Analog Ammeters and Voltmeters Questions and Answers

Answer: D

A square-law torque against a linear spring gives divisions that widen and then narrow, so the useful part of the scale is the middle and upper portion. Shaping the moving vane improves it but does not make it uniform.

Answer: A

Because the torque follows the square of the instantaneous current, the mean torque follows the mean square and the scale can be marked in root mean square units. It therefore reads correctly on any waveform, which a rectifier instrument does not.

Answer: B

Correct answer: fixed coils in place of the permanent magnet

Both the field and the moving coil carry the current, so the torque is proportional to the product of the two - that is, to the square of the current if they are in series. Reversing the current reverses both fields, so the deflection is unchanged, which is what allows the instrument to read alternating quantities and to measure power.

Answer: C

Calibrating it on direct current, where a potentiometer gives great accuracy, and then using it on alternating current transfers that accuracy across. Its low sensitivity and square-law scale are the offsetting drawbacks.

Answer: D

The rectifier gives the movement a unidirectional current whose average it indicates, and the scale is marked in root mean square values on the assumption of a sinusoid. High sensitivity is what makes the arrangement so common in a multimeter.

Answer: A

Correct answer: 1.11 for a sinusoidal waveform

The rectifier instrument responds to the average of the rectified waveform, but the scale is wanted in rms, so it is marked using the form factor of a sinusoid: \[ \frac{\mathrm{rms}}{\mathrm{mean}} = \frac{0.707}{0.637} = 1.11. \] The calibration is therefore correct only for the waveform it assumes.

Answer: B

Correct answer: high, because the square wave's form factor is smaller

A square wave has a form factor of 1.00, since its rectified mean equals its rms value. The instrument nonetheless multiplies the mean it measures by 1.11, so it reads about 11 per cent above the true rms. A waveform with a form factor above 1.11, such as a triangular wave, would be read low.

Answer: C

Heating follows the square of the current, so the instrument reads true rms whatever the waveform, and it works to very high frequencies because the heater is a simple resistance. Its overload capability is very small.

Answer: D

Correct answer: radio frequency current measurement

A thermocouple instrument measures the heating effect, which depends on the true rms value and is unaffected by frequency up to very high values - the heater has no appreciable inductance or capacitance. That makes it the classical instrument for radio frequency current, where other movements fail entirely.

Answer: A

Because it responds to voltage directly rather than to a current, it draws practically nothing from the circuit and reads true rms. Its low sensitivity confines it to high voltages, where those two properties matter most.