Chemical and Physical Papers. 89 



a fairly sensitive microphone. The ordinary transmitter is a de- 

 vice on the same principle. A number of years ago Wm. Weber 

 suggested the use of the variable current thus produced as a meas- 

 ure of sound intensity. The currents in the secondary circuit of a 

 local battery telephone set are so exceedingly small, however, that 

 there has until recently been no reliable instrument to measure 

 them, though several have been proposed. Even now it is not cer- 

 tain that the instruments suggested for this purpose will be suc- 

 cessful. This method will be touched upon later. 



In 1878 and 1881 Vierordt published several papers in which he 

 describes methods of comparing sound intensities by the distances 

 to which they were audible. He found that the law of inverse 

 squares does not hold as in the case of light, but that the intensity 

 decreases as the inverse first power of the distance. Here he de- 

 pends upon the ear, which is very unreliable, especially at the 

 threshhold of sensation. Rayleigh has found that, under certain 

 circumstances, no relation seems to exist between the intensity of 

 the stimulus and that of the resulting sensation. Certainly the ear 

 is much more sensitive to sounds of some pitches than to others. 

 Even if the method of Vierordt were trustworthy it could not well 

 be used, as considerable space must be had out of doors, and quiet. 



Next in the point of time came the suggestion of Overbeck. He 

 used a microphone with but two carbon contacts, the one resting 

 lightly on the other and the whole upon a resonance board. He 

 showed that, to a certain degree of approximation, the increase of 

 the resistance of this arrangement, when acted upon by a sound vi- 

 bration, was proportional to the intensity of the sound falling upon 

 it. It was therefore put into one arm of Wheatstone's bridge, a 

 balance obtained when the microphone was at rest, and the de- 

 flection of the galvanometer produced by a sound wave falling upon 

 the apparatus measured. This was useful only for sounds of mod- 

 erate intensity and for the range of pitch c to c'. 



Dvorak describes a number of contrivances depending upon the 

 repulsion of resonators and of light aluminum vanes perforated 

 with holes larger on the one side than on the other. In a similar 

 way Professor Wood, of Baltimore, has made a sound pedometer, the 

 speed of which depends upon the intensity of the tone, but which 

 cannot be used as a measure of it. Lord Rayleigh suggested placing 

 a light mica vane within a tube at an angle of forty-five degrees to 

 the axis of the tube. At the open end of the tube was an elastic 

 diaphragm at the node of a standing wave of the sound to be 

 studied. When the sounding body was in action the vane tended 



