V. TEMPERATURE DETERMINATIONS 



157 



The oxides are usually enclosed within ceramic or glass fired under 

 accurately controlled temperature and atmospheric conditions. Lead 

 wires are attached and the resistance unit enclosed or mounted in a 

 manner suitable to the thermal measuring reciuirements. Stability 



Fig. 10. Thermistors made in the form of a bead, rod, disc, washer, and flakes. 

 (Courtesy Bell Telephone Laboratories.) 



tests on thermistor units have disclosed no appreciable change in re- 

 sistance characteristics through 500,000 heating and cooling cycles. 



2. Electrical Circuits for Resistance Thernionieters 



Figure 11 shows a diagrammatic presentation of the most com- 

 monly used circuit for resistance thermometers of the three lead type. 

 The galvanometer is used in this circuit as a null indicator to show 

 when the bridge is in balance. Current flows through the galvanom- 

 eter only when electrical balance is not present. This method is 

 independent of voltage surges from the current source and a dry cell 

 or low voltage alternating current with a copper oxide or selenium 

 rectifier can be satisfactorily used as this source. 



A double slide wire Wheatstone bridge is represented in the dia- 

 gram. Accuracy of measurement is assured by this circuit design, 

 which places all moving contacts in the galvanometer or batter}^ cir- 

 cuits. Both slide wires are mounted on a single drum and have re- 

 sistance values such that, at all points on the slide wire, the resistances 

 of arms 1 and 2 are equal so that the ratio arms of the bridge will be 

 equal in resistance. Resistance AB will vary as the dial is rotated 

 and since arm 1 equals arm 2, the bridge will be balanced when arm 3 



