Question

During an experiment with a metre bridge, the galvanometer shows a null point when the jockey is pressed at $40.0cm$ using a standard resistance of $90\Omega$, as shown in the figure. The least count of the scale used in the metre bridge is $1mm$. The unknown resistance is

• A. $60\pm 0.15\Omega$
• B. $135\pm 0.56\Omega$
• C. $60\pm 0.25\Omega$
• D. $135\pm 0.23\Omega$

Answer 1

Answer: (C)

$60\pm 0.25\Omega$

Wheatstone meter bridge formula,

$\frac{R_1}{R_2}=\frac{l_1}{l_2}$

$\therefore \frac{R_1}{90}=\frac{40}{60}$

$R_1=60\Omega$

Now,

we also have,

$\frac{dR}{R}=\frac{L_1+L_2}{L_1.L_2}dL$

$\frac{dR}{60}=\frac{60+40}{60.40}0.1$

$\therefore dR=0.25\Omega$