the Filar Anemometer 



87 



A regular speed of the wire favours the occurrence of tones, 

 particularly those which are awakened by resonance. 



5. The beneficial effects obtained in § 3, as the result of 

 filing, induced me to try a form of microphone in which the 

 sensitive contacts are produced by a pulverulent conductor*. 

 After many attempts I finally succeeded in devising an 

 arrangement which is far superior to the forms just described, 

 inasmuch as the thunder is altogether absent, and other 

 extraneous noises are excluded. The sounds heard in the 

 telephone are not loud, but clear and flute-like, and the range 

 of pitch obtained is enormous. I passed almost from utter 

 cessation of motion to the highest speed which I could give 

 the whirling arm, and heard sounds quite inaudible to the 

 unaided ear. 



The annexed figure (3) shows the form which seemed to 

 give the best results. Here aa and bb are two tin-plates about 



Fig. 3. — Granular Microphonic Contact for Air- Wire. 



h 



6 



8 



a 



d- 



a 



f J> \ 



la-v 



f 



0*016 cm. thick and 8 cm. in diameter, held apart by an 

 interposed flat ring of hard rubber dd about 0*1 cm. thick or 

 less. An elastic band of soft rubber ee^ is stretched quite 

 around the circumference and secures the plates by pressure 

 against the hard rubber ring. The upper plate is centrally 

 perforated, and a tube, g, about 1 cm. wide and 2 cm. long 

 communicates with the perforation. A stylus c, is soldered 

 to the middle of the lower plate and passing axially through 



* Originally devised by Hunning-, I believe. See Preece or Du Moncel, 

 I. c. 



