166 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 2 



susceptibility of the salt. The coils are usually kept in liquid 

 hydrogen to keep their resistance low and constant. 



The susceptibility method is the primary method for the measure- 

 ment of lowest temperatures. However, the experimental require- 

 ments are considerable and the results appear in "curie temperature" 

 and have to be corrected for the thermodynamic temperature scale. 



For general information and references, see C. F. Squire, "Low Temperature 

 Physics," pp. 147, 156-157, McGraw-Hill Book Company, Inc., New York, 

 1953; W. A. Allis and M. A. Herlin, "Heat and Thermodynamics," pp. 180- 

 181, McGraw-Hill Book Company, Inc., New York, 1952. 



b. Permeability and Eddy-current Losses. An empirical method of 

 measuring temperatures by measuring the change of permeability 

 and eddy-current losses in ferromagnetic materials is described by 



Keinath. 1 The method is primarily 

 applicable for temperature determina- 

 tions in rotating parts, such as heated 

 steel cylinders or rollers, and does not 

 require direct physical contact between 

 the rotating part and the measuring 

 device. 



A schematic diagram is shown in 

 Fig. (2-1)10. A steel cylinder of varying 



Fig. (2-1)10. Inductive tempera- , . r j.i i r 



. i , r temperature lorms the yoke ol an lron- 



ture transducer. 1 - J 



core inductor L from which it is sepa- 

 rated by a small air gap. A variation of the permeability or of the 

 eddy-current losses resulting from a temperature change of the 

 steel cylinder affects the magnitude of the magnetic flux and is 

 reflected in a change of the reactive and resistive component of L, 

 which can be detected in subsequent stages. Empirical calibration is 

 required. No data on the performance of this method are available. 



2-13. Capacitive Temperature Transducer 



The dielectric constant of some insulating or semiconducting 

 materials varies with temperature; a capacitor made up with such a 

 material as a dielectric changes capacitance with temperature and 

 can be used as a temperature transducer. 



A material of this kind, a titanate named Thermacon, is de- 

 scribed by Dranetz, Howatt, and Crownover. 2 Depending upon its 



1 G. Keinath, Arch. tech. Messen, J 215-2, January, 1934. 



2 A. I. Dranetz, G. N. Howatt, and J. W. Crownover, Tele-Tech, June, 1949, 

 p. 36. 



