of Determining the Elementary Electrical Charge. 219 



the instant of passage across the middle cross-hair, the other 

 at the instant of passage across the lower one. It will be 

 seen that this method of observation furnishes a double check 

 upon evaporation : for if the drop is stationary at first, it is 

 not evaporating sufficiently to influence the reading of the 

 rate of fall, and if it begins to evaporate appreciably before 

 the reading is completed, the time required to pass through 

 the second space should be greater than that required to 

 pass through the first space. It will be seen from the 

 observations which follow that this was not, in general, the 

 case. 



It is an exceedingly interesting and instructive experiment 

 to w 7 atch one of these drops start, and stop, or oven reverse 

 its direction of motion, as the field is thrown off and on. I 

 have often caught a drop which was just too light to remain 

 stationary and moved it back and forth in this way four or 

 five times between the same two cross-hairs, watching it first 

 fall under gravity when the field was thrown off, and then 

 rise against gravity when the field was thrown on. The 

 accuracy and certainty with which the instants of passage 

 of the drops across the cross-hairs can be determined is pre- 

 cisely the same as that obtainable in timing the passage of a 

 star across the cross-hairs of a transit instrument. 



Furthermore, since the observations upon the quantities 

 occurring in equation (4) are all made upon the same drop 

 .all uncertainties as to whether conditions can be exactly 

 duplicated in the formation of successive clouds obviously 

 disappear. There is no theoretical uncertainty whatever left 

 in the method unless it be an uncertainty as to whether or 

 not Stokes' law applies to the rate of fall of these drops under 

 gravity. The experimental uncertainties are reduced to the 

 uncertainty in a time determination of from three to five 

 seconds, when the object being timed is a single moving 

 bright point. This means that when the time interval is say 

 5 seconds, as it is in some of the observations given below, the 

 error which a practised observer will make with an accurate 

 stop-watch in any particular observation will never exceed 

 2 parts in 50. The error in the mean of a considerable 

 number of concordant observations will obviously be very 

 much less than this. 



Since in this form of observation the v 2 of equation (I) is 

 y.ero, and since X is negative in sign, equation (4) reduces 

 to the simple form 



e=3-422 x 10 ~* x ? ( Wl )B .... (G) 



