JAMB Physics 2019 Paper

Concept:A slide wire bridge works on the Wheatstone bridge principle. At balance, the unknown resistance and the standard resistance are in the same ratio as the two lengths of the wire.Explanation:The total wire length is $1\text{ m} = 100\text{ cm}$.The balance point is 25 cm from the end where the unknown resistance $R$ is connected.So, the length on the unknown side is $25\text{ cm}$.The length on the standard resistance side is:$$100 - 25 = 75\text{ cm}$$At balance:$$\frac{R}{3.6} = \frac{25}{75}$$Simplify the ratio:$$\frac{R}{3.6} = \frac{1}{3}$$Cross-multiply:$$3R = 3.6$$$$R = \frac{3.6}{3} = 1.2\, \Omega$$Answer:The unknown resistance is $1.2\, \Omega$, which is option D.