Sec. 2-1] TEMPERATURE TRANSDUCERS 157 



the usable range is about ">") times that of an ordinary resistance 

 thermometer. The effect seems to be caused by small amounts of 

 lead impurity in the wire, which gradually become superconducting. 1 

 At low temperature the resistance is affected by the presence of mag- 

 netic fields. Varying amounts of lead in the wires allow their use in 

 intermediate or strong fields. 2 



Several forms of resistance thermometers have been developed for 

 the measurement of surface temperatures. The resistance wire is 

 mounted in grid form on the flat end surface of an insulating cylinder; 

 the diameter of the probe can be made as small as 1 mm diameter. 3 

 Other constructions consist of a Connecting 



grid of nickel wire bonded be- 

 tween two pieces of a flexible insu- 

 lating material, as shown in Fig. 

 (2-1)3. The resulting paper-thin 



Sandwich is fastened by an appro- J ">• ( 2 -D3. Resistance thermometer 



. for the measurement of surface tem- 



priate cement, like a strain gauge, perature. 

 to the surface to be measured. 



Surface thermometers have also been made in the form of ribbons or 

 blankets 4 or in the form of "spaghetti." 5 They are applied to the 

 surface under investigation by wrapping or by an adhesive tape. 

 The lower limit of temperature for which surface thermometers can 

 be used is about — 200°C, the upper limit +250°C, although surface 

 thermometers of this type that can be operated in the vicinity of 

 500°C have been described. 



The platinum resistance-thermometer element usually consists of 

 a platinum wire wound in the form of a free spiral or held in place by 

 an insulating carrier. It is frequently enclosed in a tube for protec- 

 tion from mechanical injuries and chemical alteration. Of all metals, 

 platinum is most appropriate for a resistance thermometer, because 

 it is commercially available in pure form and is relatively stable 

 under different environmental conditions. The relation between its 

 resistance and temperature is simple and holds over a wide tempera- 

 ture range with high precision. The platinum resistance ther- 

 mometer forms the basis of the International Temperature Scale 

 from —190 to +660°C, but can be used for measurements up to 1000 

 and down to — 264°C (9°K) when corrections are applied. 



1 J. D. Babbit and K. Mendelssohn, Phil. Mag., 20, 1025 (1935). 



2 H. Van Dijk and W. H. Keesom, Pkysica, 7, 970 (1940). 



3 A. C. Ruge Associates, Inc., Cambridge, Mass. 



4 Minco Products Inc., Minneapolis, Minn. 



5 Trans-Sonics, Inc., Burlington, .Mass. 



