27, 28] Modern View of Electricity 21 



A piece of ordinary matter in its unelectrified state contains a certain 

 number of electrons of this kind, and this number is just such that two 

 pieces of matter each in this state exert no electrical forces on one another 

 this condition in fact defines the unelectrified state. A piece of matter 

 appears to be charged with negative or positive electricity according as the 

 number of negatively-charged electrons it possesses is in excess or defect of the 

 number it would possess in its unelectrified state. 



Two important consequences follow from these facts. In the first place it 

 is clear that we cannot go on dividing a charge of electricity indefinitely a 

 natural limit is imposed as soon as we come to the charge of one electron, 

 just as in chemistry we suppose a natural limit to be imposed on the divisi- 

 bility of matter as soon as we come to the mass of an atom. And, secondly, 

 in charging a body with electricity we either add to or subtract from its mass 

 according as we charge it with negative electricity (i.e., add to it a number of 

 electrons), or charge it with positive electricity (i.e., remove from it a number 

 of electrons). Since the mass of an electron is so minute in comparison with 

 the charge it carries, it will readily be seen that the change in its mass is 

 very much too small to be perceptible by any methods of measurement which 

 are at our disposal. Maxwell mentions, as an example of a body possessing 

 an electric charge large compared with its mass, the case of a gramme of gold, 

 which may be beaten into a gold-leaf one square metre in area, and can, in 

 this state, hold a charge of 60,000 electrostatic units of negative electricity. 

 The mass of the number of negatively electrified electrons necessary to carry 

 this charge will be found, as the result of a brief calculation from the data 

 already given, to be 2xlO~ 13 grammes. The change of weight by electrification 

 is therefore one which it is far beyond the power of the most sensitive balance 

 to detect. 



On this view of electricity, the electrons must repel one another, and 

 must be attracted by matter which is devoid of electrons, or which has a 

 deficiency of electrons. The electrons move about freely through conductors, 

 but not through insulators. The reaction which, as we have seen, must be 

 supposed to occur at the surface of charged conductors between " matter" and 

 4t electricity," can now be interpreted simply as the systems of forces between 

 the electrons and the remainder of the matter. Up to a certain extent these 

 forces will restrain the electrons from leaving the conductor, but if the electric 

 forces acting on the electrons exceed a certain limit, they will overcome the 

 forces acting between the electrons and the remainder of the conductor, and 

 an electric discharge takes place from the surface of the conductor. 



Thus an essential feature of the modern view of electricity is that it 

 regards the flow of electricity as a material flow of charged electrons. Good 

 conductors and good insulators are now seen to mean simply substances in 

 which the electrons move with extreme ease and extreme difficulty re- 

 spectively. 



