Introduction:
A dynamometer type wattmeter is an instrument used for measuring electric power. It consists of two coils, namely the current coil and the pressure coil. The current coil is connected in series with the load current, and the pressure coil is connected across the load voltage. The pressure coil is responsible for measuring the power factor and the reactive power.

Explanation:
The pressure coil of a dynamometer type wattmeter is highly resistive. This means that it has a high resistance value compared to its inductance value. There are several reasons for this:

1. Measurement of true power: The primary function of the pressure coil is to measure the true power in the circuit. True power is the actual power consumed by the load and is given by the product of the load voltage, load current, and the power factor. To accurately measure true power, the pressure coil should have a high resistance. This ensures that the voltage drop across the pressure coil is directly proportional to the load voltage and current.

2. Minimization of errors: The presence of inductance in the pressure coil can introduce errors in the power measurement. Inductance causes a phase shift between the voltage and current, which results in inaccurate power readings. By keeping the pressure coil resistive, the effects of inductance are minimized, leading to more accurate power measurements.

3. Prevention of self-induction: When a current flows through a coil, it creates a magnetic field around it. This magnetic field induces a voltage in the coil itself, known as self-induction. Self-induction can affect the accuracy of power measurements. By having a highly resistive pressure coil, the self-induction effects are reduced, leading to more accurate power measurements.

4. Compensation for load power factor: The pressure coil is also responsible for measuring the power factor of the load. The power factor is the ratio of true power to apparent power. By having a resistive pressure coil, the power factor can be accurately determined, as the resistive component of the load voltage and current is directly proportional to the true power.

In conclusion, the pressure coil of a dynamometer type wattmeter is highly resistive to accurately measure true power, minimize errors, prevent self-induction, and compensate for the load power factor.