but in delicate instruments, where magnetic needles or electro-magnets are used for recording, measuring, or detecting, iron and steel parts should be carefully avoided, except where their use is expressly indicated.

An Astatic Current-detector

Astatic current-detectors and galvanometers are those having two magnetic needles arranged with the poles in opposed directions.

The ordinary magnetic or compass needle points to the North, and in order to deflect it a strong magnetic field must be created near it. For strong electric currents the ordinary single-needle current-detector meets all requirements, but for weak currents it will be necessary to arrange a pair of needles, one above the other, with their poles in opposite directions, and placed within or near one or two coils of fine wire. This apparatus will be affected by the weakest of currents, and will indicate their presence unerringly.

The word “astatic” means having no magnetic directive tendency. If the needles of this astatic pair are separated and pivoted each will point to North and South, after the ordinary fashion. For all astatic instruments we must employ two magnetic needles in parallel, either side by side or one above another, as shown in Fig. 6, with the N and S poles arranged as indicated. This combination is usually called Nobili’s pair. If both needles are of equal length and magnetic strength, they will be astatic, for the power of one counterbalances that of the other. As a consequent neither points to North.

A compound needle of this form requires but a very feeble current to turn it one way or the other, and this is the theory upon which all astatic instruments are constructed.

A simple astatic current-detector may be made from a single coil of fine insulated wire, a pair of magnetic needles, and a support from which to suspend them, together with a base-block.

For the base-block obtain a piece of white-wood, pine, or cypress, four inches square and three-quarters of an inch thick. Sand-paper it smooth, and then give it two or three coats of shellac. From a strip of copper or brass (do not use tin or iron) make a bridge, in the form of an inverted [V], seven inches high, using metal one-sixteenth of an inch thick and half an inch wide. This bridge is to be screwed to the outside of the block, as shown at Fig. 7, so that it will be rigid and firm. A small hole is drilled through the top of the bridge to admit a screw-eye for the tension.

Make a coil of No. 30 insulated wire, using ten or fifteen feet, and wind it about the base of a drinking-glass to shape it; then remove it and tie the coil, in several places, with cotton or silk thread, so as to hold the strands together. Shape it in the form of an ellipse and make it fast to the middle of the base-board with small brass or copper straps, and copper tacks or brass screws. Be very careful not to use iron, steel, or tin about this instrument, as the presence of these metals would deflect the needles and make them useless.

Separate the strands at the top of the coil so that one of the needles may be slipped through to occupy a position in the middle of the coil. Ordinary large compass needles may be employed for this apparatus, or magnetized pieces of highly tempered steel piano-wire will answer just as well.

A short piece of brass, copper, or wood will act as the carrier-bar for the needles. These should be pushed through holes made in the bar, and held in place with a drop of shellac or melted paraffine. A small hole is drilled at the top of the bar, or a small eye can be attached, through which to pass the end of a thread. The upper end of the thread is tied in a screw-eye, the screw part of which passes up through the hole in the bridge and into a wooden button or knob, which can be turned to shorten or lengthen the thread, and so raise or lower the needles. The lower needle must be pivoted at an equal distance between the upper and lower parts of the coil.

Two binding-posts are arranged at the corners of the base, and the ends of the coil wires are attached under the screw-heads. The in-and-out wires are to be made fast under the copper washers on the screw-eyes.

Owing to the astatic qualities of the needles, the base-block does not have to be turned so that the coil may face North and South, as in the current-detector. When the slightest current of electricity passes through the coil it instantly affects the needles, turning them to the right or left according to the way in which the current is running through the coil.

An Astatic Galvanometer

The sensitiveness of an astatic detector may be increased by the added strength of the coil-field for a given current.

There are two ways of accomplishing this result. The number of turns of wire may be increased in the coil, or two coils may be used, placed side by side. The latter method is the more satisfactory, since then the coil does not have to be opened at the top to admit the lower needle, the latter being dropped down between the coils. This apparatus is shown in the illustration of an astatic galvanometer, Fig. 8. The general arrangement of needles, bridge, and coils, is the same as described for the astatic current-detector.

Each coil is made separately of ten feet of No. 30 insulated copper wire, wound about the base of a drinking-glass to shape it; then pressed into elliptical shape, and fastened to a base-block with a brass or copper strip, and held down with small brass screws.

The base-block should be four inches square, with the corners sawed off. Smooth the block with sand-paper, and then give it several good coats of shellac.

The bridge is made from brass one-sixteenth of an inch thick and half an inch wide. The coils of wire are arranged about half an inch apart, and at both ends a small separator-block is placed between the coils, and then bound with silk or cotton thread. A circular indicator disk of bristol-board should be cut out and marked and attached to the top of the coils with a few drops of sealing-wax or paraffine; then the needles are suspended so as to hang properly, one above the card, the other between the coils.

Three binding-posts are placed at one end of the block, and to them the end wires of the coils are led and attached. To the first binding-post (at the left) the strand of wire leading to the first coil is attached. It leads in and is coiled as the hands move on a clock, from left to right. The leading-out wire from the coil is made fast to the middle post. The leading-in wire to the second coil is also made fast to the middle post. The coil wires should have the turns in the same direction as the first coil; then the last wire is attached to the right-hand post.

