CHAP, r.] ELECTRICITY AND MAGNETISM. 17 



be found to be only one quarter as great as the force 

 at one inch ; and at ten inches it will be Only T ^th 

 part as great as at one inch. This law is proved by the 

 following experiment with the torsion balance. The 

 two scales were adjusted to o, and a certain charge was 

 then imparted to the balls. The ball n was repelled 

 round to a distance of 36. The twist on the wire 

 between its upper and lower ends was also 36, or the 

 force of the repulsion was thirty-six times as great as the 

 force required to twist the wire by i . The torsion-head 

 was now turned round so as to twist the thread at the 

 top and force the ball n nearer to m, and was turned 

 round until the distance between n and m was halved. 

 To bring down this distance from 36 to 18, it was 

 found needful to twist the torsion-head through 126. 

 The total twist between the upper and lower ends of the 

 wire was now 126 + 18, or 144; and the force was 

 144 times as great as that force which would twist the 

 wire i. But 144 is four times as great as 36 ; hence 

 we see that while the distance had been reduced to one 

 half, the force between the balls had become four 

 times as great. Had we reduced the distance to one 

 qttarter^ or 9, the total torsion would have been found 

 to be 576, or sixteen times as great; proving the 

 force to vary inversely as the square of the 

 distance. 



In practice it requires great experience and skill to 

 obtain results as exact as this, for there are many 

 sources of inaccuracy in the instrument. The balls 

 must be very small, in proportion to the distances between 

 , them. The charges of electricity on the balls are found, 

 moreover, to become gradually less and less, as if the 

 electricity leaked away int6 the air. This loss is less 

 if the apparatus be quite dry. It is therefore usual to 

 dry the interior by placing inside the case a cup con- 

 taining either chloride of calcium, or pumice stone 

 soaked with strong sulphuric acid to absorb the moisture. 

 c 



