90 Kansas Academy of Science. 



to set itself along the axis of the tube. This motion was balanced 

 by the action of a magnet upon the vane and held in position by 

 the field of the earth. E. Grimsehl used a similar arrangement. 



None of the methods so far described, however, with the excep- 

 tion of that of Weber, is of more than incidental interest. All de- 

 pend upon assumptions and have very limited possibilities. Prof. 

 Max Wien, in 1889, constructed his optical telephone. Upon the 

 diaphragm of a special receiver he placed a small mirror which 

 vibrated with the varying current in the magnet spools, thus giving 

 a measure of the amplitude of the vibration of the diaphragm. 

 Later, he placed the mirror directly upon a membrane actuated by 

 the vibration to be studied, without the intervention of the tele- 

 phone. If the tone under consideration is pure, that is, if it can be 

 represented by a simple sine function, the intensity in absolute 

 measure may be calculated from the amplitude thus found. For com- 

 plex tones, and those with which we have most to do are very com- 

 plex, this method is of no direct value. 



G. Stern, in 1891, used a special microphone to study the inten- 

 sity of sound in the various parts of a room. The method is merely 

 qualitative and gives us little of any value. 



Mr. B. F. Sharpe, working with Professor Webster, at Clark 

 University, estimated the intensity of sound as follows : One total 

 reflecting mirror of a Michelson refractometer is made small and 

 light and mounted upon a thin plate forming part of the wall of a 

 Helmholtz resonator. The objective of the observing telescope was 

 a lens mounted upon one prong of a tuning-fork of the same period 

 as the tone to be studied. A system of oblique bands was formed, 

 the inclination of which is a function of the intensity of the sound. 

 Here also the method is inapplicable to anything but a pure tone. 



As shown mathematically by Rayleigh, radiation must exert a 

 steady pressure upon a surface that opposes its advance. Nichols 

 and Hull demonstrated the existence of this pressure in the case of 

 light several years ago. More recently Altberg constructed an ap- 

 paratus consisting of a reflecting wall in which there was a circular 

 hole. Within this hole was a plunger supported upon one arm of 

 a torsion balance. In this way he succeeded in measuring the ac- 

 tual energy given out by a glass tube set in longitudinal vibration 

 by the friction of a rapidly rotating rubber of cloth wet with al- 

 cohol. With a sound so loud that he was obliged to stop his ears 

 while working with it he found a radiation pressure of 0.25 dynes 

 cm^. If this method could be applied to sounds of ordinary in- 

 tensity it would certainly be very valuable, but this noise was prob- 



