Electrical Engineering ⇒ Topic : Crompton D.C. Potentiometer

Crompton D.C. Potentiometer

Fig. (a) shows the general arrangement of Crompton d.c. potentiometer. A graduated slide wire AC is connected in series with 14 (or more) resistance coils, each of which has a resistance exactly equal to the resistance of the whole slide wire. There are two moving contacts P₁ and P₂.The contact P₁ can slide over the slide wire and contact P₂ can slide over the studs connected to the resistance coils. B is the supply battery (2 volts) and R₁ and R₂ are two variable resistors. The resistor R₁ consists of a number of coils in series and is meant for coarse adjustment of potentiometer current.The resistor R₂ is in the form of a slide wire and is meant for fine adjustment of potentiometer current. The connections C₁, C₂ are taken to selector switch S (double-pole, three position switch) for connecting either the standard cell (S.C.) or cell of unknown e.m.f. The terminals for cell connections are marked Positive (+) and negative (-) for correct polarity connection.

                                                                  figure (a)

Working. The potentiometer is first "standardised" (i.e., made direct reading) by adjustment of the current from the supply battery B. For this purpose, we connect Weston type standard cell whose e.m.f. is 1.0183 volts to the terminals marked S.C. (Be sure to connect correct polarity). The moving contacts P₁ and P₂ are set to a reading equal to the e.m.f. of the standard cell i.e., contact P₂ is placed on stud 1.0 and contact P₁ on 0.0183 on the slide wire. The regulating resistances R₁ and R₂ are varied until the galvanometer reads zero. Now the potentiometer is standardised i.e., reading of the potentiometer (= 1.0 + 0.0183 = 1.0183) is directly read as the voltage being measured. Therefore,the potentiometer becomes direct reading. Note that after standardisation of the potentiometer, the setting of R₁ and R₂ is not to be changed.

The source of e.m.f. whose e.m.f. is to be measured is connected between the terminals + 1 and -1 (or + 2 and -2) and the selector switch is switched over to the position + 1 and -1. The moving contacts P₁ and P₂ are adjusted until the galvanometer reads zero. The reading of the potentiometer will give the e.m.f. or potential difference to be measured directly