al to the relative weights of the different kinds of atoms 

 involved. Electrons, like atoms, can today be detected 

 singly, and Faraday's conclusion as to the atomicity of 

 electricity has been fully confirmed. Later in the nine- 

 teenth century, Thomson showed that all atoms contain 

 electrons, and he measured the mass of the electron, finding 

 it to be about 1/1850 of the mass of the lightest atom 

 (the hydrogen atom). 



The third great advance in physical knowledge 

 achieved last century was the prediction of, and then the 

 actual generation of electromagnetic (radio) waves. The 

 suggestion that rapidly accelerating electric charges should 

 be expected to give rise to electric and magnetic dis- 

 turbances in their neighborhood was made by Maxwell 

 in 1873. He found, as the result of a mathematical analy- 

 sis of the conditions in the region surrounding a conductor 

 carrying an oscillating electric charge, that when the fre- 

 quency of oscillation was high enough, the more remote 

 parts of the electric field associated with the charge would 

 not be able to follow the changes in the field close to the 

 conductor. Consequently, these regions of the field would 

 break away from the oscillating electric charge, and would 

 move outwards into the surroundings. The changing elec- 

 tric field would produce a corresponding changing mag- 

 netic field and vice-versa, perpendicular to the electric 

 field, and to the direction in which this "electromagnetic" 

 wave was travelling. Maxwell predicted that these electro- 

 magnetic waves should move at the same speed as light, 

 but they would have frequencies much lower than those 

 of light, with correspondingly longer wavelengths than 

 light waves possessed. The experimental verification of 

 Maxwell's theory was performed by Hertz in 1887: Hertz 

 showed that the rapidly oscillating electric charges in a 



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