Analog Ammeters and Voltmeters Questions and Answers

Answer: A

The answer is both (A) and (R) are true and (R) is the correct explanation of (A).

A hot wire ammeter measures the expansion of a wire heated by the current. The heating power is \(I^{2}R\), so the deflection is proportional to the square of the current. Equal steps of current therefore produce small deflections near zero and large ones near full scale: the scale divisions are crowded at the low end, which is what a cramped (non-uniform) scale means.

The square law that R states is the direct cause of the cramped scale, so R explains A. The same square law is why the instrument reads true rms values and works on both ac and dc.

Answer: A

Correct answer: the deflection settles at a definite value for a given input

Without a controlling torque the pointer would swing to the end of the scale for any input at all, since nothing would oppose the deflecting torque. The controlling torque grows with deflection, so the pointer stops where the two balance - and it also returns the pointer to zero when the input is removed.

Answer: B

It depends on the velocity of the moving system, so it opposes the motion and dies away as the pointer settles, without shifting the final reading. Too little gives oscillation and too much gives a sluggish instrument.

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.