66 STATIC ELECTRICITY 



which must be a very good insulator, with a base having an 

 index mark which slides along the lower rule. On the- upj. 

 is a slider with an index mark, and from the slider depends a pith 

 ball B hung by a cocoon fibre, the ball being on a bid "it 

 centre of A. A telescope sights B when it is uncharged, and 

 position of the slider is read. Then A is brought into the middle 

 of the field of view, and the position of its base is read. A 11 

 charged, B touches it, is charged and repelled. A is then removed 

 say, 10 cm. to the left, and the slider is moved to the left till 

 the middle of the telescope field. If the slider has ben m 

 the force on B is for small angles proportional to d. Now n 

 to, say, 20 cm. from the central position and move the slider till 

 is in the middle of the field ; let its displacement be f. It ii 



' ' ' ' j ' ' ' ' J" J I i I i I i 



FlQ. 58. 



found that with fair accuracy d : d = 4 : 1. If A be now touched 

 to another equal insulated sphere, is charge is halved and the 

 forces at the same distances are huht <l. 



General statement of the inverse square law. Both 

 directly and indirectly, then, it has been proved thut the force on a 

 small electrified body placed at any point in a field containing 



charged surfaces separated by air may be calculated by supposing 



body with 

 a force proportional to the product of the element of charge and 



that each element of charged surface acts dim tl\ on the 



the charge on the small body, and inversely as the square of the 

 distance between them, the actual force being the resultant of all 

 such elementary forces. Since the force is thus proportional to the 

 charge on the small body, it is the same per unit of charge on that 

 body whatever the total charge and whatever the size of the body, 

 both being very small. We have, therefore, a definite and i 

 sistent measure by which to describe the field, in the force acting 

 per unit charge on a small body. This method of measuring has 

 already been employed in Chapter HI, but we shall here repeat 

 some of the definitions and propositions of that chapter, with 



