Stevens — Sympathetic Vibrations of Small Amplitude. 231 



Akt. XXVI. — The Detection of Sympathetic Vibrations of 

 Small Amplitude by Interference Methods ; by J. S. 

 Stevens. 



Duking the past few years a number of attempts have heen 

 made in the Physical Laboratory of the University of Maine 

 to apply interference methods to the detecting of sympathetic 

 and forced vibrations of small amplitude. The sensitiveness 

 of the experiment has heretofore prevented the obtaining of 

 any satisfactory results. During the past semester the exper- 

 iment was assigned to Mr. R. M. Holmes, an assistant in the 

 department, and a beginning was made toward solving the 

 problem. It is well known that if two tuning forks of the 

 same pitch are mounted side by side, the vibrations of one fork 

 will cause sympathetic vibrations in the other. These may be 

 detected by the sense of hearing, or if a small mirror is attached 

 to the prong of the second fork a spot of light may be made 

 to trace its path on a screen. The object of the work described 

 in this note was to detect the presence of sympathetic vibrations 

 of extremely small amplitude. A mirror was rigidly attached 

 to the prong of one fork, and this mirror was used as the ad- 

 justable mirror of a Michelson interferometer. Another fork 

 was adjusted so that it had nearly the same pitch as the first 

 fork, and this was used as the exciting source. When the two 

 forks were so nearly alike in pitch that no beats could be heard, 

 the exciting fork was set in vibration and held near the fork 

 which had the mirror attached. There was no contact between 

 the two forks, and the vibrations of one passed through the air 

 directly to the other. In this case vibrations were readily 

 detected by rhymic motion of the fringes whenever the fork 

 was struck. When the exciting fork was weighted, so that it 

 had a velocity of 2606 vibrations per second (the pitch of the 

 mirror fork being 265), it was possible to produce vibrations 

 in the second fork. To do this, however, it was necessary to 

 give the first fork a great amplitude and hold it very close to 

 the second fork. When its pitch was changed to 258*2 no 

 vibrations could be detected. 



It is suggested that here is a field of considerable interest. 

 If the experiment were performed in a chamber from which 

 the air could be exhausted, the relation between the power of 

 the air to carry sound-waves and its density could be deter- 

 mined. The substitution of other gases for air would prove an 

 interesting piece of work. The sensitiveness of the apparatus 

 used is illustrated by various tests which were made by the two 

 forks. The interferometer was mounted on a slate slab sup- 



