RADIUM AND THE ELECTRON—RUTHERFORD. 195 
that the charge carried by the ions in gases was equal to the charge 
carried by the hydrogen atom in the electrolysis of water and made 
the first measurements of this fundamental unit. Other methods of 
attack were developed by Sir J. J. Thomson and H. A. Wilson, and 
by a skillful adaptation of methods Millikan was able to demonstrate 
in a very direct way the unitary nature of electricity and to measure 
the value of the unit charge, probably the most important and 
fundamental constant in physics, with an accuracy, it is believed, of 
one in a thousand. By combining the value of this constant with 
electro-chemical data, the number of molecules in a cubic centimeter 
of gas and the mass of the atoms can be deduced with equal accuracy. 
The convincing proof of the atomic nature of electricity and the 
accurate measure of the fundamental atomic and molecular magni- 
tudes are two of the greatest triumphs of the new era. 
One of the most important properties of X-rays is their power of 
making gases a temporary conductor of electricity. The study of 
this small conductivity led to a clear idea of the transfer of elec- 
tricity through gases by means of charged ions, and the nature and 
difference of the positive and negative ions have been closely studied. 
The proof by Townsend of the production of ions by collision in 
electric fields opened up a new field of investigation and gave us 
for the first time a clear idea of the processes leading up to an 
electric spark. The ionization theory was found to explain the con- 
ductivity produced by radium rays and the conductivity of flames. 
The laws controlling the escape of electricity from glowing bodies 
were closely examined by H. A. Wilson and O. W. Richardson. 
It is a striking fact that these purely scientific researches on the 
conductivity of gases, which had their inception in the Cavendish 
laboratory, and appeared at first to have only an academic interest, 
should so soon have resulted in important practical applications. 
We may instance the use of a hot filament in a low vacuum as a 
rectifier of alternating currents and a detector of electrical waves. 
The supply of electrons from a glowing filament coupled with the 
generation of ions by collision has led to the production of powerful 
electric oscillators and amplifiers for magnifying minute currents to 
any desired degree. These amplifiers have not only been of great 
service in war, but have also rendered possible radiotelephony across 
the Atlantic. Last, but not least, we have the invention of the 
Coolidge X-ray tube, which has played such an important part in 
research and radiography. 
While the mechanism of ionization of gases by X-rays and radium 
rays and the transfer of electricity in ordinary electric fields is in 
the main well understood, it is a striking fact that the passage of 
the disruptive discharge through a vacuum tube, which was the 
