224 PHYSICAL SCIENCE 



of an electro-magnetic wave through free space 

 determined the relative magnitudes of certain 

 electric units, so that by comparing the values of 

 the units the velocity could be calculated. Experi- 

 ment showed that the velocity was the same as 

 that of light ; light became an electro-magnetic 

 phenomenon, and optical science a branch of elec- 

 tricity. Many years afterwards, Maxwell's great 

 work was confirmed by the direct experiments of 

 Hertz, who detected the existence, and measured 

 the speed of electro-magnetic waves, thus laying 

 the foundations on which the practical art of 

 wireless telegraphy is based. 



Details of the technical applications of science 

 are outside the scope of this work, but a short 

 description of the theory of wireless telegraphy 

 and telephony may not be out of place. 



The work of main theoretic interest was done 

 by Maxwell and Hertz, but much development by 

 other men was needed before their achievements 

 could be turned to practical account. The radia- 

 tion and reception of sufficient energy for signal- 

 ling at a distance was first made possible by 

 Marconi's introduction of the aerial wire, which is 

 used to emit the waves at one station and catch 

 them at another. 



Each single electric spark from an induction 

 coil consists of a few electric oscillations, rapidly 

 dying away. It was these damped oscillations 

 which were used by Hertz and in all the early 

 methods of wireless telegraphy. But nowadays 

 continuous waves, the vibrations of which are 

 maintained constant except when purposely 

 interrupted, are used. 



