In electrical engineering, a dynamometer type wattmeter is suitable for measuring power in which type of circuits?

Introduction / Context:
A dynamometer type wattmeter is a commonly used instrument in electrical engineering laboratories and power systems to measure electric power. This question checks whether you know the important practical fact that such wattmeters are versatile instruments that can work in both direct current (DC) and alternating current (AC) circuits. Knowing where a particular measuring instrument can be used is essential for safe and correct power measurements in real installations.

Given Data / Assumptions:

• The instrument mentioned is a dynamometer type wattmeter.
• We are concerned with the type of circuit in which it can measure power.
• No special frequency limitation or power factor condition is given.
• We assume normal laboratory or power distribution usage conditions.

Concept / Approach:
A dynamometer wattmeter works on the principle of the interaction between the magnetic fields produced by two coils: a current coil and a pressure (voltage) coil. The deflecting torque is proportional to the product of the instantaneous current and voltage and hence proportional to the power. This principle is valid for both DC and AC, as the instrument essentially responds to the average value of power over time, not just to instantaneous values. Therefore, the same instrument can be used for DC as well as single phase AC power measurement.

Step-by-Step Solution:
Step 1: Recall that in DC circuits, voltage and current are constant with time, so the power is P = V * I and a dynamometer wattmeter can easily measure this product. Step 2: For AC circuits, both voltage and current vary sinusoidally and may have a phase difference. The wattmeter deflection is proportional to the average value of v(t) * i(t) over a cycle, which gives the real or active power. Step 3: The magnetic interaction between the current coil and pressure coil exists for both DC and AC because the torque depends on instantaneous current and flux, whose average over time gives a steady deflection. Step 4: Therefore, the instrument is suitable for measuring power in both DC and AC circuits without any change in basic construction.

Verification / Alternative check:
In most standard electrical engineering textbooks, dynamometer type wattmeters are listed as the standard laboratory wattmeters precisely because they can be used for both DC and AC. Permanent magnet moving coil instruments are limited mostly to DC, whereas dynamometer instruments, with air cored coils, avoid problems of hysteresis and eddy currents and therefore are suitable for AC as well. This confirms our conclusion.

Why Other Options Are Wrong:
Direct current (DC) circuits only: This is incorrect because it ignores the ability of the wattmeter to handle alternating current systems. Alternating current (AC) circuits only: This is also incorrect; nothing in the working principle prevents DC usage. Neither DC nor AC circuits, only for calibration work: This is incorrect because the instrument is specifically built for power measurement in live circuits, not only for calibration.

Common Pitfalls:
Students sometimes confuse the dynamometer wattmeter with other instruments such as permanent magnet moving coil meters that are restricted mainly to DC. Another confusion is between devices that measure only AC because of their construction and those that can work with both types of currents. Remember that it is the nature of the magnetic circuit and coils that decides this capability.

Final Answer:
The dynamometer type wattmeter can measure power in both direct current and alternating current circuits. Therefore, the correct option is Both direct current (DC) and alternating current (AC) circuits.