424 THE POPULAR SCIENCE MONTHLY 



when an electrical charge is placed upon a body, the medium about the 

 body becomes the seat of new forces, and this may be described by say- 

 ing that the medium about the body has been thrown into a state of 

 strain. But it is one thing to say that the electrical charge on the 

 body produces a state of strain in the surrounding medium, and quite 

 another thing to say that the electrical charge is nothing hut a state of 

 strain in the surrounding medium, just as it is one thing to say that 

 when a man stands on a bridge he produces a mechanical strain in the 

 timbers of the bridge, and another thing to say that the man is noth- 

 ing more than a mechanical strain in the bridge. The practical differ- 

 ence between the two points of view is that in the one case you look 

 for other attributes of the man besides the ability to produce a strain 

 in the bridge, and in the other case you do not look for other attri- 

 butes. So the strain theory, although not irreconcilable with the 

 atomic hypothesis, was actually antagonistic to it, because it led men to 

 think of the strain as distributed continuously about the surface of the 

 charged body, rather than as radiating from definite spots or centers 

 peppered over the surface of the body. Between 1850 and 1900, then, 

 the physicist was in the following anomalous and inconsistent position : 

 When he was thinking of the passage of electricity through a solution, 

 he pictured to himself definite specks or atoms of electricity as travel- 

 ing through the solution, each atom of matter carrying an exact 

 multiple of a definite elementary electrical atom; while, when he was 

 thinking of the passage of a current through a metallic conductor, he 

 gave up altogether the atomic hypothesis, and attempted to picture 

 the phenomenon to himself as a continuous " slip " or " breakdown of 

 a strain " in the material of the wire. 



About 1900, however, a great stride forward was taken when the 

 atomic hypothesis began to be applied to metallic conductors as well as 

 to solutions, and electrical currents, even in wires, began to be looked 

 upon as due to the transport through the wire of discrete units of elec- 

 tricity, now beginning to be called electrons, these units being either 

 handed on from atom to atom or else being pushed along through the 

 interstices between the atoms. This point of view, which was a return 

 to Franklin's way of thinking, found its new justification in the fact 

 that it was found possible in vacuum tubes of the X-ray type to ob- 

 tain from all kinds of matter very minute electrically charged bodies 

 of negative sign, which under all circumstances showed exactly the 

 same behavior in electrical and magnetic fields and which had a mass 

 which was computed to be but 1/1,760 the mass of the atom of hydro- 

 gen, the smallest known atom of matter. There was indeed no direct 

 proof that the charges of these bodies were all the same, since no 

 method had been found of examining them individually, nevertheless, it 

 was pretty conclusively shown, as early as 1899, by Townsend of Ox- 



