AND MODK11X PHYXIfti 218 



Similarly when electrical vibrations full tin a re- 

 Hector, a largo flat surface of metal, for example, 

 stationary vibrations duo to the interference between 

 the incident and reflected waves are produced, and 

 these give rise to electrical nodes and loops. The 

 position of such nodes and loops can be found by the 

 use of Hertz's apparatus, or in other ways, and hence 

 the length of the electrical waves can be found. The 

 existence of tho nodes and loops shews that the 

 electric effects are propagated by wave motion. The 

 length of tho waves is found to be definite, sinco the 

 nodes and loops recur at equal intervals apart. 



If it. bo assumed that, the frequency is known, the 

 velocity of wave propagation can be determined. 

 Hertz found from his experiments that in air the 

 waves travelled with the velocity of light It appears, 

 however, that there were two errors in the calculation 

 which happened to correct each other, so that neither 

 tho value of the frequency given in Hertz's paper 

 nor the wave length observed is correct 



By modifying the apparatus it was possible to 

 measure the wave length of the waves transmitted 

 along a copper wire, and hence, again assuming the 

 period of oscillation, to calculate the velocity of wave 

 propagation along the wire. Hertz made the experi- 

 ment, and found from his first observations that the 

 waves were propagated along the wire with a finite 

 velocity, but that the velocity differed from that in 

 air. The half-wave length in the wire was only about 

 2*8 metres ; that in air was about 4'5 metres. 



Now, this experiment afforded a crucial test 

 between tho theories of Maxwell and Von Hclmholtz 



