6 Prof. R. A. Millikan on a new 



25 cm. distant from the drop, which was brought into 

 focus by advancing or withdrawing the whole telescope 

 system by means of a nut and screw. The distance through 

 which the drops were timed was 10220 cm. It was mea- 

 sured precisely as in the 1912 work, and could be duplicated 

 in successive readings to one part in a thousand. The mean 

 value obtained from ten readings varied from 1*0218 to 

 1*0223 — a maximum difference of one part in two thousand. 

 This factor in the determination of e shares with the 

 coefficient of viscosity of air in introducing the largest 

 uncertainty into the final result. 



The velocities of the drops both under gravity and in the 

 field were measured with a most convenient and reliable 

 printing chronograph made by William Gaertner & Co.,, 

 of 5545 Lake Avenue, Chicago, and kindly loaned to the 

 Laboratory for this determination. This instrument is 

 controlled by a standard astronomical clock, and prints 

 on a tape the hour, minute, second, and hundredth of a 

 second at which the key is pressed as the drop crosses 

 the cross-hair in the eyepiece — the maximum error, so 

 far as the recording mechanism is concerned, being never 

 more than a hundredth of a second. Some slight errors 

 were found in the calibration of the Hipp chronoscope 

 used in the previous determination ; but with the Gaertner 

 Printing Chronograph the uncertainty in the time-mea- 

 surements was reduced to a wholly inappreciable amount. 



The electrical field strengths were determined for each 

 drop with the aid of a 7 50-volt Weston Laboratory Standard 

 voltmeter, and contain no uncertainty larger than 1 part in 

 3000. For this voltmeter was repeatedly calibrated in the 

 midst of the observations against three different standard 

 Weston cells, with results which never differed by as much 

 as the limit indicated. The volts are then actually mea- 

 sured in terms of a Standard Weston cell, the above limit 

 being merely the limit of certainty in reading the pointer 

 in the part of the scale used. All the other elements of 

 the problem were looked to with a care which was the 

 outgrowth of six years of experience with measurements 

 of this kind. This work was concluded in August 1916, 

 and occupied the better part of two and a half years of 

 time. 



4. The Validity of the Method. 



That portion of the investigation which has had to do 

 with the testing of the general validity of the method and 

 the endeavour to discover the causes of the disagreement 



