MODERN PHYSICS 15 



what we call an electrical current consists simply in the 

 journey along- the conductor of these electrical specks, 

 which we may call with perfect justice definite material 

 bodies. Now, I can give you in just a word the proof of 

 that statement. There are half a dozen ways in which it 

 could be approached. I will mention the one with which 

 I am most familiar, because it is the particular proof which 

 we worked out in our laboratory. 



We took these plates with a field of 10,000 volts be- 

 tween them, with a little hole in the top plate, and blew 

 an oil spray above the top plate so as to get an electrically 

 charged body just as small as we could ; for we expected 

 that the frictional process involved in blowing the spray 

 would charge the drops ; and this, indeed, it was found to 

 do. We let one of those drops come into the space be- 

 tween the plates, and then moved it up and down by an 

 electrical field, throwing it on the field as it came close to 

 the bottom plate, and throwing it off as it approached the 

 upper one. And so we kept that oil drop going up and 

 down between the 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. It captured ions frequently and sig- 

 nalled 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 in- 

 creases 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 fric- 

 tional 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, you cannot get in a given field anything ex- 

 cept a definite number of speeds, which will make an arith- 



