Prof. R. A. Millikan 



on a new 



From this equation * it is seen at once that the intercept 



of the git— line on the e^ axis is the value of eL and that 

 pa 



the slope of this line divided by the above-mentioned intercept 

 is the constant b, the significance of which has been pointed 

 out before t and will be more fully discussed in a following 

 paper. 



3. 17ie Apparatus. 



The apparatus is new throughout, and every constant 

 involved in this method of determing e has been re-evaluated 

 with the aid of improved and refined methods. The old air- 

 condenser i had consisted of ribbed brass plates, tested merely 

 mechanically and found flat to about '01 mm. They were 

 held apart by ebonite posts 16 mm. long. These posts were 

 found to change in length slightly through the absorption of 

 oil. The new plates M and N (Hg. 1) were made optically 

 flat by polishing and then testing them with the aid of 

 mercury fringes against a standard optical test-plate. They 

 were nowhere in error by more than two wave-lengths of 

 green mercury light. They were 22 cm. in diameter and 

 were separated by three small pieces of echelon plates about 

 1 cm. square and 14*9174 mm. thick, placed at points 60 a 

 apart about the circumference. These echelon plates, of 

 course, had optically perfect plane-parallel surfaces. The 

 dimensions of the condenser, therefore, no longer introduced 

 an error of more than about 1 part in 10,000 instead of 

 about 1 part in 1000 as in the previous w r ork. 



The oil droplets from the atomizer A, blown by a puff of 

 air through r (fig. 1) entered the condenser MN through five 

 minute holes \ mm. in diameter in the middle of the upper 

 plate, and were observed as in the former work by means of 

 light from the arc, a, filtered through a trough of water, w, 

 and one of cupric chloride, d, for the removal of heat rays. 

 The temperature was held constant to within one or two 

 hundredths of a degree and very close to 23° C. by the oil- 

 bath, Gr. The charge on the drop, />, was changed by X-rays 

 from the bulb X passing through the window, g. The 



* Equations (3) and (4) are perhaps more easily visualizable if the- 



correction term il-\ j is written in the form (l+A-b in which I is- 



the mean free path of the gas molecule. Since the exact value of / is 

 uncertain, I have for simplicity chosen to compute it uniformly from 

 r) = '3502nmcl. This gives at 23° 76 cm., Z aip = -000009417 cm. 



t Millikan, Phys. Eev. xxxii. p. 381 (1911). 



X Millikan, Phys. Kev. ii. p. 122 (1913). 



