THE PRESIDENTS ADDRESS ,25 



As Helmholtz pointed out clearly in 1881. the acceptance of the 

 atomic theory forces upon us the recognition of a unit quantity 

 of electricity which is just as definite as the atom itself and which 

 Helmholtz very properly said "behaves like an atom of elec- 

 tricity." But the physicists of that day and for long afterwards 

 were accustomed to think of electricity from the standpoint of 

 energy, and the suggestion remained unheeded : indeed, it is very 

 doubtful if Helmholtz himself grasped its full significance. 



In 1897 Professor J. J. Thomson took up again the study of 

 the cathode rays in a Crookes tube. These rays had then acquired 

 an extraordinary interest from their use in the production of 

 X-rays. Professor Thomson devised a very ingenious experiment 

 in which he deflected the cathode rays into an insulated hollow- 

 vessel by an electromagnet. Knowing the capacity of the vessel, 

 he could determine the quantity of electricity carried into it by the 

 electrons in a given time. He also arranged to have the electrons 

 fall on a thermocouple of known heat capacity, and determined 

 the energ}- developed on stopping them. From the data obtained 

 he calculated that the velocity of the particles was of the order 

 of ten thousand miles a second, or even as high as i/io the 

 velocity of light. The quantity of electricity carried by the par- 

 ticles was found to be very considerable in proportion to the heat 

 energ}' shown by the thermocouple. These facts seemed to allow 

 of only two alternatives : either the mass of the particles is 

 extremely small or each particle must carry an enormous charge 

 of electricity. Further experiments confirmed the choice of the 

 first of these possibilities, and physicists are generally agreed 

 that the mass of the electron is about one eighteen hundredth part 

 of the mass of a hydrogen atom. Physicists are not altogether 

 agreed, however, as to whether this electromagnetic mass is the 

 same in nature as the mass of an atom w^hich may be measured by 

 reference to the force of gravity. H it is affected by gravity, an 

 electron having a velocity of lo.cxx) kilometers a second would 

 fall toward the earth possibly two millimeters in flying 800 kilo- 

 meters, say, as far as from here to Pittsburgh. The difficulty of 

 preparing a straight, evacuated tube of that length is evidently 

 rather great, and we shall probably have to wait some time for 

 experimental evidence on this point. The question has become 

 one of unusual speculative interest in the light of the discussion 

 by J. T. Thomson and the measurements of Kaufmann and 

 Bucherer, which seem to show that the mass of the electron is 

 wholly dependent on the velocity with which it moves. 



