METEOROLOGICAL EQUIPMENT FOR SHORT WAVE 233 
METEOROLOGICAL 
ELEMENTS 
FREQUENCY 
GENERATOR 
ELEMENT SWITCHING 
AND 
SERIES 
LIMITER 
RECORDER 
Ficure 4. Basic components of amplifier-recorder. 
tundamental that voltages should be measured across 
the hygrometer or temperature elements. Auy lock-in 
device must be inherent in the equipment as remote 
from the voltage appearing across the elements. This 
principle has been incorporated in the design of the 
equipment. 
Since the electrolytic hygrometer element can be 
designed so that it will not polarize under direct cur- 
tents approaching 100 pa, it appears desirable to design 
the recording equipment for possible adaptation to 
this type of element. In other words, the amplifiers 
must be capable of handling direct currents as well as 
alternating currents. 
CaBLe Error 
If the standard temperature and hygrometer curves 
are used originally for calibrating the recorder, it is im- 
portant to note that the cable resistance will introduce 
a positive error. For average Southwest Pacific climate 
and sounding heights up to about 3,000 ft, the positive 
temperature error is roughly 1.5 F' while the positive 
hygrometric error is about 5 per cent RH. 
These errors, unfortunately, cannot be compensated 
without complete recorder calibration at the outset or 
by mathematically adjusting the standard calibration 
curves. Thus, since the cable is a fixed resistance, the 
standard curve can be recomputed to allow for any 
fixed cable resistance, with the result that no error 
will be introduced into the recorder. 
In consideration of the above requirements, it will 
be necessary, in adapting the standard U. S. Weather 
Bureau electric hygrometer and temperature elements, 
to provide (1) a means of developing a stable low- 
frequency voltage across the elements, (2) to switch 
from one element to another in measuring the voltages 
across these elements, (3) to amplify such voltages, 
+150 
HYGROMETER 
ELEMENT, 6SJ7 
> INPUT 
TEMPERATURE 
ELEMENT 
(4) to provide a means of controlling the sensitivity 
and range of the recorder, and (5) to supply the output 
of the amplifier to a 0 to 500 microammeter auto- 
matic recorder. 
ELECTRONIC AMPLIFIERS 
The basic components of the amplifier-recorder are 
shown in Figure 4. The frequency generator operates 
at 10 c and is composed of three units, (1) a phase- 
shift oscillator, (2) a paraphase amplifier, and (3) a 
cuntrollable push-pull output amplifier. 
The amplifier-recorder unit is composed of (1) a 
series limiter, (2) a cathode follower, and (3) a two- 
story amplifier, as shown in Figure 5. For the sake 
of simplicity, the automatic switching device that 
changes the current from hygrometer to temperature 
element has been shown schematically. The switching 
takes place at any rate between approximately 1.0 to 
0.1 ¢; this rate is not critical. 
Since the amplifier must be able to handle either 
direct or alternating current, the balanced two-story 
amplifier has been constructed, wherein the top tube 
is the plate load for the lower tube of the two-story 
amplifier. The output of this amplifier is approximate- 
ly equal to one-half the amplification factor of the tube. 
As shown in Figure 5, this amplifier is fed by a 
cathode follower which has impressed upon it from the 
series limiter only the positive peaks of the a-c voltage 
drop across the hygrometer or temperature element. 
The voltage across the elements can be as low as %4 
to % v, depending upon the sensitivity adjustment of 
the amplifier. It will operate with voltages as high 
as 30 to 60 v across the elements. 
RepuctinG AMPLIFIER OUTPUT TO GROUND 
The amplifier output is at one-half the positive volt- 
+150 
OUTPUT TO RECORDER 
Vicure 5. Automatic recorder circuit. 
