CHAPTKK V 



THE FORCE ON A SMALL CHARGED BODY 

 IN THE FIELD, AND THE PULL OUT- 

 WARDS PER UNIT AREA ON A CHARGED 

 CONDUCTING SURFACE 



The inverse square law for the force deduced from that for electric 

 strain Coulomb'* direct measurements General statement of the 

 inverse square law Electric intensity Unit quantity Relation be- 

 tween electric intensity and electric strain Intensity just outside the 

 surface of a conductor Lines and tubes of force Outward pull per 

 unit area on a charged conducting surface Note on the method of 

 inrwtigatiitfthe field in Chapters II I- V 



The force on a small body in the field. Electric 



intensity. In the fore^oin^ inNe-ti^.ition- of the mode in which 

 barges are di-tri: nature of electric- induc- 



tion, we have not consider* <1 tin- magnitude of the forces acting on 

 <l bodic* or the eon thr-e forces and the 



strain existing where they aie manifest -<1. It is true that our 



tical method of filing the direction of strain d- pends on the 



' tion of the forces 01 pointer, and our measure of 



tin- magnitude of strain depend-* ultimately on fore- I exerted in an 



roscopc fit lu r directly or indirectly by a body charged by 

 induction, lint these forces tell us nothing as to the forces acting 

 on the body when in the electric field Ix-fore it was brought into 



lectroscope. We shall now investigate the field with regard 



to the forces exerted in it. We may suppose that we explore the 



field by the aid of a small insulated conductor carrying a charge 



which i% however, so small that it does not appreciably alter the 



bution of the charges on the surrounding conductor-. 



The inverse square law for the force deduced from 

 the law for electric strain. We may without further ex- 

 periment -how that the force on a small charged body may be 

 calculated in a similar way to the strain, i.e. by the application of 

 the inverse square law to each element of charge. For from the 

 fact that the strain and the force on the -mall ch.irgc-d body every- 

 where coincide in d. we can show that they must be pro- 

 portional to each otl 



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