308 THE REALITIES OF MODERN SCIENCE 



by short wires and thus the polarity of the condenser 

 could be reversed. 



Above the plates was a closed chamber, which re- 

 ceived the spray from the atomizer ; and occasionally 

 a small drop would find its way through the pinhole, 

 p, in the center of the upper plate and thus be avail- 

 able for observation. Powerful beams of light were 

 directed through the space between the plates from 

 the windows c and c on the right and left. The illu- 

 minated drop was observed by a small telescope through 

 a window in the apparatus. In the eyepiece of the 

 telescope were small parallel hairs which served as 

 fixed reference lines for noting the position of the 

 " droplet." This droplet appeared merely as a speck of 

 light, like a bright star on a black background. 



If it was desired to alter the charge on an oil drop 

 the air between the plates was ionized by exposure to 

 some ionizing agent, either X-rays or radium. If it 

 was desired to give a drop an additional negative charge 

 it was arranged that it should be near the positive 

 plate of the condenser while the air was being ionized. 

 Toward this plate there came, of course, a shower of 

 electrons produced from the air by the ionization. 

 Conversely, when it was desired to give the droplet 

 an additional positive charge it was held near the nega- 

 tive plate, and thus it suffered collisions from positive 

 ions. 



The position of the drop could be controlled at will 

 because it was so small that it fell very slowly indeed. 

 For example, a matter of ten or twelve seconds would 

 be required for the droplet to fall under the action of 

 gravity through the distance of about one centimeter 



