The sensitive elements used in these tests are tungsten 
wires 0.0003L inch in diameter. and 0.20 inch in length. Similar 
wires used by Schubauer at the Bureau of Standards (4) had measured 
time constants of approximately 0.002 second, Freliminary measure- 
ments of the same wire in water exhibit time constants in the order 
of 50 to 100 microseconds, The difference is probably accounted for 
by the fact that the thermal conductivity of water is about 25 times 
greater than that for air. The present tests have been conducted 
without compensation for thermal lag. 
The hot-wire circuit shown in Figure 2 is identical with 
those described in Reference (1). From the theory of this circuit 
it is possible to measure the correlation coefficients of a turbu- 
lent stream by proper combination of the outputs of the two wires. 
Assuming that the velocity fluctuations are small, then the voltage 
fluctuations across each wire are proportional to’ changing velocities. 
By means of the reversing switch S, the voltages of the two wires 
can be added or subtracted before amplification. This combined 
output, amplified, can then be measured by a vacuum-type thermo- 
couple and microammeter. The reading of the meter is proportional 
to the mean square of the voltage input. If M, is the microammeter 
reading for the voltages added; and M,, that for the voltages sub- 
tracted, then 
M, = K(e,+e,)? K'(u, +u,)? and 
M, = K(e)-e,)* = K' (a) a5)” 
where @, and ep are the instantaneous voltages of the respective 
wires, and u, and uo the respective turbulent velocities at 
each. It cat be shoe by algebra that the correlation coefficient 
= ease. LP for isotropic turbulence. 
u 
M atMp 
For measurement of turbulent intensity, only one wire is 
required. It can be shown that the per cent of intensity is 
Vue = __2 Bw gt 
ir (hon 
where R, is the resistance of the wire at water temperature, 
ais the root-mean-square voltage fluctuation as measured 
by the calibrated microammeter, 
R is the mean operating resistance of the wire, 
i is the heating current, 
F is the slope of the static calibration of the wire, 
Sethe Pig 
