Sec. 2-1] 



TEMPERA T URE TRANSD UCERS 



it:, 



Amplifier 



Bondposs 

 filter 



# 



electrons produces randomly varying potentials between the termi- 

 nals of a conductor which increase in magnitude with the tempera- 

 ture of the conductor. 1 The mean square voltage V 2 between the 

 terminals of a conductor with the resistance R which is held at 

 the absolute temperature t in degrees Kelvin is proportional to the 

 product Rt. 



Such randomly occurring potentials contain frequency com- 

 ponents from zero to the highest 

 measurable frequencies. If meas- 

 ured within a limited frequency 

 band A/, the mean square voltage 

 is 



V 2 = tictR A/ 



where A; is the Boltzmann constant, 

 1.37 x 10 -23 joule/deg. A resistor in the kilohm range at room 

 temperature will have an rms noise voltage of a few microvolts 

 measured in a frequency interval of 10 4 cps. 



A schematic diagram of a noise thermometer is shown in Fig. 

 (2-1)18. In principle, a measurement of the noise voltage generated 

 in a resistor would be sufficient to 



Resistor probe 



Fig. (2-1)18. Xoise thermometer, sche 

 matic diagram. 



Wollaston wire 

 0.0001" 



Platinum wire 

 carriers 



Quartz tube 



"'Platinum tube 

 0.002" thick 



determine its temperature. How- ,„„„„* .„ 



ever, the accurate determination 

 of a mean square voltage in the 

 microvolt range is technically diffi- 

 cult. 



Garrison and Lawson 2 and 

 Hogue 3 have successfully built 

 noise thermometers for the 

 1000°K range in which they com- 

 pare the unknown temperature of 

 one resistor with the known 

 temperature of another. Their 



method is the following: A resistor R x at the known temperature t x 

 is adjusted in value until the noise voltage from it is the same as 

 that from another resistor R 2 at the unknown temperature t 2 . When 

 this adjustment is made, the temperature t 2 is given by 



Fig. (2-1)19. Resistance probe of a 

 noise thermometer [from J. B. Garrison 

 and A. W. Lawson, Rev. Set. Instr., 20, 

 785, (1949); by permission]. 



t. 



-ffl 



1 J. B. Johnson, Phys. Rev., 32, 97 (1928). 



2 J. B. Garrison and A. W. Lawson, Rev. Sci. Instr., 20, 785 (1949). 



3 E. W. Hogue, Natl. Bur. Standards Rept. 3471, July, 1954. 



