184 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1919. 
After various arrangements were tried the current for the telephone 
transmitter was obtained by shunting this transmitter about filaments 
of two vacuum tubes which gave approximately the correct voltage 
for operating this transmitter. The 24-volt and 275-volt power is 
obtained from a double voltage direct current generator which is 
driven by an air fan, the complete generator being mounted on one 
of the struts of the landing gear of the airplane and hence being 
directly in the propeller blast. The most difficult problems involved 
in the design of the generator were those related to the excessive 
speed variation and the extremely high maximum speed at which 
the generator was required to operate. The air speed of the plane 
being subject to extremely wide variations and the air fan rotational 
speed varying in 
almost exactly the 
same ratio, it was 
necessary for the 
generator to fur- 
nish its normal 
voltages over a 
speed range of 
4,000 to 12,000 rev- 
olutions per minute. 
This required the 
use of a special 
voltage regulator, 
the circuits of 
which are shown in 
figure 1. This reg- 
iat era ulator depends for 
Fig. 1.—Circuit diagram, wind-driven generator. its operati on upon 
the relation existing between the filament current and the electron 
emission from the filament in a special vacuum tube which has been 
designated as a regulator tube. At the lower limit of the operating 
speed range, namely, 4,000 revolutions per minute, the filament of 
this regulator tube operates at a temperature which gives practically 
no emission of electrons. As the speed of the generator increases 
above this value the voltage rises, and accordingly the temperature 
of the regulator tube filament increases. This results in an extremely 
rapid increase in the number of electrons emitted by the regulator 
tube filament, which in turn causes a rapid increase in the current 
flowing through the differential field and the plate circuit of the 
regulator tube, as shown by the characteristic curves of figure 2. 
This current tends to reduce the total magnetization of the generator, 
and as a result, the voltage regulation is maintained within extremely 
close limits, 
