Sec. 2-1] 



TEM PERA Tl RE TRANSDUCERS 



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temperature of the gas. The relation between the degree of ionization 

 and the temperature has been derived by Saha. 1 The conductivity 

 of the gas becomes measurable in the region above about 4000°K. 



The relationship between ionization and temperature has been 

 used by Suits 2 to measure the temperature in an electric-arc dis- 

 charge operating between 1 and 10 atm of pressure. The accuracy 

 is of the order of 10 per cent. The method is inherently suitable for 

 the measurement at very high temperatures above ten thousand 

 degrees Kelvin. 



2-12. Inductive Temperature Transducers 



a . Variation of Susceptibility of Paramagnetic Salts. The magnetic 

 susceptibility % of certain paramagnetic materials changes with tem- 

 perature according to Curie's law: 



C 



where C is the curie constant (a characteristic of the paramagnetic 

 material) and T the absolute temperature. These changes with 

 temperature of the susceptibility of such 

 salts as iron ammonium alum or iron 

 ammonium sulfate which have a very 

 low curie point, can be used to measure 

 extremely low temperatures in the range 

 below 4°K. 



The susceptibility is usually determined 

 by means of an arrangement as shown in 

 Fig. (2-1)9. A current source produces 

 in a primary coil P a magnetic field of 

 several gauss. Two secondary windings FlG - ( 21 ) 9 - System used for 



~ , „ n . -j. J* J.- inductive temperature trans- 



S^ndS 2 are wound in opposite directions ducer> wd upon suscepti . 



and Connected, as shown, to a galvanom- bility variation of a specimen 



eter. In the absence of a paramagnetic B - E > voltage source; A, 

 ,, .. -, l-i-i switch: P, primary, S, and S- 



specimen, the voltages induced in both sec0 ndary t windings of a trans- 

 secondary windings are equal and of former; G, galvanometer. 

 opposite sign, and the galvanometer 



should show no deflection when the switch A is closed. If a sample 

 B of a paramagnetic salt is introduced in one coil, the voltage 

 induced in this coil is increased, and upon closing the switch A 

 the galvanometer receives a ballistic impulse proportional to the 



1 M. N. Saha, Phil. Mag., 40, 472 (1920). 



2 C. G. Suits, J. Appl. Physics, 10, 728 (1939). 



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