20 Electrostatics -Physical Principles [CH. i 



27. Two-fluid Theory. A further attempt to explain electric phenomena 

 was made by the two-fluid theory. In this there were three things concerned,, 

 ordinary matter and two electric fluids positive and negative. The degree 

 of electrification was supposed to be the measure of the excess of positive 

 electricity over negative, or of negative over positive, according to the sign 

 of the electrification. The two kinds of electricity attracted and repelled, 

 electricities of the same kind repelling, and of opposite kinds attracting, and 

 in this way the observed attractions and repulsions of electrified bodies were 

 explained without having recourse to systems of forces between electricity 

 and ordinary matter. It is, however, obvious that the two-fluid theory wa& 

 too elaborate for the facts. On this theory ordinary matter devoid of both 

 kinds of electricity would be physically different from matter possessing 

 equal quantities of the two kinds of electricity, although both bodies would 

 equally shew an absence of electrification. There is no evidence that it is 

 possible to establish any physical difference of this kind between totally 

 unelectrified bodies, so that the two-fluid theory must be dismissed a& 

 explaining more than there is to be explained. 



28. Modern view of Electricity. The two theories which have just been 

 mentioned rested on no experimental evidence except such as is required 

 to establish the phenomena with which they are directly concerned. The 

 modern view of electricity, on the other hand, is based on an enormous mass 

 of experimental evidence, to which contributions are made, not only by the 

 phenomena of electrostatics, but also by the phenomena of almost every 

 branch of physics and chemistry. The modern explanation of electricity is 

 found to bear a very close resemblance to the older explanation of the one- 

 fluid theory so much so that it will be convenient to explain the modern 

 view of electricity simply by making the appropriate modifications of the 

 one-fluid theory. 



Let us suppose the " electric-fluid " of the one-fluid theory replaced by a 

 crowd of small particles " electrons," it will be convenient to call them all 

 exactly similar, and each having exactly the same charge of negative electricity 

 permanently attached to it. The electrons are almost unthinkably small, 

 about 10 27 of them being required to make a gramme about as many in fact 

 as would be required of cubic centimetres to make a sphere of the size of our 

 earth. The charge of an electron is enormously large compared with its 

 mass the charge of each being about 3 x 10~ 10 in electrostatic units, so that a 

 gramme of electrons would carry a charge equal to about 3 x 10 17 electrostatic 

 units. To form some conception of the intense degree of electrification 

 represented by these data, it may be noticed that two grammes of electrons, 

 if placed at a distance of a metre apart, would repel one another with a 

 force equal to the weight of about 10 21 tons. Thus the electric force out- 

 weighs the gravitational force in the ratio of about 10 42 to 1. 



