ISOLATION OP AN ION MILLIKAN. 



233 



oil is blown with the aid of dust-free air into the dust-free chamber 

 C. One or more of the droplets of this cloud is allowed to fall 

 through a pinhole p into the space between the plates M, N of a hori- 

 zontal air condenser and the pinhole is then closed by means of an 

 electromagnetically operated cover not shown in the diagram. If 

 the pinhole is left open air currents are likely to pass through it and 

 produce irregularities. The plates M, N are heavy, circular, ribbed 

 brass castings 22 centimeters in diameter having surfaces which are 

 ground so nearly to true planes that the error is nowhere more than 

 0.02 millimeter. These planes are held exactly 16 millimeters apart 

 by means of three small ebonite posts a held firmly in place by ebo- 

 nite screws. A strip of thin sheet ebonite c passes entirely around 



Fig. 1. 



the plates, thus forming a completely enclosed air space. Three glass 

 windows, 1.5 centimeters square, are placed in this ebonite strip at the 

 angular positions 0°, 165°, and 180°. A narrow parallel beam of 

 light from an arc lamp enters the condenser through the first window 

 and emerges through the last. The other window serves for observ- 

 ing, with the aid of a short focus telescope placed about 2 feet distant, 

 the illuminated oil droplet as it floats in the air between the plates. 

 The appearance of this drop is that of a brilliant star on a black 

 background. It falls, of course, under the action of gravity, toward 

 the lower plate ; but before it reaches it, an electrical field of strength 

 between 3,000 volts and 8,000 volts per centimeter is created between 



