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To determine the internal resistance of a primary cell

  • Initially in secondary circuit, key K’ remains open and balancing length (l1) is obtained (Fig. 12). Since cell E is in open circuit, so its emf balances on length l1, i.e., E = xl1(i)
  • Now key K’ is closed; so cell E comes in closed circuit. If the process of balancing is repeated again then potential difference V balances on length l2, i.e., V = xl2(ii)
Fig. 12
  • By using internal resistance formula, we get 75351.png75357.png

Comparison of emfs of two cells

Let l1 and l2 be the balancing lengths with the cells E1 and E2, respectively, then E1 = xl1 and E2 = xl2 75386.png
Fig. 13
Let E1 > E2 and both are connected in series. Let the balancing length is l1 when cells assist each other and it is l2 when they oppose each other as shown in Fig. 14. Then
Fig. 14
⇒ 75415.png or 75402.png

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