272 LECTURES TO SCIENCE TEACHERS. 



ciple of perfectly general application, that if we have one or 

 any number of electrified "bodies completely inclosed in a 

 hollow conductor which is insulated and possesses no electri- 

 fication independent of the bodies inclosed, the outside of the 

 conductor possesses a charge equal to the sum of all the charges 

 which are inside. Therefore if this conductor is a sphere we 

 have on the outside of that sphere a quantity of electricity 

 exactly equal to all that is inside, and we may measure that 

 quantity by allowing it to be divided between the hollow 

 sphere and a small external one which can be put into the 

 balance. Thus in a very great number of cases the measure- 

 ment of the quantity of electricity in any electrified body can 

 be reduced to the measurement of the electricity of a charged 

 sphere. 



"We have next to speak of Density, or the quantity of 

 electricity on unit of surface. As you are aware, the general 

 method of measuring this is to touch the conductor, on which 

 we want to know the density by a small plane conductor. 

 If the conductor with which we touch the body to be tested 

 is small enough, the outer surface of the plane conductor 

 which may conveniently be a small disc of gilt cardboard 

 becomes, for the time being, practically part of the surface of 

 the body to be examined, so that the quantity of electricity 

 which was previously upon the part of the surface covered 

 by the gilt cardboard is transferred to the cardboard. When 

 we take this away we carry off the quantity of electricity 

 previously on that part of the surface, and can measure it 

 in a torsion-balance ; we can also ascertain the amount of 

 surface upon which it was; then, having a measured 

 quantity and measured surface, the ratio of quantity to surface 

 gives the average density. There is another method which 

 may be employed in special cases for determining the 

 density of a charge. It is founded on this principle, which I 

 have not time to prove, but which is probably known to many 

 of you, that the electrical force just outside a charged con- 

 ductor is equal to four times TT (in its ordinary meaning as 

 representing the ratio of the circumference of a circle to the 

 diameter) multiplied by the electrical density. 



In speaking of the electric force just outside a conductor 

 I mean this : Imagine a unit of electricity close to the surface 

 of the conductor, and suppose it can be put there without 

 disturbing the actual distribution of electricity previously 



