406 
DR. W. S. TUCKER AND MR. E. T. PARIS ON 
of attention from physicists. The most complete investigation available is that contained 
in the well-known paper by L. Y. King.* King shows that, for fine wires in air-currents 
of low velocity, the heat-loss per cm. is given by an equation of the form 
H = aOjlog^, 
where a and /3 are constants, 0 O is the excess temperature of the wire above its 
surroundings, Y is the velocity of the air-current, and d the diameter of the wire. 
This equation is theoretically applicable whenever 
Yd < 0-0187, a condition which is amply fulfilled in the 
present case with d = 0 -0006 cm. and V not greater 
than 6 cms. per second. We have not, however, been able 
to adapt this equation in any way which leads to useful 
results in the case of the Hot-Wire Microphone. It may 
be remarked that both the diameter of the wire and the 
magnitude of the air-currents with which we are concerned 
are considerably smaller than those used in the experi¬ 
ments of King, or of other investigators to whose work 
reference will be found in King’s paper. In view of this 
mounting, it was thought desirable to determine experi¬ 
mentally the relation between R and U for such small 
values of IT as are likely to be required to account for 
the behaviour of the grid under the influence of alter¬ 
nating air-currents. 
The arrangement of the apparatus used in the experi¬ 
ments is shown diagrammatically in fig. 9. A micro¬ 
phone grid is mounted in the holder at A. The interior 
of the small brass container (B), carrying the microphone 
and holder communicated by means of the short tube (C) 
with the reservoir (D), which was partly filled with water. 
A current of air could be produced past the microphone 
grid by opening the tap (T), which allowed the water in D 
to escape through the tube (E) into a second reservoir 
from 4 to 5 feet below I). A current in the reverse 
direction could be produced by allowing water to siphon into D from another reservoir 
at a higher level. 
The average velocity of the air-current passing the grid was deduced from a knowledge 
of the area of the aperture in which the grid lies, the area of the cross-section of D. 
Fig. 9. 
* ‘ Phil. Trans.,’ A, vol. 214, pp. 373AL32 (1914). 
