Electrical Engineering ⇒ Topic : Direct Reading Potentiometers

Direct Reading Potentiometers

The simple potentiometer described above for determining or comparing e.m.f.s involves some computations. Therefore, it is used for educational purposes only. For convenience, modem potentiometers are always made direct reading i.e., the potentiometer wire is marked in terms of voltages. Therefore, the voltages being measured can be directly read off from the calibrated potentiometer. The calibration of a potentiometer (i.e., making it direct reading) is called standardisation ofpotentiometer.

Standardising the potentiometer. The calibration (i.e., standardisation) of a simple potentiometer is done with the help of a standard cell as shown in Fig.(a). The standard cell employed is Weston cadmium cell whose e.m.f. is 1.0183 volts. A high variable resistance is put in the standard cell circuit to keep the current through it to permissible limit This high resistance is gradually cut off as the null point condition is approached. Ultimately, this high resistance is entirely cut off by short-circuiting it with key K when final null point is obtained

                                                                         figure (a)

Suppose balance (i.e., null point) is obtained when jockey is at point D on the slide wire. Let AD= l_s If I is the steady current through the slide wire AC and r is the resistance per unit length of the slide wire, then,

Potential drop across AD = Il_s

This potential difference is equal to the e.m.f. of the standard cell.

Thus potentiometer wire can be marked in terms of voltages and the instrument becomes direct reading. Note that standardisation of the potentiometer remains the same as long as the current I through the slide wire is constant. Therefore, once the potentiometer is standardised, the setting of rheostat should not be changed in any case. After standardising, the potentiometer can be used for various purposes.