DETERMINATION OF G

For the determination of the value of universal gravitation constant G, it is necessary to measure the force of attraction between two bodies each of mass 1 kg separated by 1m is only 6.6×10ˉˡˡ N(extremely small to measure) the experiment must have a very high precision. The first accurate measurement of G was made by a English physicist lord Cavendish in 1798 using a very sensitive torsion balance. Since the significant improvements have been made in determining G particularly by J. pointing and c. v. boys in the 19’Th century. Here, we describe Boys experiment.

BOYS EXPERIMENT

The experimental arrangements in shown in figure 7.3 It consists of two identical gold balls A and B each of the mass m and diameter 0.5 cm suspended from the ends of the mirror RS by fine quartz Fibbers. The plane mirror RS itself is suspended from torsion head H by a fine Quartz fiber W. To avoid the suspended system form external disturbance, it is kept centrally inside a vertical glass tube T.

In order coaxial tube, two identical lead balls c and d each of mass M and of diameter 1.125 cm are suspended in level with A and B respectively in a way that the distance between the centers of pairs A and C and B and D are equal.

Two rubber pads p, p are placed just below the leads balls C and D to prevent damage to the apparatus in case they fall. The apparatus is mounted on a platform such that component of the set up are capable of rotating about their common axis. The reflection of RS is measured by telescope and by scale methods.

PROCEDURE: First of all, the lid of the outer cylindrical tube is rotated so those lead balls lie on the opposite sides of the gold ball but not in line with RS. The gravitational force of attraction between their pairs, being equal and opposite, form a couple causing RS to rotate. Due to this rotation of mirror strips RS the suspension fiber W is twisted and an elastic restoring couple develops in it. As soon as the restoring couple balances defecting couple, the stripe RS will attain equilibrium position. The position of C and D are so adjusted that the deflection of the stripe RS is maximum. The angle of deflection is measured by lamp and scale arrangement.

Next the outer tube is rotated so that the lead spheres lie on the other sides of the gold spheres in exactly similar position to produce maximum deflection. The mean of two deflections is taken.