The voltage ratio box is used to step-down the voltage to a suitable value. The accurate value of the voltmeter is determined by measuring the value of the voltage to the maximum possible range of the potentiometer. The potentiometer measures the maximum possible value of voltages. The negative and positive error occurs in the readings of the voltmeter if the readings of the potentiometer and the voltmeter are not equal. The figure below shows the circuit for the calibration of the ammeter.
The standard resistance is connected in series with the ammeter which is to be calibrated. The potentiometer is used for measuring the voltage across the standard resistor.
The below mention formula determines the current through the standard resistance. Where, V s — voltage across the standard resistor as indicated by the potentiometer. S — resistance of standard resistor. This method of calibration of the ammeter is very accurate because in this method the value of standard resistance and the voltage across the potentiometer is exactly known by the instrument.
The standard resistance is connected in series with the Wattmeter which is to be calibrated. The low voltage supply is given to the current coil of the Wattmeter. And as the sliding contact moves further away from zero, the magnitude of this current flow decreases.
This is because, as the contact area increases, the voltage drop across the scaled area will get close to the voltage of the standard cell. So at a certain point, the voltage drop across the scaled area will be equal to the voltage of the standard cell and at that point, there will no current flow between two circuits.
Now that a galvanometer is connected in the secondary circuit, it will show a deviation on its display because of current flow and higher the current more will be the deviation. Based on this, the galvanometer will show no deviation only when both the circuits are balanced and this is the state we will be trying to achieve for calibrating the potentiometer. Now by measuring this exact length at which galvanometer shows zero, we can calibrate the potentiometer scale based on the standard cell voltage value.
After knowing the voltage drop per centimeter in the potentiometer scale, connect the unknown voltage to the secondary circuit and slide the contact to measure the length at which we will have zero deviation. After knowing this length of scale at which balance takes place, we can measure the value of unknown EMF as,.
In addition to the measurement of unknown voltage, the potentiometer can also be used to measure the current and power, it just needs a couple of extra components for measuring them. Other than measuring voltage, current, and power, the potentiometers are mainly used for calibration of voltmeters, ammeters, and wattmeter.
Also, since the potentiometer is a DC device, the instruments to be calibrated must be DC moving iron or electrodynamometer types. In the circuit, the most important component for the calibration process is a suitable stable DC voltage supply. This is because any fluctuations in supply voltage will cause an error in the voltmeter calibration thereby leading to an entire failure of the experiment. So standard voltage cell with stable terminal value is taken as a source and connected in parallel with voltmeter which needs to be calibrated.
A voltage ratio box is also connected in parallel with the voltmeter to divide the voltage across the voltmeter and get appropriate value suitable for connecting the potentiometer.
With the entire setup in place, we are ready for testing the accuracy of the voltmeter. So to start, just provide the power to the circuit to get a reading on the voltmeter and an unknown voltage at the voltage ratio box output. Now we will use a calibrated potentiometer to measure this unknown voltage.
After getting the potentiometer reading, check whether the potentiometer reading matches the voltmeter reading. Since potentiometer measures the true value of voltage, if the potentiometer reading does not match with the voltmeter reading, then a negative or positive error is indicated.
And for correction, a calibration curve can be drawn with the help of the readings of voltmeter and potentiometer. Also, for accuracy of measurements, it is necessary to measure voltages near the maximum range of the potentiometer as far as possible. As mentioned above, we will use a suitable stable DC supply voltage to avoid the errors in calibration which do not produce voltage fluctuations during the entire experiment.
A rheostat is used for adjusting the magnitude of the current flowing through the entire circuit. Now we will use a potentiometer to measure the voltage across the standard resistor and then use ohms law to calculate the current through the standard resistance.
Since we are using the standard resistor, the resistance will be accurately known and the voltage across the standard resistor is measured by the potentiometer.
The calculated value will be the accurate value of the current flowing through the loop. Then compare this calculated value with ammeter reading to check the accuracy of the ammeter. If there are any errors, we can make necessary adjustments for the ammeter to rectify the errors. As mentioned above for an accurate calibration process, we will use two suitable stable DC voltage power supplies as sources. Usually, low voltage supply is connected in series with the current coil of a wattmeter and a moderate voltage supply is connected to the potential coil of the wattmeter.
A rheostat in the top circuit is used for adjusting the magnitude of the current flowing through current coil and trim pot in the bottom circuit is used for adjusting the voltage across the potential coil. Do remember that a trim pot is preferred for adjusting the voltage and rheostat is preferred for adjusting the current in a circuit.
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