.242 BELL SYSTEM TECHNICAL JOURNAL 



been used for recording temperatures and differences between two 

 temperatures by means of resistance thermometers and a Wheatstone 

 bridge arrangement. As it seemed feasible to adapt this instrument 

 to meet our requirements, the double Wheatstone bridge circuit 

 shown in Fig. 1 and the auxiliary' wind tunnel equipment with resist- 

 ance thermometers shown in Fig. 2 were developed. Fig. 3 is an 

 illustration of the Leeds & Northrup recorder used. 



This recorder was arranged to measure the resistance of the dry 

 bulb thermometer and the difference between the resistances of the 

 dry and wet thermometers, and to record these values upon a chart. 

 Referring to the circuit diagram Fig. 1, it may be seen that, by means 

 of a relay whose operation is controlled by the commutator on the 

 recorder mechanism, the two Wheatstone bridges, one containing the 

 dry bulb thermometer, and the other containing both the dry and 

 wet bulb thermometers, may be balanced alternately by the recorder. 

 After a sufficient interval has elapsed in each case for the bridge to 

 become balanced the siphon pen is lowered into contact with the 

 chart by a cam mechanism and the point of balance thus recorded. 

 The record thus produced consists of dotted curv'es showing the 

 successive indications of dry bulb temperature and difference be- 

 tween dry and wet bulb temperatures. 



In order to secure the desired accuracy and sufficient sensitivity 

 to follow the changes in temperature, the resistance thermometers 

 used consist of platinum wire wound on mica cards and encased 

 in flat nickel silver tubes with hard rubber ferrules. These are 

 attached to a brass junction box in which is terminated the four 

 conductor cable leading to the recorder mechanism. 



The thermometers are enclosed in slotted brass tubes through wliich 

 the air is drawn by a small blower driven by a universal motor. 

 Mounted below these tubes is a shallow, covered water tank having 

 a slot in the cover beneath the wet bulb thermometer through which 

 the wick projects into the water. The desired water le\'el in the 

 tank is secured by an inverted water bottle, the neck of which projects 

 into another opening in the cover of the tank. 



The wind tunnel equipment^ containing the resistance thermometers 

 may be placed at any desired distance from the recorder mechanism, 

 as the resistances of the thermometer leads have no effect upon ilic 

 measurements provided they are equal. Leads consisting of a four 

 conductor rubber insulated lead covered cable from 50 feet to 100 feet 

 in length have been used. 



'The wind tunnel and cqiiipniciU is (|iiilc siinil.ir in opiT.itiiin to tin- "disl.ince 

 hygrometer," Sci. Am. June 6, l')14, p. 468. 



