# Wattmeter calibration using DC Potentiometer

Objective:

To calibrate wattmeter using D.C. potentiometer kit.

Theory:

A potentiometer is an instrument which is used for measurement of potential difference across a known resistance or between two terminals of a circuit or network of known characteristic. it is also used for comparing the EMF of two cells. A potentiometer is extensively used in measurement where the precision is required, is higher then that can be obtained by ordinary deflecting instrument. power can also be measured using a potentiometer. Some important for characteristic of potentiometer for which it is used for wattmeter calibration are: –

1. Potentiometer measured the unknown voltage by comparing it with a known voltage source rather than by the actual depletion of the pointer. This is capable of high accuracy.
2. As a potentiometer measures using null or balance condition, hence no power is required for the measurement.
3. Determination of voltage using potentiometer is quite independent of the source resistance.

In this calibration process, the current coil of two-watt meter is supplied from low voltage supply and potential coil from the normal supply through potential divider. The voltage v across the potential coil of the wattmeter under calibration is measured directly by the potentiometer. The current through the coil is measured by measuring the voltage drop across a standard resistor connected in series with the current coil divided by the values of the standard resistor.

The true power is then VI where v is the voltage across the potential coil and I is the current through the current coil or wattmeter. The wattmeter reading may be compared with this value. This type of arrangement is known as “phantom loading”.

Circuit diagram:

Procedure:

1. Apply voltage to the pressure or wattmeter & simultaneously apply to the current to the current coil or wattmeter. The current can be varied with the help of the rheostat provided. The wattmeter reads the power.
2. The volt ratio box converts the applied voltage in the ratio of 1V:1mV. Observed the step-down voltage in the digital devise connect to the potentiometer kit.
3. Similarly, the step-down ratio of current ratio box is 1A:10mA. Observed this reading on the current display panel of the kit.
4. Check the null point by the potentiometer scale which has the specification of 1cm=1mV/Div.
5. Finally calculate the power from the readings on potentiometer kit (v × I=p) & then calculates the error.