VI. Retell the text. Unit 20 Electrical Measuring Instruments

Electrical measuring instruments depend for their ac­tion on one of the many physical effects of an electric cur­rent or potential, and they are usually classified according to which of these effects is used. In addition, instruments are grouped as to whether they are 1) Indicating, 2) Recording or 3) Integrating.

Indicating instruments are those which indicate the magnitude of a quantity, and generally make use of a dial and a pointer. Ordinary ammeters, voltmeters, and watt­meters fall into this class.

Recording instruments give a continuous record of the quantity being measured, as for instance by an ink on paper record or graph, extending over a selected period of time.

Integrating instruments give the total amount of energy or quantity of electricity over a period of time. The summation they give is the product of time and an electrical quantity, ampere-hour and watt-hour meters being examples. The sum­mation values are generally given by a register, consisting of a set of dials and pointers.

In every indicating instrument there are essentially two systems, one being fixed and the other being capable of rela­tive movement. It is desirable that the movement of this latter system should bear some simple relation to the quantity being measured, that its movement should follow the changes in the quantity in such a way that readings can be obtained in a minimum of time, and that the expenditure of energy within the instrument proper shall be as small as possible.

To fulfill these conditions it is therefore necessary that the action of the instrument should depend upon a simple law, that there should be critical damping to the motion, and that such quantities as mechanical friction, heating, and magnetic hysteresis should be reduced to the lowest possible values. The reduction of mechanical friction is entirely a matter of correct design, and is dependent upon the method of supporting the movement. In general three methods are employed in commercial instruments: (a) pivoting, (b) knife-edge suspension, (c) pillar or thread suspension.

Exercises

I. Find in the text equivalents for the following;

in this manner, according to, in addition to, to make use of, consequently.

II. Make up sentences illustrating the use of the above expressions.

III. Translate the following words and groups of words:

to desire, desirable, as desirable; relation, in relation with; avail­able, to be available; entire, entirely; account, on the account of, to take account of.

IV. Find in the text synonyms for:

generally, to show, amount, example, to take advantage of, to be composed of, to get, to choose, motion, fully, to carry out, to decrease, hence, namely.

V. Mind the following pairs of words:

to read — reading, to point — pointer, to record — recorder, to set — a set, to produce — product.

VI. Find in the text sentences with the Subjunctive Mood and ana­lyze them.

VII. Put questions to the text and answer them.

VIII. Give a general description of different kinds of electrical instruments and compare them.

IX. Supplementary reading.