A SELECTIVE HOT-WIRE MICROPHONE. 
417 
however, quite characteristic of the form of microphone under discussion, and cannot be 
attributed entirely to adventitious circumstances, such as the bending of the wire loops 
of the grid out of their normal positions. 
Another point which may be noted is that, since the disposition of the loops about the 
axis of the microphone is not symmetrical, a rotation of the microphone about its own 
axis produces a change in resistance. It follows from this that when observations 
such as those shown in fig. 13 are continued for values of 0 between 0 and — 7 r, the curve 
is not in general symmetrical about the line 0 = 0. 
It is clear from these results that the resistance of the grid depends, not only on the 
magnitude of the current of air by which it is cooled, but also on the direction of this 
current relatively to the plane in which the grid lies. This is due to the free convection 
from one part of the grid acting upon another part, and is essentially the same phenomenon 
as that used by J. S. G. Thomas in the construction of a “ Hot-Wire Inclinometer.’ * 
When the Amplifier method is used it is found, as stated previously, that the effect 
of tilting the microphone is to change the character of the sound heard in the telephones. 
As 6 is increased from 0 to - the fundamental note becomes gradually weaker, while 
* “ An Electrical Hot-Wire Inclinometer,” by J. S. G. Thomas, ‘ Proc. Phys. Soc.’, Loud., vol. XXXII., 
pp. 291-314 (1920). 
