V. T E M P 10 R A T U R E D E T E R M I X A T I O N S 155 



thermometers, and (-i) thermistor or semiconductor resistance ther- 

 mometers. 



All three types consist of a resistance element and an electrical 

 circuit for measuring resistance changes with temperatiu-e. For 

 temperatures above 0°C., Callendar (22) formulated the relationship 

 between temperature and resistance as: 



' ' 100 + 5 — - - 1 — - (5) 



Vi^ioo - Ro/ VlOO / 100 



where t is the temperature in degrees Centigrade, Rt is the resistance 

 of the resistor at temperature t, R^ and i^ioo are its resistance at 0° and 

 100° C, respectively, and 5 is a constant characteristic of the indi- 

 vidual thermometer and must be determined by calibration at a fixed 

 point. The value of 8 usually lies between 1.49 and 1.50°C. 



1. Types of Resistance Thernioineters 



Noble Metal Resistance Thermometers. Platinum wire is 

 usually employed in noble metal thermometers. This metal is 

 readily obtainable in a high degree of purity. Its purity is assured 

 if the mean temperature coefficient, a, (equation 4) between and 

 100° C. is not less than 0.00388 and the constant 5 of the Callendar 

 equation (5) is not greater than 1 .52. The resistor of the thermometer 

 is usually supported in a framework so that it is protected from strain 

 and injury. It is enclosed in a protecting tubing or sheath and con- 

 nected by insulated copper wires to resistance measuring apparatus. 

 A resistor of this character requires about 2 m. of wire. The wire is 

 wound into a coil around a frame so that it can expand or contract on 

 heating, with a minimum of mechanical stress and time lag. The 

 heating effect of the measuring current should be minimal. Mica 

 can be used for the mounting frame, gold for the connecting leads in- 

 side the protecting tube, which may be of metal, glass, porcelain or 

 fused silica, and copper wire for the leads to the measuring instru- 

 ment. 



Base Metal Resistance Thermometers. Nickel, Nichrome, or 

 similar alloys are most commonly used in base metal resistance 

 thermometers. A large number of different alloys with a wide 

 range of temperature coefficients are available. Selection of a par- 

 ticular alloy will depend upon cost and the particular resistance- 



