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State the working principle of a potentiometer. With the help of the circuit diagram explain how apotentiometer is used to compare the emfs of two primary cells. Obtain the required expression used for comparing the emfs. |
Answer» Solution : COMPARISON of emfs of two cells: A labelled circuit diagram arrangement to compare the emfs of two primary cells using a potentiometer is shown in Fig.  Plug is applied in KEY K and by adjusting the rheostat Rh a constant CURRENT I is allowed to flow through potentiometer wire due to the driver cell Ba. Put the plug in key `K_1` so as to connect cell &z and SLIDE the pencil jockey J till a balance (null) point is OBTAINED. Let the length of potentiometer wire AJ in this position be `l_1` , then `epsi_1 = kl_1` , where k is the potential gradient along the wire. Now plug is removed from key `K_1` and, in turn, plug is inserted in key `K_2` so as to bring `epsi_2`in the electric circuit. Again slide the pencil jockey and obtain the position of jockey for null deflection in galvanometer. Let the length now be `l_2` , then `epsi_2 = kl_2` ` therefore epsi_1/epsi_2 = l_1/l_2` |
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