[boyle] 



COM PRESSION AL WAVES IN METALS 



299 



TO AMPLIFIEf^ AND 

 TELEPHONE 



Ô 



PRIMARY 

 AOJUSrABLE 



MICROPHONE 



Fis. 2 



fact by having the capacity C accurately calibrated as a Hertzian 

 wave-meter, the pitch of the note from the rod could easily be ascer- 

 tained. This was the case in the experiments. 



It was found that when the note emitted by the rod was of audible 

 pitch, e.g., of 10,000 or 15,000 vibrations per second, both this funda- 

 mental note and the beat note could be heard in the telephones, but 

 when the emitted note was above the limit of audibility, above 20,000 

 vibrations per second (about), the beat note only could be heard. 



In this way the beat could be detected with a frequency of emitted 

 note up to about 42,000 vibrations per second, i.e., from a rod six cms. 

 long. Above this frequency no beat could be heard. Hence the train 

 of waves for each blow of the hammer on the clamped bar was sus- 

 tained enough, even at this high frequency, to give detectable beats 

 with a continuous source. 



The arrangement can be used as a simple method of finding the 

 elastic modulus (£) of a bar of metal. For, finding the pitch in) 

 of the note emitted by the struck rod as just described, and the length 

 of the rod (/) being half a wave-length, we have the relation 



V= 1/ — = n.2l 

 ^ P 



:.E=p{2ln)\ 



where p is the density of the material. 



It is hoped to develop the experiment further. 



