METHODS AND APPARATUS. 9 



Part II. Methods and Apparatus. 



An analysis of the measurements to be made as outlined in the preceding 

 section shows that the method of measurement must comply with the following 

 requirements : 



Several temperatures must be observed at practically the same time, and 

 this process repeated at intervals of a few minutes for protracted periods. 



The precision of the measurement must approximate 0.01 C. 



The thermometers must be capable of being read without being disturbed; 

 they must also be small and flexible, so as not to cause undue discomfort even 

 if in position for a considerable length of time. 



Only two methods were considered as being able to meet these conditions, 

 namely, those employing (1) electrical resistance thermometers, 1 and (2) 

 electrical thermal-junction thermometers. 2 



COMPARISON OF METHODS. 



The many advantages of the resistance method have led to its extended 

 application. In the first place, it has inherently a greater sensitiveness than 

 can be obtained by the thermal-junction method. The reason for this might 

 be suggested by the fact that in the resistance method the amount of energy 

 expended in the thermometer is left entirely to the discretion of the designer, 

 while in the thermal-junction method the source of energy is in the thermome- 

 ter itself and therefore is limited by the physical properties of the junction. 

 This implies, also, that the electromotive forces involved in the resistance 

 method will, in general, be far greater than those used in measurement with 

 the thermal junction, so that with the resistance method comparatively little 

 trouble will be experienced from extraneous electromotive forces, which are so 

 common a source of annoyance in thermo-electric measurements. Another 

 advantage of the resistance method is that the measurement is completed by 

 performing a single operation, usually that of balancing a bridge; there is no 

 second temperature to be read, no potentiometer current to be adjusted, and 

 no routine observations necessary to correct for the effect of extraneous 

 electromotive forces. Also, the apparatus for measuring the temperature of 

 the resistance thermometer is simpler than that required for the thermal 

 junction, as in the former case no constant temperature need be maintained, 

 also no standard cell is necessary. 



In spite of these manifest advantages, there are certain qualities inherent 

 in the resistance method which make it less fitted than the thermal-junction 

 method for meeting the requirements previously outlined. For example, the 

 measuring current flowing through the resistance thermometer produces an 

 appreciable heating of the wire, the resistance of which is being measured. 

 In measurements with the thermal junction, however, the current taken from 



'Benedict and Snell, Archiv f. d. ges. Physiol., 1901, 88, p. 492; and 1902, 90, p. 33. 

 -Gamgee, Phil. Trans. Royal Soc. of London, Ser. B., 1908, 200, p. 219. 



