Measuring the Intensity of Aerial Vibrations. 



187 



upon the mirror at an angle of 45°, and, after reflection, 

 escapes from the tube through a glass window at E. It then 



B A 



1 



\ 



D 



H 





\ 





E 





F <=: 





falls upon a lens F, and throws an image of the slit upon a 

 scale G-. At a distance DH, equal to DO, the tube is closed 

 by a diaphragm of tissue paper, beyond which it is acousti- 

 cally prolonged by a sliding tube I. 



AY hen the instrument is exposed to sounds whose half wave- 

 length is equal to C H, H becomes a node of the stationary 

 vibrations, and the paper diaphragm offers but little impedi- 

 ment. Its office is to screen the suspended parts from acci- 

 dental currents of air. At D there is a loop ; and the mirror 

 tends to set itself at right angles to the tube under the influ- 

 ence of the vibratory motion. This tendency is opposed by 

 the magnetic forces; but the image upon the scale shifts its 

 position through a distance proportional to the intensity of 

 the action. 



As in galvanometers, increased sensitiveness may be ob- 

 tained by compensating the earth's magnetic force with an 

 external magnet. Inasmuch, however, as the effect to be 

 measured is not magnetic, it is better to obtain a small force of 

 restitution by diminishing the moment of the suspended 

 magnet, rather than by diminishing the intensity of the field 

 in which it works. In this way the zero will be less liable to 

 be affected by accidental magnetic disturbances. 



So far as I have tested it hitherto, the performance of the 

 instrument is satisfactory. "What strikes one most in its use 

 is the enormous disproportion that it reveals between sounds 

 which, when heard consecutively, appear to be of the same 

 order of magnitude. 



June, 1882. 



