218 Prof. Millika.ii : New Modification of the Cloud Method 



an expansion I have seen drops which at first seemed 

 stationary, but which then began to move slowly down in the 

 direction of gravity, then become stationary again, then finally 

 began to move slowly up. This is probably due to the fact 

 that large multiply-charged drops are not in equilibrium with 

 smaller singly-charged drops near them, and hence, instead 

 of evaporating, actually grow for a time at the expense of 

 their small neighbours. Be this as it may, however, it is by 

 utilizing the experimental fact that there is a considerable 

 period during which the drops are essentially stationary that 

 it becomes possible to make measurements upon the rate of 

 fall in which the error due to evaporation is wholly negligible 

 in comparison with the other errors of the experiment. 

 Furthermore, in making measurements of this kind the 

 observer is just as likely to time a drop which has not quite 

 reached its stationary point 'as one which has just passed 

 through that point, so that the mean of a considerable number 

 of observations would, even from a theoretical standpoint, be 

 <|uite free from an error due to evaporation. 



§ 5. The Method of Observation. 



The observations on the rate of fidl were made with a 

 short-focus telescope T (see figure p. 212) placed about 2 feet 

 away from the plates. In the eyepiece of this telescope were 

 placed three equally spaced cross-hairs, the distance between 

 the extreme ones corresponding to about one third of the 

 •distance between the plates. A small section of the space 

 between the plates was illuminated by a very narrow beam 

 from an arc-liffht, the heat of the arc being absorbed bv three 

 water cells in series. The air between the plates was ionized 

 by 200 mg. of radium, of activity 20,000, placed from 3 to 

 10 centimetres away from the plates. A second or so after 

 expansion the radium was removed, or screened off with a 

 lead screen, and the field thrown on by hand by means of a 

 double-throw switch. If drops were not found to be held 

 suspended by the field the P.D. was changed or the expansion 

 varied until they were so held. The cross-hairs were set 

 near the lower plate, and as soon as a stationary drop was 

 found somewhere above the upper cross-hair, it was watched 

 for a few seconds to make sure that it was not moving, and 

 then the field was thrown off and the plates short-circuited 

 by means of the double-throw switch, so as to make sure that 

 they retained no charge. The drop was then timed by means 

 •of an accurate stop-watch as it passed across the three cross- 

 hairs, one of the two hands of the watch being stopped at 



