Outside the plates we have opposite effects in the same direction, hence they annul. Between the plates we have opposite effects in opposite directions and the effects are equivalent and hence cumulative. The tension will depend upon the number of lines and the closeness together. Each positive line will increase the tension of a positive line and a negative line will diminish the tension of a positive line. If a positive line increases the tension of a positive line just as much as a negative line decreases it, and if a negative increases the tension of a negative just as much as a positive decreases it, then the resultant tension between the plates will be zero, since there are just as many positive as negative lines. But if the effect (increase) of a positive on a positive is not the same as a negative on a positive there will be a resultant tension between the plates.
Now take an unelectrified body which we consider an assemblage of positive and negative charges. Lines of force will start on the positive charges and terminate on the negative ones, and just as there was a tension between the plates if the effect of a positive on a positive was not the same as a negative on a negative, so there will be a tension around this unelectrified body, if the above is true. Hence by making the assumption that the lines of force from a positive charge are not the same as those from a negative charge, the increase in tension of a positive on a positive is not the same as the decrease of a negative on a positive and we shall have a resulting tension. This might give rise ta forces in the body of which the most important is gravitation. Here are Professor Thomson's own words taken from my class notes: "Matter I regard as made up of positive and negative charges. . . . Each unit charge is the termination of a line of force. I do not regard the positive and negative lines side by side as the same."
If you are looking for a Herculean task put the theory to test. Before starting let me remind you that the effect of the earth's field on an electron is something like 30 million times that of gravity and that it is not easy to screen a magnetic force. We see then that this theory is beset with difficulties before which the experimenter at present is helpless. Again, it complicates the simplicity of the electronic theory of electricity. Again, electrical attraction depends upon the medium. On this theory should not gravitational attraction also depend upon the medium? A summary of Professor Thomson's lectures on gravitation was published in Cambridge Philosophical Society Proceedings during the spring of 1909.
Various other theories have been promulgated but they must be passed by. Those interested in the subject will want to read Osborne Reynold's theory. Kelvin's hydrodynamical theory, which involves both the creation and destruction of matter, is rather unique.
Some despairing of ever finding a physical explanation have taken