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To calibrate a voltmeter using Crompton Potentiometer.

OBJECT: To calibrate a voltmeter using Crompton Potentiometer.



Crompton potentiometer, Galvanometer, 24 dc battery (two quantity),Standard cell (1.0813 volts), Standard resistor, Rheostat, Voltmeter, Connecting lead



Crompton potentiometer is a DC potentiometer in which a high precision could be obtained without the use of long slide wire by adding 15extension coils each equals in resistance to the whole of the slide wire .There are two moving contacts P1&P2, contact P1 sliding over the slide wire and contact P2 sliding over the studs connected to the resistance coil. Balance condition is obtained more easily and quickly by means of the Coarse (dial) adjustment and the fine (slide-wire) adjustment. A battery ‘B’ of about 2volts is connected across the potentiometer through the resistors R1&R2 for controlling the current drawn from the battery. A multiple circuit switch ‘S’ by means of which either the standard cell, Score other emf is measured, can be connected to the galvanometer circuit.



Fig.5.1 Crompton Potentiometer


Standardization: Before putting the potentiometer in use, it is standardized i.e. the potentiometer is made to give the direct readings by an adjustment of the current from the battery ‘B’. For standardization a sell SC, whose emf is 1.0183volts is connected to the terminals of the selector switch marked as SC. The moving contact P2 is set at 1.00stad of the coils and moving contact P1 is set at 1.0183on the slide wire. Now the resistance R1 and R2 are so adjusted that there is no deflection on the galvanometer when key ‘K’ is passed. The potentiometer is now said to be standardized. After standardization the position of R1 and R2 should not be disturbed.


Calibration of voltmeter: For calibration of volt-meter a potential divider of a high resistance is connected across high voltage. The voltmeter under calibration is connected across this potential divider in such a way that potential drop across the voltmeter can be varied. The volt-ratio box is connected in a parallel with volt-meter under the calibration to reduce the voltage across to a value, which is within the range of the potentiometer. Now this reduced potential difference is measured on the potentiometer. The potential difference is measured on the potentiometer multiplied by the ratio of the volt-ratio box gives the actual potential difference across the volt-meter under calibration, to which the instrument reading can be compared.






Reading by Voltmeter



Reading by Potentiometer





















RESULT: In calibration voltmeter error is varied from …… to ….

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