How can potentiometer be used for
Calibration of ammeter:- The ammeter to be calibrated is connected in series with a variable resistance R and a standard resistance S. The standard resistance, S should be such of magnitude that with the current to be passed through it, the voltage drop across it does not exceed the range of the potentiometer. Variable resistance is including in the circuit to vary the magnitude of current flowing through the ammeter and standard resistance S.
The voltage drop across standard resistance S is measured on potentiometer. The voltage drop across the standard resistance measured in volts on potentiometer divided by the value of the standard resistance in ohms gives the value of current flowing through the ammeter in amperes. Thus the ammeter reading can be compared. The current flowing through standard resistance S and ammeter under calibration is changed by the varying magnitude of the variable resistance R in the circuit and process is repeated for various value of current over the entire range of ammeter.
Calibration of voltmeter:- The connection diagram for calibration of voltmeter is shown in Fig 15.17 for calibration of voltmeter a potential divider of a high voltage (say 250v) dc supply main show. The voltmeter under calibration is connected across this the voltmeter can be varied.
The volt ratio box is connected in parallel with voltmeter under calibration to reduce the voltage across them to a value, which is within the range of potentiometer. Now this reduced potential difference is measured on the potentiometer multiplied by the ratio of volt ratio. Box gives the actual potential difference across the voltmeter under the calibration, to which the instrument reading can be compared. The voltage across the voltmeter is changed by varying the position of the sliding contact on the potential divider and process is repeated for various values of the potential difference across the voltmeter. The voltage the across standard resistance S is measured potentiometer, the voltage drop across the standard resistance measured in volts on potentiometer divided by the value of standard resistance in ohm gives the value of current flowing through standard resistance S and ammeter under calibration is changed by varying magnitude of variable resistance R in the circuit and the process is repeated for various values of current over the entire range of ammeter potential divider. The voltage V across the potential coil and current I through current coil are measured in turn by the potentiometer employing voltage ratio box and standard resistor, as shown in figure 15.18. The true power is them VI watts and the wattmeter reading may be compared with the value this is a precise method of calibration of wattmeter.
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