The measurement of any physical quantity applies a determination of its magnitude in terms of some appropriate unit. It follows, therefore, that before we can measure we must decide upon a system of units which will be convenient for the purpose. In the case of simple fundamental quantities such as length, mass, or time, the units themselves are simple.

Electrical and magnetic quantities are, however, much less simple than length, mass, or time, and cannot be measured directly by comparison with a material standard. The units in which we express these quantities have to be defined in terms of their observable effects obtained in experimental work, e. g. the weight of silver deposited in one second by a current when it is passed through a solution of silver nitrate is a measure of the magnitude of this current.

Electrical measurements can be classified broadly as either absolute measurements, or secondary measurements, but we need concern ourselves very little with the first class because such measurements are rarely undertaken and, generally speaking, are used only for periodic checks upon the accuracy of primary standards. They are of interest only to the specialist, the very large majority of the measurements made in practice being secondary, or comparison, measurements.

Instruments and Meters

Component Parts of Indicating and Recording Instruments Classification. — Electrical measuring instruments can be divided into three classes: indicating instruments, recording instruments and integrating instruments.

Indicating instruments, such as ammeters, voltmeters and wattcmeters, constitute the largest of the three classes. These are fitted with a pointer which moves over a fixed scale and their characteristic is that they give an immediate indication of the value of the current, voltage or other quantity being measured. Such an instrument might therefore be compared with, say, a weighing machine or a barometer, giving an immediate reading of the weight or pressure existing at any instant, but making no permanent record of such a measurement.

Recording instruments, or graphers, as they are sometimes called, instead of being fitted with a pointer and scale, carry a pencil or pen, which presses on to a traveling ribbon of paper, and thus makes a continuous chart or record of the values measured. Such an instrument could be compared with the recording barometer often exhibited in an instrument maker's window. It will be noted that these two types do not differ in principle, since they are both used to measure the same kinds of things; but in the one case the indication is momentary and must be read by an observer on the spot, whilst in the other case the values are recorded on a chart for future observation and reference.

The third group, consisting of integrating instruments or electricity supply meters, differs fundamentally from the other two groups, since instead of indicating or recording, these instruments add up the total amount consumed over any given period. Thus, instead of reading, say, the current or the power flowing at any instant, they measure the product of current and time (in ampere-hours) or of power and time (in watt-hours), and so add up the electrical quantity or energy consumed. An integrating instrument is, therefore, like the gas meter, which registers the quantity of gas consumed. Instead of a pointer and scale with a limited arc of movement, they are usually made to revolve and carry a train of gearing and a register which counts the number of revolutions made. In such instruments, the   rate of revolution being proportional to the current (in an ampere-hour meter) or to the power (in a watt-hour meter), the total number of revolutions is proportional to the ampere-hours or watt-hours respectively.

All indicating instruments have three essential features: an operating force or mechanism, a controlling force or mechanism, and a damping force or mechanism. It must be realized that all measurement is comparison, and just as a length can be measured by putting a foot rule against it, or a weight can be measured by balancing it against another weight, so an electrical effect can only be measured by allowing it to act against some known force or control. Thus, the process of electrical measurement can be said to consist of a "tug of war" between two opposing forces — the operating force, or torque, generated by the electricity which is being measured, and the controlling force, or torque, which opposes it. When the instrument comes to rest the pointer indicates the position of stability reached by these two opposite pulls. Although it does not enter into the actual measurement by influencing this position of stability, damping is essential to bring the moving system of the instrument to rest in a reasonably short time. When an electric current flows, it gives rise to various effects — heating, electrostatic, electro-magnetic and chemical, and any one of these effects can be utilized to furnish the operating force of a measuring instrument.

М.А. Беляева и др. «Сборник технических текстов на англ. языке»