512 



Mr. W. H. Preece. On the Conversion of [Mar. 10, 



9. A delicate microphone was fixed in varions ways on the case, 

 fig. 3. Although the sounds emitted in the hearing tube were as 

 intense as indicated in Experiment 1, scarcely any perceptible effect 

 was detected on the microphone. Had the disk sensibly vibrated, its 

 vibrations must have been taken up by the case. A microphone never 



| fails to take up and magnify the minutest mechanical disturbances. 

 It was thus evident that the disk did not play a prime part in this 

 phenomenon, but it appeared as Professor Hughes suggested, that the 

 result might be due wholly to an expansion and contraction of the 

 t air contained in the air space c, fig. 3. 



An entirely new set of cases and disks were now constructed, but of 

 the same dimensions. 



10. With a new clean case and an ebonite disk, the sonorous effects 

 were feeble, viz., 15 on the sonometer scale. 



11. If, however, a lens (d) were placed close in front of the ebonite 

 disk a, (fig. 5), the sonorous effects were magnified considerably, rising 

 at once to about 40. 



12. A case was constructed similar to that shown in fig. 5, but with 

 a tube at the side, as well as one at the back, communicating with 

 the air cavity in front of the disk, as well as that behind, and whether 

 one listened at one place or the other, the effect was equally good, 

 indicating that the results were most probably due to the expansions 

 of the contained air. 



13. When the one tube was stopped and opened by a cork while the 

 other tube was used for listening, the quality of the sounds varied 

 more than the intensity, but there was distinct evidence of variation. 



14. The ebonite disk of this case was now fitted with an extremely 

 delicate microphone, which in this case gave good indications upon 

 the telephone, but whether the vibrations were the results of the 



