A SELECTIVE HOT-WIRE MICROPHONE. 
420 
The image, however, becomes ill-defined for angles of incidence exceeding 20 degrees, 
and the tendency is then to obtain a train of maxima of nearly equal intensity when 
the microphone moves from one edge of the disc to the other. 
The above illustrations give some indication of the manner in which the microphone 
can be applied to the investigation of acoustical problems. Many of the measurements 
described in this paper were made in a locality where a certain amount of noise was 
constantly occurring, but which the microphone, being highly tuned, failed to record. 
Tuned reception for sound has all the advantages of tuned reception in “ wireless 
in distinguishing and magnifying faint signals. 
A distinct limitation of this microphone is its restriction to the measurement of low- 
frequency sounds, but it is hoped to devise a microphone of the Hot-M ire type sufficiently 
sensitive to deal with speech frequencies. 
§ 10. Summary. 
A new form of Selective Hot-Wire Microphone is described, consisting of an electrically 
heated grid of fine platinum wire placed in the neck of a Helmholtz resonator. The 
effect of a sound having the same frequency as that natural to the resonator itself 
is to produce an oscillatory motion of the air in the neck, which in turn causes a change in 
resistance of the platinum wire grid. The total resistance change comprises a steady fall 
3 N 
VOL. CCXXI.—A. 
