THE Wilt HI) SONDK 



51 



7.3 



Tin: wiKKi) soM)i; 



Temperature and humidity elements of the type 

 described arc combined in a lightweight assembly 

 which can be moved rapidly through the lower 

 atmosphere. Such equipment, when first built in 

 England, used dry and wet thermopiles,'- 27 and 

 soon thereafter the same method was adopted by 

 the State College of Washington, 228 - 232 - 234 and, with 

 slight modification, by the Navy Radio and Sound 

 Laboratory [NRSL] at San Diego. 236-238 This design 

 uses a combination of a resistance temperature 

 element and an electrolytic humidity element. The 

 instrument developed by the Radiation Laboratory 

 of the Massachusetts Institute of Technology uses 

 dry and wet resistance elements. 229 



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 instru- 

 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 Si S 2 S 3 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- /if 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 resistance of the thermal 

 element X controls the bias of one triode of the 

 double triode 6SN7 which acts as a vacuum tube 



R-H METER/'7\0-S0 MICROAMP 



HI — «= 



1.5V 



T METER\//0- 



+ 

 50 



MICROAMP 



RELAY 



6 O 

 6VDC 



Figure 1. Circuit diagram for State College of Wash- 

 ington wired sonde. 



'B 

 (105 V) 



200,000 



500 <I0,000 



Figure 2. Circuit diagram for electronic amplifier for 

 measuring temperature. (Radiation Laboratory, MIT.) 



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 

 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. 



