V. T E M P E R A T U R E D E T E R M I N A T I O N S 143 



The second therm oj unction may be used covered or bare and 

 mounted in a manner appropriate to the medium into which it is to be 

 placed. Enameled, double-cotton-covered copper wire and plain, 

 double-cotton-covered constantan wire are the most useful combina- 

 tion for general biophysical laboratory use. The wire gage should 

 be chosen to suit the physical dimensions of the medium the tempera- 

 ture of which is to be measured. The smaller the caliber of the wire 

 used, the faster is the response of the couple to thermal changes and 

 the more fragile the thermojunction. For most thermoelectric pur- 

 poses gage #28 ware wall suffice; for quickly responding, delicate 

 thermocouples, gage #40 wire can be used. 



Usually thermocouple leads require no additional insulation. If 

 desired, ho^vever, the cotton covering may be reinforced by dipping 

 the leads into latex or other pliable material. Excellent protection 

 from short-circuiting due to insulation wear is afforded by passing 

 the leads through small bore rubber or plastic tubing. This is easily 

 accomplished by attaching the tubing to a vacuum pump, drawing a 

 string through the bore, and then pulling the leads through with the 

 string. Care should be taken not to stretch the tubing because the 

 wires are easily broken. 



1. Thermoelectric Force 



With the standard junction at 0°C. gradual heating of the other 

 junction will increase the thermoelectric force to a maximum at about 

 275° C. for a copper-iron junction. Increasing the temperature above 

 275°C. will decrease the thermoelectric force as shown in Figure 2. 

 Temperatures above 550° C. will reverse the direction of current flow. 

 The temperature at which the thermoelectric force reaches a maxi- 

 mum is called the neutral temperature and that at which it changes di- 

 rection, the inversion temperature. Each pair of dissimilar metals 

 has a characteristic neutral and inversion point. The temperature 

 curve for most junctions may be represented by a parabola for the 

 full range. 



The curve of Figure 2 may be approximated either by experi- 

 mentally determining the thermoelectric force for a series of M values 

 betw^een junctions or by using an empirical equation between the 

 thermoelectric force, E, and temperature: 



E = Bt+ (CfV2) (3) 



