of Determining the Elementary Electrical Charge. 217 



o£ the upper plate are jerked up agaiust gravity to this plate. 

 The result is that after a lapse of 7 or 8 seconds the field of 

 view has become quite clear save for a relatively small 

 number of drops which have just the right ratio of charge to 

 mass to be held suspended by the electric field. These 

 appear as perfectly distinct bright points. I have on several 

 occasions obtained but one single such " star" in the whole 

 field and held it there for nearly a minute. For the most 

 part, however, the observations recorded below were made 

 with a considerable number of such points in view. Thin, 

 flocculent clouds, the production of which seemed to be 

 facilitated by keeping the water-jackets J\ and J 2 a degree or 

 two above the temperature of the room, were found to be 

 particularly favourable to observations of this kind. 



Furthermore, it was found possible to so vary the mass of 

 a drop by varying the expansion, or the charge carried by a 

 drop by varying the ionization, that drops carrying in some 

 cases two, in some three, in some four, in some five, and in 

 some six, multiples could be held suspended by nearly the 

 same field. The means of gradually varying the field which 

 had been planned were therefore found to be unnecessary. 

 If a given field would not hold any drops suspended it was 

 varied by steps of 100 or 200 volts until drops were held 

 stationary, or nearly stationary. When the P.D. was thrown 

 off it was often possible to see different drops move down 

 under gravity with greatly different speeds, thus showing that 

 these drops had different masses and correspondingly different 

 charges. 



The life history of these drops is as follows. If they are 

 a little too heavy to be held quite stationary by the field they 

 begin to move slowly down Tinder gravity. Since, however, 

 they slowly evaporate, their downward motion presently 

 ceases, and they become stationary for a considerable period 

 of time ; then the field gets the better of gravity and they 

 move slowly upward. Toward the end of their life in the 

 space between the plates, this upward motion becomes quite 

 rapidly accelerated and they are drawn with considerable 

 speed to the upper plate. This, taken in connexion with 

 the fact that their whole life between plates only 4 or 5 mm. 

 apart is from 35 to 60 seconds, will make it obvious that 

 during a very considerable fraction of this time their motion 

 must be exceedingly slow. I have often held drops through 

 a period of from 10 to 15 seconds, during which it was im- 

 possible to see that they were moving at all. Shortly after 



