Page:Popular Science Monthly Volume 80.djvu/429

Rh, that the mean value of the charge carried by these electrons was the same as the charge carried by the hydrogen atom in electrolysis; and about the same time Sir J. J. Thomson found a way of making a rough determination of the absolute value of this mean charge. This method was improved in 1902 by H. A. Wilson, now of McGill University, and actually formed the starting point some five years later of the work out of which grew, by a series of natural steps, the experiments which are herewith presented and which have made it possible to capture and make accurate measurements upon one single isolated

electron or any desired number of such electrons up to one hundred and fifty.

Imagine two circular plates M and N (Fig. 1) 22 centimeters (about 10 inches) in diameter and 16 millimeters (f inch) apart which can be electrically charged, one positively and the other negatively, by making them the terminals of a ten-thousand-volt storage battery B. Suppose also that with the aid of a switch S the plates can be instantly discharged when desired so as to possess no electrical properties at all. Now when the plates are suddenly charged the air between them is found to remain perfectly quiet and free from convection currents of any kind—a result which shows that practically all of the air molecules between the plates are electrically neutral. But if now a beam of