2dix-The Student Choice
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To measure low resistance using Crompton potentiometer.

OBJECT: To measure low resistance using Crompton potentiometer.



Crompton potentiometer, Galvanometer, 2V DC supply, Standard cell (1.0183 volts), Unknown resistor, Ammeter, Connecting load.



Crompton potentiometer is a DC potentiometer in which high precision could be obtained without the use of a long slide wire by adding 15 extension coils, each equal in resistance to the whole of the slide wire(fig.). There are two moving contacts P1 & P2 contact P1 sliding over the slide wire and contact P2 sliding over the study 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 2 volts is connected across the potentiometer through the resistor R1 & R2 for controlling the current drawn from the battery. A multiple circuit switch ‘S’ by means of which either the standard cell, SC or others emf to be measured can be connected to the galvanometer circuit.




                                                                                Fig. 6.1 Crompton Potentiometer



  • Standardization: For standardization a western type standard cell sc, whose end is 1.0183 volts, is connected to the terminals of selector switch marked as sc. The moving contact P2 is set at 1.00 stud of the coils and moving contact P1 is set at 0.0183 on the slide wire. Now the resistance R1 & R2 are so adjusted that there is no deflection in the galvanometer where key K is pressed. The potentiometer is now said to be standardized the position of R1 & R2 should not be disturbed.
  • Measurement of Resistance:

A.  Make the connection as shown in fig.

B.  The unknown resistance should be such a magnitude with the current to be passed it, the voltage drop across it does not exceed the range of potentiometer.

C.  By varying the value of rheostat and take the value of current by ammeter and voltage by potentiometer.

D.  Take the value in 4-set and calculate the R=V/I & calculate the average of R.




Current in ammeter

         I (Amp)

Voltage in potentiometer

             V (volts)

R = V/I

Actual voltage of R (Ω)

















RESULT: The value of unknown low resistance is ……………. Ω.

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