CHAPTER IX.

ELECTRIC MEASURING INSTRUMENTS.

§ 1. Classification of Measuring Instruments.— The next subject which must engage attention is that of the practical measurement of electric currents. We have already shown how a simple tangent galvanometer can be constructed and calibrated, and it is necessary to supplement this elementary information by some further knowledge of electrical testing instruments. We shall not attempt to make even a mention of all the numerous processes and instruments employed in electrical testing, but confine our attention to a few of the most practically useful appliances and methods. Electrical instruments are classified according to the purpose for which they are to be used and the scientific principle involved in their construction. We have thus one classification as follows:

Electric measuring instruments may be

1. Amperemeters, or Ammeters, for measuring the strength of electric currents.

2. Voltmeters, for measuring electric pressure, potential difference, or electromotive force.

3. Ohm-meters, for measuring electrical resistance.
4. Watt-meters, for measuring electrical power.

5. Coulomb-meters, or Ampere-hour meters, for measuring electric quantity.

6. Foulemeters, or Watt-hour meters, for measuring electric energy in joules or Board of Trade units.

They may also be classified according to the physical principles involved in their construction, as1. Electrodynamic instruments.

2. Electrostatic instruments.

3. Electrochemical

4. Electrothermal

5. Electromagnetic
6. Electro-optical

It will be convenient to consider, in the first place, the methods of making certain general electrical measurements which are fundamental.

§ 2. The Potentiometer. One of the most practically useful instruments in making electrical measurements with continuous currents is the arrangement called

the Potentiometer. The construction of this instrument is as follows:

Let A B B'A' (see Fig. 87) be a long fine bare wire of platinoid or German silver, or some material having a small resistance temperature co-efficient. Let this wire be stretched over a scale 2 metres long, divided into 2000 parts.

The ends of this wire are connected, through an adjustable resistance, with a secondary battery B, generally

of two cells, having a very constant electromotive force. The resistance of the wire A B B'A' should be at least 40 ohms, in order that the current flowing through it under a pressure of 4 volts may be only the fraction of an ampere. The wire must be of such uniform diameter and structure that the fall in potential down the wire per centimetre of length is the same at all parts of the wire. The current through it is then to be adjusted to a standard value in which the fall in potential down the 2000 divisions of length of the wire is exactly 2 volts. This is done by means of a resistance and a Clark standard cell. A Clark standard cell C, supposed to be at a temperature of 15° C., has its positive pole connected, through a sensitive galvanometer G, with that end of the divided wire in connection with the positive pole of the working battery B. The other pole of the cell is connected to a slider S, which makes contact with the wire at any desired point. The slider is first moved to touch the wire at 1434 divisions from the positive end, and a resistance in series with the potentiometer wire and battery (not shown in Fig. 87) is then varied until the galvanometer shows no current through it. When this is the case we know that the fall of potential down the 1434 divisions of the wire, due to the working battery, must be equal to the electromotive force of the Clark cell, which is 1'434 volts at 15° C. If the cell is not at 15° C., then, looking out in the table on p. 99 the electromotive force of the cell at the temperature at which it actually is at the time being, we set the slider to read that corresponding number on the scale. Thus, if the E.M.F. of the cell is 1440 volts at the time, let the slider be set to make contact at 1440 divisions on the scale. This being done, we now know that, the fall in potential or voltage drop down 1434 divisions of the wire being 1434 volts, the voltage drop down 2000 divisions must be two volts, and the voltage drop down I division must be goth of a volt. The potentiometer is then said to be set for use. Suppose, then, that we desire to measure the electromotive

force of any other voltaic cell C'. We substitute this cell for the Clark cell and find the new position S', to which the slider must be moved, so that the galvanometer G indicates no current. Suppose that it is at 1920 divisions on the scale, then we know that the electromotive force of the cell C' is 1920 volts.

Again, suppose that we require to measure a continuous electromotive force or potential difference of 100 volts, or some such value. We connect across the terminals or ends of the circuit, between which exists the potential difference to be measured, a wire of considerable resistance, say of 10,100 ohms. A connection is made to the ends of a fraction of this which may be, for instance, 100 ohms, or one-hundredth of the larger resistance. The wire is contained in a box, usually called a

Volt-box (see Fig. 88). Thus, if there exists a potential difference of 101 volts at the extremities ac of the whole wire of 10,100 ohms resistance, the potential drop down. the 100 ohms a b, in series with the resistance of 10,000 ohms, will be one volt. If we measure, as above described, by means of the potentiometer when set for use, the drop in potential down this one-hundredth of the volt-box resistance, we know that the whole potential difference between the ends of the volt-box wire must be just 10 times as great.

The divided resistance, therefore, enables us to measure on the potentiometer any voltage by measuring a known fraction of it as above described. By a suitable divided resistance any continuous potential difference or electromotive force may be measured on the potentio

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meter. In using the potentiometer two or three standard Clark cells should be carefully checked against each other, to ensure accuracy in setting the potentiometer. The potentiometer is also used to measure continuous electric currents in the following manner. A series of low resistances are provided, made of strips of platinoid or manganin, or some material not changing its resistance much with temperature. To these resistances

terminals are fixed for sending through them large electric currents, and smaller wires, called potential wires, are soldered to the strips at such places that the resistance between the potential wires is o'I ohm, o'01 ohm, or o⚫ooi ohm, as the case may be. These low resistances must be accurately adjusted, and have a cooling surface so great that they do not become sensibly heated when 10, 100 or 1000 amperes are sent through them respectively.

Suppose, then, that a current of 10 amperes is sent through the one-tenth of an ohm, and potential wires are taken from its potential terminals to the potentiometer. It is clear that there will be a fall of potential of one volt down this resistance, and this voltage may be measured on the potentiometer just as if it were the electromotive force of a cell. If, then, an unknown current is sent through the strip, and we find the terminal voltage of the strip to be 951 volt, we know the current is 951 amperes. In this manner, by the use of two or three different low resistance strips, we can make measurements of continuous currents over an enormous range of values, and measure, by means of a Clark cell and a potentiometer, any continuous current or any continuous voltage.

In using the potentiometer in a workshop, it is desirable to employ a galvanometer of the type called a movable-coil galvanometer, because this is not disturbed by other electric currents in its neighbourhood, as is the fixed-coil type of galvanometer. The potentiometer is an exceedingly useful instrument for calibrating other