Types of Ammeters and Voltmeters

In order to produce the necessary deflecting torque for the opera­tion of ammeters and voltmeters the various effects of electric current and of potential — the heating effect, electrostatic effect, magnetic effect, and electro-magnetic induction effect — are used leading to a num­ber of different types of instrument. The resulting instruments are -called: (a) hot wire, (b) electrostatic, (c) moving iron, (d) moving coil, and (e) induction. They are dealt with-in some detail in succeeding pages where their advantages, disadvantages, and specialized charac­teristics are given. By way of introduction ( В якості передмови) , however, it may be well to point out that the various types are usable on both d. c. and a. c. circuits with two exceptions, namely, the permanent-magnet, moving-coil instrument which can be used only with d. c, and the induction instru­ments which are limited to a. c. operation. For general purposes the moving-iron instruments are by far the most commonly used, while for d. c. work the permanent-magnet, moving-coil instrument is the best, being specially suitable for use with shunts and multipliers for multi-range purposes. The other types are uncommon for general work, al­though each has advantages under certain conditions. Thus the order in which the instruments are described below is not that of relative importance, but merely such as to fit in with the general arrangement of the presentation.

Unit 22 Wattmeters

Instruments designed to measure the amount of power passing in a circuit are known as wattmeters.

In the case of D.С circuits the power taken is the pro­duct of the voltage and the current flowing. Ammeter readings become almost a sufficient indication of the power taken and the installation of wattmeters an unnecessary expense.

Due to a factor which must be taken into account, namely, the power factor or phase relationship between the voltage and current, the power taken by even the simplest two-wire A. C. circuit is not so easily estimated. In some A.C. circuits, then, the installation of a wattmeter is necessary if the po­wer is to be measured.

Common forms of wattmeter, as installed on switchboards or as portable instruments, are of the direct-indicating type with pivoted moving elements. Other forms used as standard for calibration and classed as laboratory equipment have a torsion head suspension and are "indirect" reading.

Since their modes of operation are basically the same as certain types of ammeter and voltmeter, it is only to be expect­ed that their individual mechanical and electrical component parts are identical with or bear a close resemblance to those already considered. They will, in general, be restricted by the same errors and disadvantages, and will gain by the same advantages.

Exercises

I. Read fluently:

the amount of power, the product of the voltage and current, a sufficient indication of the power taken, must be taken into account, by even the simplest circuit, installed on switch-boards, the instal­lation of wattmeter, direct-indicating type, their modes of operation.

II. Add suffixes and prefixes to the following word and trans­late the words into Russian:

advantage, sufficient, to estimate, to install, power, use, to ex­pend.

III. Make nouns from the following verbs and translate them:

to produce, to relate, to estimate, to suspend, to resemble, to expend.

IV. Give the different meanings of:

reading, product.

V. Memorize the following expressions and compose sentences with them:

to take into account, to bear a resemblance to, due to.

VI. Answer the following questions:

1. What is the power taken in a D.С circuit? 2. What is the indi­cation of the power taken?

3. Is it easily estimated? 4. What types of wattmeters are used? 5. Are their modes of operation different from those of ammeters and voltmeters?

VII. Retell the text.

VII. Supplementary reading.