390 
DR, W. S. TUCKER AND MR. E. T. PARIS ON 
pitches. The highest note with which we have experimented hitherto was one of 512 
vibrations per second. 
During the course of a large number of experiments with various types of sound- 
collectors and transmitters, we have found this selective form of hot-wire microphone 
to be of great assistance. It is very simply constructed and easily manipulated, and 
for many purposes the only electrical circuit needed is a Wheatstone’s Bridge. If, 
however, it is desired to use aural methods, or if the sound to be observed is exceedingly 
faint, it is necessary to amplify (by means of thermionic valves) the electrical effects 
occurring in the microphone. When amplification is used it is possible to detect and 
render audible a pure tone which is quite inaudible to the unaided ear. 
This form of microphone provides us with a very convenient instrument for comparing 
the efficiencies of various forms of sound-collector—particularly when these are considered 
in relation to the wave-length of the sound employed. It can also be used for deter¬ 
mining the distribution of intensity at the focus of an acoustical lens or mirror, and 
(what is very important in many practical problems) the manner in which sound is 
diffracted by obstacles of various shapes and sizes. In addition to these and similar 
experiments, the microphone can be employed to estimate the relative strengths of 
the harmonics in an impure sound such as that produced by the usual form of Seebeck’s 
siren. Some examples of these applications of the microphone will be given in the last 
section of this paper. 
So far as we are aware, there is no other instrument of a selective character which 
could be used for making observations of the kind indicated above. In nearly all 
cases where attempts have been made to measure or analyse sounds, the instruments 
employed have depended on the setting in vibration of some form of diaphragm. Such 
instruments are generally insensitive to notes of moderately low pitch, and are, moreover, 
easily disturbed by vibrations communicated through the mounting of the diaphragm. 
For this reason methods of amplification are often of little service if the mountings are 
to be moved during the experiment. 
The hot - wire instrument here described seeks to avoid this disadvantage by 
measuring directly vibrations which are set up in the air itself, but the displacements 
in progressive waves are so extremely small that they have been increased by resonance. 
This employment of resonance naturally limits the scope of the microphone (so that it 
cannot, for example, be employed for telephony), but it has the advantage not only of 
magnifying the sound to be recorded, but also of isolating from a complex sound the 
particular tone which it is desired to measure. 
The closed vessel with a single orifice (in which the platinum wire grid is mounted) 
forms the well-known Helmholtz resonator. The advantages possessed by this form of 
resonator are 
(1) That the resonance is sharp. 
(2) That the overtones are all relatively high. 
