830 The Outline of Science 



wave is called an electromagnetic wave. It is a double system of 

 lines of magnetic and electric force moving onward as we have 

 described. These electric waves form the foundation of wireless 

 telegraphy. 



These waves are sometimes described as "ether waves" which 

 are propagated more or less in all directions. As we have said 

 elsewhere (see "Matter, Ether, and Einstein," p. 288), there is 

 less justification than formerly for assuming the existence of 

 the ether. It is merely a hypothesis, and no observations have 

 so far enabled the physicist to determine finally that ether exists. 

 Some maintain its existence, others have abandoned the hypo- 

 thesis. The former maintain that ether is a universally diffused 

 medium and that it forms the connecting link by which forces 

 like radiant energy are transmitted across space. It is assumed 

 that light and heat rays are propagated by ether vibrations. 

 Other physicists conceive that a great part in the transmission 

 of energy is played by corpuscular radiators, the corpuscles being 

 the electrons we have spoken of. Sir William Bragg says, "it 

 seems that we must admit the importance of each view, and to a 

 certain extent we can accurately define the part that each must 

 play." We need not for the purpose of explaining our subject 

 consider the relations between the energy carried by ether waves 

 and the energy carried by electrons. There are distinctive fea- 

 tures of each of these two forms of radiation, and they may have 

 some extraordinary connection which has not been explained. 



Every time an electron changes its motion it sends out the 

 electromagnetic waves. In the case of the electrons shot out in a 

 Crooke's tube, for instance, their sudden stoppage by the walls of 

 the tube, or by a plate put in their path, produces those very 

 short electromagnetic waves we call X-rays (see p. 253). Now 

 if we can cause a large number of electrons all to change their 

 motion in the same way, the effects due to each will unite to pro- 

 duce a powerful electromagnetic wave. Such a wave could be 

 detected, by suitable electrical appliances, at a considerable dis- 



