228 RADIO WAVE PROPAGATION EXPERIMENTS 
The Gregory humidiometer’* uses a lithium chlo- 
ride solution soaked in a clean cotton cloth. The re- 
sistance of the element changes from over 100,000 
ohms at 30 per cent RH to as little as 50 ohms at 100 
per cent RH. It undergoes pronounced aging during 
the first several days and then remains sensibly con- 
stant for a number of weeks. The instrument is in an 
experimental stage and is at present being tried out at 
the Rye towers in Sussex (see page 229). 
Circuit Design for Resistor Elements 
Thermocouples or thermopiles are commonly used 
in a conventional bridge circuit. In connection with 
the electrolytic and ceramic type of resistance ele- 
ments, circuits have recently been developed that in- 
clude certain features novel in the technique of atmos- 
pheric measurements. 
Tn the equipment developed by Washington State 
College** the standard radiosonde temperature ele- 
ment was originally used, fut in a more recent type 
they have combined the Sanborn temperature element 
and the radiosonde electrolytic humidity element. The 
electric equipment (Figure 1) consists of a dry cell 
with potentiometer supplying about 14 volt, two 
double-pivot microammeters, one in series with each 
of the elements, and a 6-volt d-c motor. The relay re- 
verses the current through the elements at a rate of 
50 cycles (100 reversals) per minute while maintain- 
4Information supplied to the U. S. Propagation Mission to 
England. 
Instruments made by Negretti and Zamba, Ltd., London. 
20x 
ing constant polarity at the meters. The current is 
smoothed by large condensers in parallel with the 
meters. The commutation eliminates polarization of 
the electrolytic elements and greatly increases their 
accuracy and useful life. The commutation period is 
so selected that it is long enough to prevent inductive 
and capacitative interaction between the two circuits 
but is short enough to allow of smoothing the currents 
through the meters. 
The cireuit illustrated in Figure 2 has been devel- 
oped by the Propagation Group at the Radiation Labo- 
ratory, MIT.* The apparatus includes two Sanborn 
Tesistance elements, one of them surrounded by a mois- 
tened wick. The current flowing through the resistors 
originally was fed into an amplifier which drove a 
recording milliammeter. However, after a number of 
amplifiers had been tried, the simple scheme shown in 
Figure 2 was adopted and, at the time of the writing 
of this report, is being used for all measurements made 
by the Radiation Laboratory, those from planes as 
well as those from captive balloons which will he 
described later. 
The dry and wet elements are placed in the circuit 
alternately by means of a hand-operated switch. The 
device can be calibrated by means of a set of fixed 
precision resistors and the balance of the bridge is 
checked before each flight. An advantage of this 
method is the possibility of using a commercial d-c 
recorder (0 to 1 ma) immediately at the plate ter- 
minals of the amplifier tube. This is particularly 
favorable for use in airplanes and dirigibles. 
+8(10S v) 
RECO ROER 
x 
ELEMENT 
FicurE 2. Schematic circuit of electronic amplifier. 
The resistance of the thermal element, X, controls the bias of one triode of the double triode, 6SN7, which acts as a vacuum tube voltmeter 
to compare the resistance of the thermal element with a standard resistance. A 1-ma recording meter 1s placed between the two plates. The 
resistance in the grid circuits is so chosen as to place 10 v across the thermal element at the lowest temperature of each range. This voltage 
decreases as the temperature rises. The zero is set by means of a 100-ohm potentiometer in the cathode circuit. Calibration of the amplifier is 
obtained by switching a series of precision resistors in steps of 1,000 ohms into the circuit in place of the thermal element. A range of roughly 25 C 
for full scale is used, and ¢hanges of 0.25 C can be measured. Sufficient overlapping is provided so that both wet and dry bulbs can record on 
a single setting. 
The stability is such that with a change in line voltage between 95 and 120 v there is no readable change in the meter deflection at 0 (when the 
tubes are balanced) or at full scale reading. When tubes are replaced there is, at worst, a change of 1 per cent of full scale deflection tapering to 
no deflection at 0. 
