Galvanometers, Ammeters, Voltmeters. How to Make a Galvanometer.

If several turns of wire are wrapped about a compass, and a current of electricity sent through the coil, the compass needle will be deflected. Such an instrument is called a galvanometer and may be used for detecting very feeble currents.

Figure 67 shows a very simple form of galvanometer which may be made by winding about fifty turns of No. 36 B. & S. Gauge single silk covered wire around an ordinary pocket compass. The compass is set on a block of wood and the wire wound around both the compass and the block. The terminals of the wire are connected to two binding posts.

A galvanoscope of this sort may also be used for a polarity indicator to distinguish between positive and negative poles. Connect an ordinary dry cell to the galvanometer and the needle will immediately swing to one side or the other, depending upon which way the current flows. The carbon of the dry cell is positive and the zinc is negative. Note which way the needle swings, whether toward the binding post connected to the positive pole or the one connected to the negative pole. It will swing towards the corresponding pole when any other source of current is connected to it and by knowing whether it points towards positive or negative it is very easy to determine the polarity.

The Construction of Ammeters and Voltmeters.

The experimenter will find a voltmeter and an ammeter to be very useful instruments about the workshop and laboratory. A voltmeter is an instrument designed to measure electro-motive force or electrical pressure. An instrument designed to measure rate of flow of current is called an ammeter.

Ammeters and voltmeters are really, in principle, galvanometers, the scales of which have been calibrated to read in amperes or volts, as the case may be.

The little meters described below are very simple but quite sensitive.

The wooden bobbin which holds the wire is shown in Figure 68. The exact dimensions are best understood from the illustration. The wood can be easily secured from an old cigar box. In laying out the work, scratch the lines on the wood with the point of a darning needle. Pencil lines are too thick to permit of accuracy in small work. The bobbin should be perfectly square and true when finished. Finish by rubbing with fine sandpaper and then give it a coat of shellac. Two bobbins will be required, one for the voltmeter and one for the ammeter. Do not use any nails in putting the bobbins together. Use strong glue only.

The bobbin for the ammeter should be wound full of No. 14 B. & S. Gauge double cotton covered magnet wire. The voltmeter requires much smaller wire. No. 37 B. & S. Gauge single silk covered wire will serve the purpose satisfactorily, but a finer size such as No. 38 or 40, is better. Such fine sizes will probably, however, prove more difficult for the experimenter to obtain. Sufficient wire should be used to fill the bobbin up. The wire should be wound on carefully in smooth even layers.

A small hole should be bored in one of the flanges, through which to pass the end of the wire when starting the first layer. About six inches of wire should be left at both ends of the coils to make connection to the terminals with. The whole winding should be given a coat of shellac when finished.

The armature is a piece of soft steel, one and one-eighth inches long, and one-quarter of an inch thick. A one-sixteenth inch hole is bored, slightly above the centre of the armature, to receive the shaft. The centre of gravity is thus thrown below the centre of mass and the pointer attached to the armature will always return to zero if the instrument is level. The shaft is a piece of one-sixteenth inch bessemer steel rod, three-eighths of an inch long. The ends are filed to a sharp point as shown in the upper pert of Fig. 69. A small hole is bored in the top of the armature almost directly over the shaft as shown in Fig, 69 to receive the lower end of the pointer, which is a piece of No. 16 aluminum wire, three and one-quarter inches long. The other end of the pointer should be flattened slightly by tapping with a hammer and then filed into a shape resembling a spear head.

After all the holes have been bored and before the shaft and the pointer are in place, the armature must be tempered so that it will retain its magnetism. In order to temper the armature, heat it to a bright red and drop it immediately into a basin of strong salt water. It may then be magnetized by rubbing one end against the pole of a strong magnet.

The bearings are formed by two 6-32 brass screws, one-half an inch long and having a small hole in the end to accommodate the end of the shaft. These screws pass through the upper flange of the bobbin from the opposite sides. The holes in the flanges should be slightly smaller than the outside of the screw so that the latter will fit snugly and "take hold" as if the wood were threaded.

Figures 70 and 71 show two different methods of assembling and completing the instruments. In one, the bobbin is mounted on the base and the scale is at the top. In the other, this relation is just turned abound and the bobbin is at the top and the scale at the bottom. In the latter case the pointer must be attached to the bottom of the armature instead of the top.

Figure 73 shows the shapes and dimensions of the wooden parts which compose the case.

A glass front slides in two shallow grooves cut in the wooden sides, one-eighth of an inch from the front. Glue and brass screws should be used in putting the case together. Do not use iron or iron screws.

The two binding posts connected to the terminals of the wire on the bobbin should be mounted on the base. A small, round-headed brass screw, long enough to pass all the way through the base will serve to level the instrument and bring the pointer exactly at zero. If a little brass strip is placed in the slot in the screw head and soldered there so as to form what is known as a "winged screw", the adjustment may be made with the fingers and without the aid of a screw driver.

The scale is formed on a piece of stiff white cardboard directly under the pointer. The scale is supported by gluing it to two small wooden blocks. The various values are marked on the scale with a pen and ink. The glass front should therefore not be put in place until the instrument has been calibrated.

To properly calibrate the meters, they must be compared with a suitable standard.

The zero value on the meters is normally in the centre of the scale. When a current is passed through the bobbins, the armature tends to swing around at right angles. But since the armature is pivoted above its centre of mass, the centre of gravity is displaced when it swings and exerts a pull in opposition to that of the bobbin. The amount of swing will be greater as the current is correspondingly stronger. The pointer will swing either to the right or to the left, depending upon the direction in which the current passes through the coil of wire on the bobbin.

The zero point on the two instruments shown in the accompanying illustration is at the extreme left of the scales. The pointers are bent to the left so that the current may be passed through the meters in only one direction and the scales will have a greater range of values.

In order to calibrate the ammeter, place it in series with a standard ammeter having a calibrated scale which is known to be correct. A set of strong batteries and a rheostat should be included in the circuit. The arrangement is illustrated in Figure 74. The rheostat is adjusted so that various current readings are obtained. The positions of the pointer on the standard meter are carefully noted and corresponding graduations made on the scale of the other meter for each value.

In order to calibrate the voltmeter it must be compared to a standard voltmeter. The voltmeters must be placed in parallel or shunt with each other as shown in Figure 75.

A switch is connected to the battery so that the voltage of a varying number of cells may be passed through the meters. In order to secure a close adjustment of the voltage, a rheostat is placed across the battery and switch so that it shunts the cells which are in circuit. By adjusting both the rheostat and the switch, any voltage within the maximum range of the battery may be secured.

After the meters have been calibrated and assembled, they are ready for service and will form a very useful and valuable part of the experimenter's laboratory.

When using the meters remember that it is always necessary that the ammeter shall be in series and the voltmeter in parallel or shunt with the circuit.