Analog Ammeters and Voltmeters Questions and Answers

Answer: C

The former moving in the strong field of the permanent magnet has currents induced in it whose reaction opposes the motion, which is both effective and free of any moving contact. Air vane damping is used where no such field is available.

Answer: D

Correct answer: its field is too weak and would be distorted by a damping disc

Eddy current damping needs a strong magnetic field for the disc to move in, and the moving iron instrument’s own field is weak. Introducing a permanent magnet for damping would distort that field and upset the reading, so a light vane moving in a sealed chamber is used instead.

Answer: A

A weight on an arm gives a controlling torque proportional to the sine of the deflection, which needs the instrument level and gives a cramped scale at the lower end. Springs work in any position, which is why portable instruments use them.

Answer: B

Spring torque grows in proportion to the deflection, so a deflecting torque proportional to the measured quantity gives equal divisions. A square-law deflecting torque gives the cramped scale of a moving iron instrument.

Answer: C

The torque follows the instantaneous current and the inertia of the moving system averages it, so on an alternating current of zero mean the pointer does not move at all. That is why such an instrument is a direct current instrument only.

Answer: D

Correct answer: proportional to the current

With a permanent magnet supplying a constant field, the torque on the coil is proportional to the current in it, and the spring torque is proportional to the deflection. The two balance at a deflection proportional to the current, which gives an evenly divided scale - one of the movement’s principal advantages.

Answer: A

Averaging a sinusoid over a cycle gives nothing, so the pointer sits at zero while quivering slightly. Adding a rectifier makes the movement usable on alternating current at the cost of a waveform-dependent calibration.

Answer: B

Correct answer: ohms per volt

\[ \mathrm{Sensitivity} = \frac{1}{I_{fsd}}, \] expressed in ohms per volt. It states how much resistance the instrument presents for each volt of range, so a higher figure means less current drawn from the circuit and less disturbance to what is being measured.

Answer: C

Correct answer: 20,000 ohms per volt

\[ \frac{1}{50\times10^{-6}} = 20{,}000\ \Omega/\mathrm{V}. \] The sensitivity is the reciprocal of the full-scale current and nothing else, so a more sensitive movement gives a better voltmeter - which is why the classic multimeter movement was made as sensitive as the mechanics allowed.

Answer: D

Correct answer: 6 MΩ

\[ 20{,}000 \times 300 = 6\times10^6\ \Omega. \] The instrument therefore draws 50 microamperes at full scale on this range, whatever the range, since that is the movement’s own current. The resistance rises in proportion to the range and the current does not.