82 



THE ROYAL SOCIETY OF CANADA 



In order to eliminate the effect of aerial vibrations a special 

 microphone having the sensitive capsule enclosed in lead, and having 

 at the same time a high ratio of mechanical leverage, was designed for 

 the purpose of detecting rock vibrations. It was, unfortunately, not 

 found possible to execute or test this design before it was necessary 

 to discontinue the experiments. 



Section 4. — Experiments on the Magnification of Telephone 



Currents. 

 It was quickly recognized in the experiments just described that 

 there would be a great advantage in having some means of magnifying 

 the feeble electric currents which are reproduced as sound in the 

 telephone receivers connected to the oscillator or to the microphones. 

 Especially is this the case when it is recognized that the telephone 

 is an extremely inefficient instrument, its use as a sensitive detector of 

 feeble electric currents being successful largely owing to the extreme 

 sensitiveness of the ear to sound. 



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Figure 2 



For this reason experiments were carried out on the use of the 

 hot-filament "amplifiers" now generally employed in wireless tele- 

 graphy and telephony. The type of amplifier available was a small 

 De Forest "Audion" designed for amateur use (single unit, type 

 4J5). It was quickly discovered that the best conditions for sensitive- 

 ness consisted in the use with the audion, of a high-ratio open-core 

 transformer as shown in Fig. 2. The microphone circuit, or the 

 oscillator armature, is connected into the primary circuit of the 

 induction-coil, while the secondary is connected to those terminals 

 of the audion, usually connected to the aerial when employed in radio- 

 telegraphy, i.e., across the tantalum filament and the grid. As the 

 effective resistance of the ionized gas across this gap is estimated 

 to be between 10,000 and 30,000 ohms, it is evident that the size 



