Chapter 5

HOW TO DETECT AND MEASURE ELECTRICITY

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.

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).

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.

Now make a bridge (E, Fig. 38) of a strip of brass 1/2 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).

The magnet is made of a bar of steel (K, Fig. 39) 1-1/2 inches long, 3/8 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.

HOW TO PLACE THE DETECTOR.--If the detector is placed north and south, as shown by the two markings, N and S (Fig. 37), the magnet bar will point north and south, being affected by the earth's magnetism; but when a current of electricity flows through the coil (B), the magnet will be deflected to the right or to the left, so that the pointer (O) will then show the direction in which the current is flowing through the wire (R) which you are testing.

The next step of importance is to _measure_ the current, that is, to determine its strength or intensity, as well as the flow or quantity.

DIFFERENT WAYS OF MEASURING A CURRENT.--There are several ways to measure the properties of a current, which may be defined as follows:

1. THE SULPHURIC ACID VOLTAMETER.--By means of an electrolytic action, whereby the current decomposes an acidulated solution--that is, water which has in it a small amount of sulphuric acid--and then measuring the gas generated by the current.

2. THE COPPER VOLTAMETER.--By electro-chemical means, in which the current passes through plates immersed in a solution of copper sulphate.

3. THE GALVANOSCOPE.--By having a coil of insulated wire, with a magnet suspended so as to turn freely within the coil, forming what is called a galvanoscope.

4. ELECTRO-MAGNETIC METHOD.--By using a pair of magnets and sending a current through the coils, and then measuring the pull on the armature.

5. THE POWER OR SPEED METHOD.--By using an electric fan, and noting the revolutions produced by the current.

6. THE CALORIMETER.--By using a coil of bare wire, immersed in paraffine oil, and then measuring the temperature by means of a thermometer.

7. THE LIGHT METHOD.--Lastly, by means of an electric light, which shows, by its brightness, a greater or less current.

THE PREFERRED METHODS.--It has been found that the first and second methods are the only ones which will accurately register current strength, and these methods have this advantage--that the chemical effect produced is not dependent upon the size or shape of the apparatus or the plates used.

HOW TO MAKE A SULPHURIC ACID VOLTAMETER.--In Fig. 41 is shown a simple form of sulphuric acid voltameter, to illustrate the first method. A is a jar, tightly closed by a cover (B). Within is a pair of platinum plates (C, C), each having a wire (D) through the cover. The cover has a vertical glass tube (E) through it, which extends down to the bottom of the jar, the electrolyte therein being a weak solution of sulphuric acid. When a current passes through the wires (D), the solution is partially decomposed--that is, converted into gas, which passes up into the vacant space (F) above the liquid, and, as it cannot escape, it presses the liquid downwardly, and causes the latter to flow upwardly into the tube (E). It is then an easy matter, after the current is on for a certain time, to determine its strength by the height of the liquid in the tube.

HOW TO MAKE A COPPER VOLTAMETER.--The second, or copper voltameter, is shown in Fig. 42. The glass jar (A) contains a solution of copper sulphate, known in commerce as blue vitriol. A pair of copper plates (B, B') are placed in this solution, each being provided with a connecting wire (C). When a current passes through the wires (C), one copper plate (B) is eaten away and deposited on the other plate (B'). It is then an easy matter to take out the plates and find out how much in weight B' has gained, or how much B has lost.

In this way, in comparing the strength of, say, two separate currents, one should have each current pass through the voltameter the same length of time as the other, so as to obtain comparative results.

It is not necessary, in the first and second methods, to consider the shapes, the sizes of the plates or the distances between them. In the first method the gas produced, within a given time, will be the same, and in the second method the amount deposited or eaten away will be the same under all conditions.

DISADVANTAGES OF THE GALVANOSCOPE.--With the third method (using the galvanoscope) it is necessary, in order to get a positively correct reading instrument, to follow an absolutely accurate plan in constructing each part, in every detail, and great care must be exercised, particularly in winding. It is necessary also to be very careful in selecting the sizes of wire used and in the number of turns made in the coils.

This is equally true of the fourth method, using the electro-magnet, because the magnetic pull is dependent upon the size of wire from which the coils are made and the number of turns of wire.

OBJECTIONS TO THE CALORIMETER.--The calorimeter, or sixth method, has the same objection. The galvanoscope and electro-magnet do not respond equally to all currents, and this is also true, even to a greater extent, with the calorimeter.