Question

Which symbols in figure represent devices used to measure currents?

A. (a) and (c)
B. (a) only
C. (a) and (b)
D. (c) only

Answer 1

In order to answer this question, we will understand first what is current. Current is rate of change of charges across a particular area and to detect it we have equipment used in electric circuits. These equipment’s have a scale measuring different parameters. These scales have least count which means minimum value of parameter that could be measured. These equipment can be used to control current, voltage and other parameters.

Complete answer:
We will know about each symbol and its uses. First symbol (a) is of Ammeter. Ammeter is a device used to measure magnitude of current in electric circuits. It is connected in series with resistance so that it can measure current. Ammeter contains low resistance so that current could pass through it without interference so the voltage drop in circuit is minimum.Ideal ammeter has $0\Omega$ resistance.

Next is (b) Voltmeter Figure. Voltmeter is a device used to measure magnitude of voltage in electric circuits. It is always connected in parallel with resistance in electrical circuits so that it measures voltage difference across the resistance. Voltmeter contains high resistance so that no current passes through it which may create error in measurement. Ideal Voltmeter has $\infty \Omega$ resistance.

Next is (c) Galvanometer Figure. Galvanometer is a device used to measure low currents in electrical circuits even of few microampere current and the only difference with ammeter is galvanometers measure not only magnitude of current but also direction of current. It measures current with less accuracy and it can only measure direct current whereas ammeter measures both direct and alternating currents.

So the correct option is B.

Note: It should be remembered that both galvanometer and ammeter can measure current but since ammeter measures current magnitude more and also it can measure all currents where galvanometer measures only direct current so only option [A] is correct. Also galvanometer could also be converted into voltmeter and ammeter both.