410 



Mr. A. G. Webster 



[June 10, 



This sensitiveness can be demonstrated by projecting the coloured 

 interference fringes on a screen and singing faintly in a remote part 

 of the room, when the fringes will disappear. Using the telescope 

 end of the apparatus, the instrument will indicate the sound of a 

 tuning-fork when one can scarcely hear it. It is obvious that the disc 

 may be made the diaphragm of a telephone and thus increase its 

 sensitiveness. In fact, Prof. King has used such a telephone to 

 record wireless messages with great success. He has also invented 

 another sort of tunable diaphragm composed of a stretched steel 

 membrane with compressed air behind it, which enables it to be 

 tuned continuously, but over a smaller range. 



Fig. 3. — Feokt View of Pho^o^ietee with Akxulae Opening. 



I now come to the source of sound — the phone. This has been 

 reduced to a reversed form of the phonometer. The disc is driven 

 1 »y an interrupted or alternating current by means of electromagnets, 

 and tuned like the phonometer. Its excursion is measured by a 

 powerful microscope, and the emission of sound is known in absolute 

 measure. It is now driven by a triode valve tube, in the manner 

 suggested by Prof. AY. H. Eccles, of Finsbury Technical College, 

 London, for a tuning-fork. This has been worked out for me by 

 Dr. Eckhardt at the Bureau of Standards in Washington. 



The third part of the investigation involves a determination of 

 the coefficient of reflection of the ground. The phone is set at a 

 convenient height, and the phonometer at a convenient distance. 



