195 



The formulae show, that in order (o attain a high degree of 

 sensitiveness the size and the weight of tlie mirror should be as siiiall as 

 possible. I also found, that we can do without the magnet if we 

 replace the quartz fibre by a flattened Wollaston-wire ^). The sen- 

 sitiveness can moreover be considerably increased by[>.ondncting the 

 sound to the mirror through a short tube whose width and length 

 are in the ratio of at least 2 : 5. The mirror is placed just in front 

 of the aperture of the tube. The diameter of the mirror should not 

 be much longer than the width of the aperture, so as to cause the 

 sound-wave to bend completely round the disc. The position of the 

 mirror is effected entirely by the pulses falling upon it on both 

 sides, not by the sound pressure, which is too feeble for it. 



The instrument discussed just now, enables us to arrange the 

 mirror so as to prevent resonance. Only the afferent tube may pos- 

 sibly resonate. Its length is 6 cm. and its width is a finger's 

 breadth. On this account I chose a mirror of 5 mm., slightly to the 

 detriment of the sensiti\'eness. Considered as ah open organ pipe 

 the peculiar tone of the tube lies somewhere near e^ and is beyond 

 the speechzone. 



Aided by such an instrument it is quite easy to determine the 

 reinforcement of sound and the sound selection of micro-telephone- 

 apparatus. 



First of all we intend to examine technical apparatus such as the 

 common telephone or that used by people hard of heaiing. Here 

 element, microphone and telephone are in circuit with each other 

 (induction apparatus being inserted only foi- long-distance telephony). 

 The conducting tube replaces the car. As close as possible, free 

 from all foreign sound disturbances, the telephone is placed before 

 the tube that transmits the sound to the mirror, which is placed 

 obliquely in the plane of the soundwave. No sooner does the 

 telephone sound than the mirror shows deviation, and a beam of 

 light directed onto it, is caught up on the scale placed at some 

 distance. The amplitude indicates the torsion of the fibre, i. e. the 

 force that was counteracted by the energy of the sound, while the 

 tone continued. 



The following illustrations represent the amplitudes generated bj' 

 a row of almost uniform stopped pipes, half lone for halftone, with 

 the aid of telephone and microphone fed by two volts. 



1) Heraeus sent us on application such a wire of 8 , thickness. The attendant 

 Stellema procures still more suitable wires by flattening round VVoUaston-wiics 

 with a hammer. 



13* 



