169 



ducted. Either method allows of altering the sound at will, the 

 number of sounds and intensities, transmissible through a long, wide 

 pipe to the diaphragm, being indefinite. The tones embraced by the 

 thermotelephone are also a great manj', from the low% non-coalescent 

 tone of an interruptor to the high hissing-sound '). Likewise the 

 intensity of the thermotelephone-sound can be varied through artificial 

 appliances within far-extended limits. Selection occurs with the latter 

 appliance only as far as the peculiar tone of the air-chamber is con- 

 cerned, but when the air-cliamber is small — as is deemed advisable — 

 it is so high, that it may be left out of calculation. 



Both methods yield progressive soundwaves, whose energy is 

 constantly procured by the generator, and emerges through the point- 

 shaped orifice of 1 mm. If the latter is in circuit with an air-chamber, 

 through which the sound is conveyed to the measuring mirror, the 

 results vary roughly according to the size of the chamber. The 

 differences are markedly perceptible with an outlet of '/j vam. in 

 diameter. As original sound-generator may be used a telephone, 

 actuated by' an electrically driven tuning fork or a large powerful 

 organ pipe. 



^ 4. Investlgntlon of resonators. 



The mode of arrangement can also be easily applied to test reso- 

 nators. When a puncture (2 mm.) is made in the wall of the reso- 

 nator, right opposite to the mouth, the sound passing through it 

 may readily be directed on to Rayleigh's mii-ror via a canal of the 

 same bore '). It will be expedient, however, to arrest by means of 

 a very small plug of cottonwool or a piece of lint, the streams of 

 air escaping, like the sound, through the fine opening of the resonator. 

 Without this precaution the mirror will never be steady, not even 

 in a perfectly quiet environment. 



The sound thus emitted through the puncture, is made up of 

 progressive waves. By means of a long tube it can also be sent to 

 a comparatively long distance, provided that fresh acoustic energy 

 be constantly supplied through the orifice. The energy collected and 

 adjusted in the resonator, emerges via the line opening, as well as 

 through the wide orifice. A mirror, subjected to these progressive 

 waves, deflects, when the amplitudes are small, proportionally to the 

 amount of acoustic energy produced. Spherical resonators yield fairly 



') According to the assistant W. v. d. Elst the pitch agrees with the tone of 

 resonance of a Cg resonator (SOOO v. d.). 

 ") H. Zwaardemaker. Multiple resonantie. Ned. Tijdsch. v. Gen. 1913. II. p. 640. 



