Measuring Instruments

The production of an electric current would not be of much value unless we had some way by which we might detect and measure it. The pound weight, the foot rule and the quart measure are very simple devices, but without them very little business could be done. There must be a standard of measurement in electricity as well as in dealing with iron or vegetables or fabrics.

As electricity cannot be seen by the human eye, some mechanism must be made which will reveal its movements.

The Detector

It has been shown in the preceding chapter that a current of electricity passing through a wire will cause a current to pass through a parallel wire, if the two wires are placed close together, but not actually in contact with each other. An instrument which reveals this condition is called a galvanometer. It not only detects the presence of a current, but it shows the direction of its flow. We shall now see how this is done.

For example, the wire (A, Fig. 35) is connected up in an electric circuit with a permanent magnet (B) suspended by a fine wire (C), so that the magnet (B) may freely revolve.

Figs. 34-36.
To the right, Compass Magnet, To the left

For convenience, the magnetic field is shown flowing in the direction of the darts, in which the dart (D) represents the current within the magnet (B) flowing toward the north pole, and the darts (E) showing the exterior current flowing toward the south pole. Now, if the wire (A) is brought up close to the magnet (B), and a current passed through A, the magnet (B) will be affected. Fig. 35 shows the normal condition of the magnetized bar (B) parallel with the wire (A) when a current is not passing through the latter.

Direction Of Current

If the current should go through the wire (A) from right to left, as shown in Fig. 34, the magnet (B) would swing in the direction taken by the hands of a clock and assume the position shown in Fig. 34. If, on the other hand, the current in the wire (A) should be reversed or flow from left to right, the magnet (B) would swing counter-clock-wise, and assume the position shown in Fig. 36. The little pointer (G) would, in either case, point in the direction of the flow of the current through the wire (A).

Fig. 37. Indicating Direction of Current

Simple Current Detector

A simple current detector may be made as follows:

Prepare a base 3' × 4' in size and 1 inch thick. At each corner of one end fix a binding post, as at A, A', Fig. 37. Then select 20 feet of No. 28 cotton-insulated wire, and make a coil (B) 2 inches in diameter, leaving the ends free, so they may be affixed to the binding posts (A, A'). Now glue or nail six blocks (C) to the base, each block being 1" × 1" × 2", and lay the coil on these blocks. Then drive an L-shaped nail (D) down into each block, on the inside of the coil, as shown, so as to hold the latter in place.

Fig. 38. The Bridge

Now make a bridge (E, Fig. 38) of a strip of brass ½ inch wide, 1/16 inch thick and long enough to span the coil, and bend the ends down, as at F, so as to form legs. A screw hole (G) is formed in each foot, so it may be screwed to the base.

Midway between the ends this bridge has a transverse slot (H) in one edge, to receive therein the pivot pin of the swinging magnet. In order to hold the pivot pin in place, cut out an H-shaped piece of sheet brass (I), which, when laid on the bridge, has its ends bent around the latter, as shown at J, and the crossbar of the H-shaped piece then will prevent the pivot pin from coming out of the slot (H).

Fig. 39. Details of Detector

The magnet is made of a bar of steel (K, Fig. 39) 1½ inches long, ⅜ inch wide and 1/16 inch thick, a piece of a clock spring being very serviceable for this purpose. The pivot pin is made of an ordinary pin (L), and as it is difficult to solder the steel magnet (K) to the pin, solder only a small disc (M) to the pin (L). Then bore a hole (N) through the middle of the magnet (K), larger in diameter than the pin (L), and, after putting the pin in the hole, pour sealing wax into the hole, and thereby secure the two parts together. Near the upper end of the pin (L) solder the end of a pointer (O), this pointer being at right angles to the armature (K). It is better to have a metal socket for the lower end of the pin. When these parts are put together, as shown in Fig. 37, a removable glass top, or cover, should be provided.

This is shown in Fig. 40, in which a square, wooden frame (P) is used, and a glass (Q) fitted into the frame, the glass being so arranged that when the cover is in position it will be in close proximity to the upper projecting end of the pivot pin (L), and thus prevent the magnet from becoming misplaced.

Fig. 40. Cross Section of Detector