DowLiNG & Haugiiey — Electrification, Phosphorus Smoke Nuclei. 57 



No possible difference in size between the smoke particles and Millikan's 

 oil-drops could account for this discrepancy. 



Fortunately, there is a very satisfactory explanation forthcoming, and it 

 is, at the same time, very interesting, introducing, as it does, a conception of 

 some novelty. 



XIV. — In Millikan's experiments a single isolated drop is under 

 observations and the velocity measured is the actual " limiting " velocity 

 through stagnant air. In our present work, on the contrary, we are con- 

 cerned with a host of particles, of nearly equal sizes, charged equally, and 

 exposed to the same electric field. By the action of the electric field these 



Eleclr 



Fiq.S 



Omoke ?( ions 



Volts 



/o 



20 



30 



particles are dragged along a narrow laneway through the air, and they carry 

 the air therein along with thein. Eelatively to the (moving) air-stream they 

 do actually travel with a velocity approximating to the limiting velocity of 

 Stokes' law ; but the air-stream, in which they are situated, is itself 

 maintained in rapid motion, being urged along by the viscous drag of these 

 particles. In fact, due to the presence of the charged nuclei in the air- 

 stream, we may think of it as being subjected throughout its whole extent 



to a force per unit volume equal to 



nqV 



where n is the number of nuclei 



per c.c. in the smoke filament, and q the charge on each. That such a 



U.I. A. PEOC, VOL. XXXVI, SECT. A. [6] 



