PHYSICS MILLIKAN. 177 



plates, in the hope that it would capture some of the ions which we 

 knew existed in the air, put there by radium or other agencies. The 

 drop met our fullest expectations as a police officer, capturing ions 

 frequently and signaling the fact of each capture to the observer by 

 the change in its speed in the field. For the oil drop is an electrically 

 charged body, and in a given field it moves with a definite speed. If, 

 however, it captures an ion, its charge increases or decreases, and 

 hence its speed increases or decreases. If the charges on ions are all 

 alike, then we can only get one particular change in speed. If the 

 charge that is already upon it, put there by the frictional process, is 

 built up out of these same units, then the total speed which the field 

 will impart must be an exact multiple of the change in speed which 

 the capture of an ion produces. In other words, if electricity is 

 atomic in structure, then in a given field it should be impossible to 

 obtain any except a definite number of speeds wmich will make an 

 arithmetical series — that is, will come up by steps, one, two, three, etc. 

 That is exactly what we found to be the case. We have experi- 

 mented with thousands of drops and scores of different substances, 

 and they always work in exactly the predicted way. Both posi- 

 tively and negatively charged drops are found to act in quite the 

 same way, showing that both positive and negative electrical charges 

 are built up of specks of electricity. Further we can count the 

 number of those specks (which we will call electrons) on a given drop, 

 with the same certainty with which you can count the number of 

 fingers that are before you now. And again since Rowland showed 

 that an electrical current is nothing but a charge in motion, you 

 have here the proof that the electrical current that goes through 

 these lamps, for example, is nothing except the motion of a certain 

 number of electrical specks through or over the filament of the lamp. 

 Add to that J. J. Thomson's discovery made in 1881, that an elec- 

 trical charge possesses inertia, the only distinguishing property of 

 matter, and you have made it perfectly legitimate to say that an 

 electrical current in a wire is a definite, material, granular something 

 which is moving along that wire. 



The fifth great discovery 'of modern physics is the bringing for- 

 ward of evidence for the electrical origin of mass. I have just said 

 that electricity is material. Can we turn it around, and say that 

 all matter is electrical in origin? The last is not exactly the same 

 as the first, and it needs evidence. When we have proved that an 

 electrical charge possesses inertia or mass we have not shown that 

 there is no inertia in matter which is not electrical in its origin. 

 Now we have a certain amount of evidence upon this point and I 

 wish to state what that evidence is. 



We can measure the inertia of the negative electron and it is 

 found to be ygVs of the inertia of a hydrogen atom, but the positive 



