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PHYSICS: R. A, MILLIKAN 
apart about the circumference. The dimensions of the condenser 
therefore now introduce an error of no more than 1 part in 10,000. The 
oil droplets from the atomizer A blown by a puff of air through r entered 
the condenser MN through 5 minute holes 0.25 mm. in diameter in the 
middle of the upper plate and were observed by means of light from 
the arc a, filtered through a trough of water w and one of cupric chlo- 
ride d for the removal of heat rays. The temperature was held con- 
stant to within one or two hundredths of a degree by the oil-bath G. 
The charge on the drop p was changed by X-rays from the bulb X pass- 
ing through the window g. The pressure was varied from 13 cm. to 76 
FIG. 1. 
cm. and was measured to 0.1 mm. by the manometer m. The ato- 
mizer A^ fed from j with the highest grade of watch-oil, density at 23°C. 
redetermined as 0.9199, was blown with carefully dried and cleaned 
air let in through the cock r, the bulbs below A being to catch excess 
oil. The observing optical system was a specially constructed tele- 
scope of 30 mm. objective and a magnification of 25 diameters. The 
distance through which the drops were timed (cross hair distance) was 
1.0220 cm. correct to 1 part in 2000. The velocities of the drops were 
measured with a most convenient and reliable printing chronograph 
made by Wm. Gaertner & Company of 5545 Lake Avenue, Chicago, 
and kindly loaned to the laboratory for this determination. It prints 
