Why is an electrodynamometer-type wattmeter used as a standard instrument in laboratories? 

Electrodynamometer instruments

• Electrodynamometer instruments are a type of measuring device used to measure both AC (alternating current) and DC (direct current) electrical quantities such as current, voltage, and power.
• Electrodynamometer-type wattmeters are widely used as standard reference instruments in laboratories because of their high accuracy and precision.
• They work on the principle of the interaction between the magnetic fields produced by fixed and moving coils.

Principle of Operation:

• Fixed coils produce a magnetic field. Moving coil interacts with the magnetic field of the fixed coils, resulting in a deflecting torque proportional to the product of the current in both coils.
• For current measurement: Both coils (fixed and moving) are connected in series to carry the same current. The torque generated is proportional to the square of the current, making the instrument respond to both AC and DC.
• For power measurement: One set of coils is connected in series with the load (to measure current), and the other set is connected across the load (to measure voltage). The deflecting torque in this case is proportional to the product of voltage, current, and the power factor, which allows it to measure true power in AC circuits.