298 THE ROYAL SOCIETY OF CANADA 



The rims, separated from one another by thin mica rings, were drawn 

 closely together by screwing the top rim to the lower with fine screws 

 through insulating ebonite bushings. In this way the steel discs were 

 electrically separated from one another, and the whole enclosed a 

 small water-tight space. On the insides of the discs, attached at the 

 centre of each, were thin, carbon, microphone buttons, about 0.3 cms. 

 in diameter, between which were packed the fine, spherical carbon 

 granules of the microphone; the rest of the space inside was filled 

 with a soft felt pad holding in the granules of the microphone. No. 

 doubt any microphone, acting by the pressure variations of sound 

 waves, could also do, but there is an advantage in a microphone of 

 the type here used ; for the small, stiff, steel discs have a natural period 

 corresponding to a high, inaudible note, and therefore the microphone 

 has sensitivity in the upper ranges of frequency, while it tends to 

 shut out the effect of ordinary low pitched sounds. 



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The microphone was connected into an electric battery circuit 

 containing the adjustable primary of an oscillation transformer. The 

 secondary, L, of this adjustably coupled transformer, with a variable 

 condenser, C, was arranged in an ionic- valve circuit to give heterodyne 

 "beat" reception; and the "beat" note was amplified by a three- 

 valve, audio-frequency, amplifier. This accompanying diagram, Fig. 

 2, shows the electrical arrangements. 



The high frequency waves produced by the struck metal rods 

 could be picked up almost anywhere in the liquid by this microphone. 

 The oscillations in the rods were found to be so sustained that the 

 electric oscillations, transformed from them, in the electric circuit 

 beat with the continuous electric oscillations from the ionic valves, 

 and the beat note was distinctly heard in the amplifier telephones. 

 The beat note could be varied in pitch in the usual way by varying 

 the adjustable capacity C, and the two gamuts of beat notes — one on 

 each side of the null point or point of unison — were present. In 



