HOT WIRE METHOD 
In view of the tediousness of the oil-drop injection 
method, other, more direct techniques were required. The hot- 
wire anemometer has provided such a technique in wind tunnel tur- 
bulence measurements, Direct application of a wind-tunnel typ 
hot wire to water has been tried, and with a fair amount of success. 
Briefly, the theory of the hot wire is as follows. From 
the fundamentals of electricity it is a well-known fact that the 
resistance of a metallic wire is approximately a linear function 
of its temperature. With this in mind, L. V. King in 1914 (3) 
discovered the relationship between the resistance of a heated 
fine platinum wire, and the velocity of the cooling air stream in 
pure it was placed. He found that for the wire placed normal to 
the stream 
He 12R,2(a + b{¥)(e.-6,) 
where H is the rate of heat lost per unit length of the wire, 
i is the heating current, 
Re is the instantaneous wire resistance, 
? is the velocity of the stream, 
6 is the temperature of the wire, and 
@, is the temperature of the stream. 
This relationship has since been successfully applied to measuring 
steady and fluctuating air velocities. 
There are two-principal methods of operating the hot 
wire, In the first, the temperature, i.e., the resistance, of 
the wire is maintained at a constant value. The heating current 
required to do this is then a function of the velocity. In the 
second method, the heating current is maintained constant, and 
the resistance allowed.to fluctuate with the velocity changes. 
This second technique has been employed in the turbulence experi- 
ments in water. 
For a repialy fluctuating velocity field, the voltage 
variations of the ideal hot wire-.should be in phase with the 
velocity, Failure of a hot wire to do this arises from the 
existence of its time constant, M, which depends upon the size 
of the wire, and the operating conditions. A value of M differ- 
ent from zero results in a voltage output lagging in phase behind 
the velocity fluctuations, and a voltage output whose amplitude 
is diminshed with increasing velocity frequency, f, in the ratio 
. Both of these effects, reduction in amplitude, and 
phase lag, can be corrected by proper compensation eircuits in 
the amplifiers used with the instrument. 
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