7. The diameter of a wire is measured with a screw gauge having 50 divisions on a circular scale and by one complete rotation of circular scale, the main scale moves 0.5mm. If the reading of the screw gauge is 0.250 cm. The minimum percentage error in the reading will be

Minimum percentage error = 20%

Explanation:

The zero error of the screw gauge is  

(5*0.5)/50 = 0.05 mm

Least count on the circular scale is 0.5/50 = 0.01 mm

Equivalent circular scale reading = 0.01 * 25 = 0.25 mm

Reading on the main scale = 2 * 0.5 = 1 mm

Actual measurement = 1 +0.25-0.05 = 1.20 mm

The minimum percentage error in the reading will be (1.20 mm -1.00 mm)/1*100 = 20%