When making connections for a strong current, use an end and middle post. This arrangement will operate but one coil. For very weak currents make the leading in and out wires fast to the end-posts. This latter plan is more clearly shown in the diagram, Fig. 9. A and B represent the coils, C, D, and E the binding-posts. The current, entering at C, passes through the coil A (as the hands move about the dial of a clock) and out at D, where connection is made with the wire leading in to coil B. The current passes through this coil in the same direction as the clock hands move, and out to post E. Be careful to arrange the wiring and connections after this exact manner, otherwise the instrument will not be of any use.

The adjustment at the top of the bridge may be made with an inverted screw-eye and a small cork into which the eye can be screwed, thus raising or lowering the needles to the proper position. Be sure to have the needles in parallel when at rest.

As the needles and coils are very sensitive it would be well to cover the instrument with an inverted glass jar. A bluestone or gravity battery jar will answer very well, and after the wires are connected to the binding-posts the glass may be placed over the entire apparatus.

A Tangent Galvanometer

For testing the various degrees of intensity of a current a tangent galvanometer is usually employed. In this apparatus the increased strength is indicated by the index-pointer as it plays over a scale or graduated circle.

A simple tangent galvanometer may be made from a flat hoop of wood-fibre or brass, mounted on a base by means of two uprights, together with the necessary compass needle, an index-card, insulated wire, and binding-posts for the electrical connections. This piece of apparatus is shown in Fig. 10. It is built on a base-block six by seven inches and three-quarters of an inch thick. The block should be of selected wood, and after it is made smooth it should be given several coats of shellac.

Two upright pieces of wood, five inches long, half an inch thick, and one inch in width, are screwed fast to the rear edges of the base-block to support the hoop on which the insulated wire is wound. Be careful not to use any iron or steel in the construction of this or any other recording instrument, except where it is expressly stated. Screws, nails, staples, or any bits of anchoring wire should be of copper or brass. String, thread, or silk may be used, especially where coils of wire are to be bound or fastened to hoops or base-blocks. The balance of the indicating needle is so delicate, and the sensitiveness of the coils is so easily affected, that nothing about or near the instruments should be of iron or steel.

The hoop may be made of very thin hickory wood, steamed and bent so as to form a ring six inches outside diameter and one inch wide. It is even possible to construct a satisfactory hoop from a ribbon of brown paper, rolled and lapped, the several thicknesses being glued as the turns are made.

If a metal hoop is to be used, solder the ends of a thin, hard ribbon of brass, copper, or zinc. This strip should be provided with holes, set in pairs about four inches apart, all around the hoop, and where the hoop is to be attached to the uprights two holes should be made close to the margins through which brass screws may pass.

Across the middle of the hoop a strip of wood six inches long, an inch wide, and a quarter of an inch thick is made fast. On this the graduated card is placed, and at the centre the balanced magnetic needle is arranged on a pivot.

After the cross-stick is in place, wind five turns of No. 24 insulated copper wire about the hoop, keeping it as nearly in the centre as possible. One end of the wire (the beginning) is to be attached to the first binding-post on the front of the base, and the other end to the second post. The wire should be wound round the hoop in the same direction as the clock hands travel about a dial.

Another coil, composed of ten turns of wire, is made over the first one, the beginning end being attached to the middle binding-post and the last end to the third post. This arrangement is shown in Fig. 11, D and E representing the coils, while A, B, and C are the binding-posts. The current enters at A, passes through coil D, and out at post B. The next passage is in at B, through E, and out at C. A current passing in at A will travel to B, thence through E, and out at C. If the leading-in wire is made fast to A, and the out wire to C, the current will travel through the entire coil.

Under this plan one or both coils may be used (the short or long one as desired) by making connections with the first and second binding-posts, the second and third, or the first and third, as the strength of the current will warrant.

Strong currents will deflect the needle when travelling through a short coil, but the weaker the current the more coils it will have to pass through to properly deflect the needle and indicating pointer.

When the coils are all on, the hoop should be attached to the uprights with small brass screws driven through holes in the hoop and into the wood. The wire is bound to the hoop by means of threads or silk passed through each pair of holes in the hoop, and then tied fast. Fine insulated wire may be used in place of the thread, but care should be taken that the insulation is in perfect shape on both the binding and coil wires; otherwise a short-circuit will quickly destroy the value of the coils.

The hoop should not touch the base-block, but should clear it by a quarter or half an inch. Make the coil ends fast (as described for the astatic galvanometer and illustrated at Fig. 9) by means of binding-posts. The wires need not be carried over the top of the block, but may run through holes under the hoop and along grooves cut in the under side of the block and leading to the foot of the binding-posts.

The graduated card should be made from a piece of stout bristol-board or heavy card-board having a smooth, hard surface. It is laid out with a pencil or pen compass, as shown at Fig. 12, and should be three inches in diameter. The card is placed on the wood strip or ledge, so that the zero marks will be at the front and rear, or at right angles to the hoop and coils of wire. The compass needle, when at rest, should lie parallel with the coils, so that the current will deflect the needle and send the indicator around to one side or the other of zero, according to the direction in which the current is passing through the coils.

This is more clearly shown at Fig. 13. The circle represents the outside diameter of the card; the dark cross-piece, the magnetic needle; and the pointed indicator, a stiff paper, or very thin brass or copper strip, cut and attached to the needle with shellac or paraffine.

When at rest the magnetic needle should be parallel to the coils. To insure this the instrument must be moved so that the lines of wire forming the coil will run North and South. Otherwise the N-seeking end of the magnetic shaft will point to North, irrespective of the position occupied by the wire coil.

The magnetic needle may be made as described for the compass (see