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THE POTENTIOMETER

10. THE POTENTIOMETER

For the accurate measurement of potential difference, current and resistance the potentiometer is one of the most useful instruments.

Its principle of action is that an unknown e.m.f. or.p.d. is measured by balancing it, wholly or in part, against a known difference of potential. 14

Fig. 34. A simple potentiometer.

 

Construction, The potentiometer, in its simplest form, consists of wire LM of uniform cross-section, stretched alongside a scale and connected across an accumulator B of ample capacity. A standard cell of known e.m.f. E1 is connected between L and terminal 1 of a two-way switch S, care being taken that the corresponding terminals of Band E1 are connected to L.

 

Working

 

-        Slider N is pressed momentarily against wire LM and its position adjusted until the galvanometer deflection is zero when N is making contact with LM. Let l1 be the corresponding distance between Land N. The fall of potential over length l1 of the wire is then the same as the e.m.f. E1·

 

-        Then move the switch S to 2, thereby replacing the standard cell by another cell, the e.m.f E2 of which is to be measured. Adjust the slider N again to give zero deflection on G. If I2 be the new distance between Land N, then 16

Applications of potentiometer. The following are the applications/uses of potentiometers:

  1. Measurement of small e.m.fs. (upto 2 volts).
  2. Comparison of e.m.fs. of two cells.
  3. Measurement of high e.m.fs. (say 250 volts).
  4. Measurement of resistance.
  5. Measurement of current.
  6. Calibration of ammeter.
  7. Calibration of voltmeter.

Example 11. Using a Weston cadium cell of 1.0183 V and a standard resistance of 0.1 Ω a potentiometer was adjusted so that 1.0183 m was equivalent to the e.m.f. of the cell; when a certain direct current was flowing through the standard resistance, the voltage across it correspond to 150 cm. What was the value of current?

Solution. E.m.f. of the standard cell, E1 = 1.0183 V

Potentiometer adjustment, I1 = 1.0183 m

and   I2 = 150 cm = 1.5 m

Voltage across the standard resistance of 0.1 Ω corresponding to I2,

E2=? 15