METEOROLOGICAL MEASUREMENTS 19 
of the Massacnusetts Institute of Technology uses 
dry and wet resistance elements. **° 
The physical assembly consists of bakelite tubing, 
in which the two elements are mounted perpendicular 
to the axis. The tube is surrounded by a radiation 
shield of aluminum foil. Wet and dry bulb instru- 
ments need artificial aeration in calm air which is 
provided by a small electric fan. Among the instrt- 
ments containing electrolytic humidity strips only 
the late model of NRSL incorporates artificial aera- 
tion. Other instruments of this type, when used in 
calm weather with a captive balloon, are aerated by 
giving the cable a few jerks of several feet amplitude. 
In both captive balloon and kite equipment only 
the measuring elements are carried aloft with fine 
wires in the cable to connect with the rest of the 
circuit. The assembly that is carried aloft is therefore 
quite light, weighing only about a pound in the case 
of nonaerated instruments and 3 to 4 pounds for 
aerated ones. 
Figure 1 shows a wiring diagram for the Washington 
State College sonde. The diagram is largely self- 
explanatory. The switches 8; S2 S83 are contained in 
the pile-up of a single relay and are-actuated by a 
miniature worm-geared motor as shown. They reverse 
the current through the elements in order to avoid 
polarization, while at the same time maintaining 
constant polarity at the meters. The period of 
reversal is 0.5 sec and the 1,000-u»f condensers in 
parallel with the meters serve to smooth the inter- 
rupted current. 
Figure 2 shows a schematic wiring diagram for the 
dry and wet bulb resistance elements of the Radiation 
Laboratory instrument. The resistanee 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 is placed between the two plates. In operation 
the dry and wet elements are switched into the 
R-H METER 
0-30 MICROAMP 
+ 
SONDE 
ELEMENTS __ 
RHO CABLE e 
+{_1.5V 
T 
Salo 
+ 
T METER 0-50 CNN 
microamp &Y OC 
Ficure 1. Circuit diagram for State College of Wash- 
ington wired sonde. 
+68 
(105 v) 
RECORDER < 10,000 
200,000 
10,000 
Ficure 2. Circuif diagram for electronic amplifier for 
measuring temperature. (Radiation Laboratory, MIT.) 
circuit alternately. 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. The stability of this voltmeter is 
such that with a change in line voltage between 95 
and 120 v there is no observable change of the 
meter at any given deflection. 
REFRACTIVE INDEX MEASUREMENTS 
The methods which have been used to make 
refractive index measurements in the lower atmos- 
phere are the following: 
1. Stationary installations on towers, usually with 
automatic recording on the ground. Aerated wet and 
dry bulb instruments are installed at several heights 
giving a continuous survey of the M curve between 
the ground and the top of the tower. 
2. Installations similar to (1), on shipboard, with 
the meters or recording equipment in the ship’s 
cabin. In order to explore the humidity distribution 
in the lowest layers adjacent to the sea surface, the 
instruments have been mounted at the end of a beam 
that pivots about a horizontal axis fastened to the 
side of the ship. This device has been used extensively 
in the Irish Sea experiments. Artificial aeration of 
shipborne installations is not usually necessary 
because in calm weather the necessary velocity of 
the air is provided by the motion of the ship. 
3. Airborne installations. The unit is mounted at 
a convenient place on the outside of the plane where 
it is not affected by motor exhaust or propeller slip 
stream, with the meters or recorders in the ship’s 
cabin. Comparatively slow-flying planes have been 
used for such measurements, not only in order to 
minimize the dynamic temperature correction, but 
also because in a fast-flying plane too long a column 
of air-will be sampled during the period of relaxation 
of the instrument. In airplane measurements it is 
