Text 8. Electrical measurements
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 ofsimple 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 measurementscan 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.
Electrical measuring instruments can be divided into three classes: indicating instruments, recording instruments and integrating instruments.
Indicating instruments,such as ammeters, voltmeters and wattmeters, 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 travelling 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 instrumentsor 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 or 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.
appropriate -придатний, властивий
to integrate -інтегрувати
to gear -приводити в рух (механізм)
revolution -обертання; цикл
to damp -послаблювати; приглушувати
to indicate -вказувати; свідчити
torque -момент обертання
1. How can electrical measurements be classified?
2. What instruments constitute the largest of the three classes?
3. What is the work principle of recording instruments?
4. In what way do integrating instruments doffer from the other two groups?
TEXT 9. ELECTRICAL MEASURING INSTRUMENTS
General Electrical Principles.To measure the magnitude of any phenomenon calls for the use of the effects produced by it. For the measurement of a mass, for example, we determine its weight; or we might utilize the force required to give it a certain acceleration. Again, for the measurement of temperature we use the expansion of solids, liquids, or gases, the change in electrical resistance of a wire, or the thermo-electromotive force produced at the junction of two conductors, etc. For the everyday measurement such indicating instruments are most convenient, in which the amount of the quantity to be measured is directly shown by the position of a pointer on a graduated scale or dial.
For the production of such instruments, some effect is employed which enables the phenomenon to be measured to produce a mechanical force tending to move the pointer along its scale; this is resisted by a controlling force which tends to move the index in the opposite direction, towards some zero position. The actual displacement of the index, or deflection, is the resultant of these two forces, and is greater of the deflecting force or of the phenomenon to be measured.
As a matter of fact, one single principle is known to underlie all electro-magnetic instruments, viz. that the current-carrying circuit tends to enclose as large a magnetic field as possible. In the moving needle galvanometers the magnetic needle turns so that more of its lines of force pass through the coil, while in the moving coil instrument the coil sets itself so as to enclose as much of the field of the magnet as possible. In the soft iron instruments the iron moves so as to increase the magnetic flux produced by the coil, and in the dynamometer the moving coil turns so that its magnetic effect increases that of the fixed coil. Should the conducting circuit be made entirely of flexible material in a uniform field, it would become circular in order to enclose the maximum possible area, while, if the current flowed in a liquid conductor — such, for instance, as mercury — it would actually try to reduce its section so as to shorten the path of the lines of force around it. To such an extent is this the case that it is difficult to pass a large current along such a conductor owing to the tendency of the mercury to contract and break the circuit.
to determine -визначати
index -стрілка (приладу)
to underlie -лежати в основі…
viz.(namely) -а саме, тобто
to enclose -оточувати
to reduce -скорочувати
1. What parameters do we use to measure the magnitude of any phenomenon?
2. What is the resultant of mechanical and controlling force?
3. What one single principle is known to underlie all electro-magnetic instruments?