He employed as a detector of this wave a simple, nearly closed circuit of wire called a Hertz resonator, but it was subsequently discovered that the metallic microphone of D.
From a pinhole burner, fed through a cavity C, one side of which is closed by a membrane m; on the other side of the membrane is another cavity C', which is put into connexion with a source of sound, as, for instance, a Helmholtz resonator excited by a fork of the same frequency.
Interesting results are obtained by singing the different vowels into a funnel substituted for the resonator in the figure.
If the harmonic corresponding to the resonator is present its tone swells out loudly.
But the same resonator will be appreciably though less affected by waves of frequency differing slightly from its own.
§ 388) thought that if the intensity of response by a given resonator in the ear to its own tone is taken as i, then its response to an equally loud tone a semitone different may be taken as about.
This rate of loss for each amplitude was determined (i) when the fork was vibrating alone, and (2) when a resonator was placed with its mouth under the free ends of the fork.
The difference in loss in the two cases measured the energy given up to and sent out by the resonator as sound.
The amplitude of the fork was observed when the sound just ceased to be audible at 27.4 metres away, and the rate of energy emission from the resonator was calculated to be 42 .
He used a spherical Helmholtz resonator resounding to the tone to be measured.
From subsidiary experiments (for which the original memoir must be consulted) the pressure variations within the resonator could be calculated from the movements of the plate.
The open orifice of the resonator was then exposed to the waves from a source of its own frequency.
Helmholtz's theory of the resonator (Rayleigh, Sound, ii.
§ 311) gives the pressure variations in the incident waves in terms of those in the resonator, and so the pressure variation and the amplitude of vibration in the waves to be measured were determined.
Physik, 1906, 21, p. 131) compared the indications of Wien's resonator manometer with those of V.
There is not much difficulty in detecting the difference tone by a resonator if it is held, say, close to the reeds of a harmonium, and Helmholtz succeeded in detecting the summation tone by the aid of a resonator.
He found that when the resonator was placed in certain positions with regard to the oscillator, small sparks were seen between the micrometer balls, and when the oscillator was placed at one end of a room having a sheet of zinc fixed against the wall at the other end, symmetrical positions could be found in the room at which, when the resonator was there placed, either no sparks or else very bright sparks occurred at the poles.