24 



ISOMORPHISM AND THERMAL PROPERTIES OF EEEDSPARS. 



element might prove too high or too low through inability to take on 

 the temperature of the surrounding mass promptly. It was to dis- 

 cover and obviate this possible source of error that the form of thermo- 

 element indicated in the adjoining diagram (fig. 2) was devised. It 

 really amounts to nothing more than the ordinary 

 form of platinum platin-rhodium element with the 

 platinum wire insulated from the other by a very 

 slender porcelain (Marquardt) tube and the platin- 

 rhodium wire broadened out and wrapped around 

 this tube like a cap over the portion which dips into 

 the charge. The hot junction is 

 then the lower extremity of the cap 

 where the platinum wire emerges 

 from its insulating tube and is welded 

 inside the platin-rhodium cap. 



This form was dictated entirely by 

 experience to meet conditions where 

 an exposed element might be neees- 

 sarv or desirable. The wires of an 

 ordinary element, if embedded with- 

 out protection, are rather frail for the 

 wear and tear of breaking or drilling 

 mineral charges out of the crucibles 

 after the measurements, and they can 

 not be strengthened without intro 

 ducing a greater error through the 

 amount of heat conducted away from 

 the junction than the one which it is 

 desired to obviate. 



It has furthermore been the almost 

 invariable experience of one of the 

 authors* that when an element, 

 through exposure to combustion products or otherwise, no longer 

 gives normal readings, the seat of the trouble lies in the 5 or 6 centi- 

 meters of the platinum wire immediately adjacent to the hot junc- 

 tion, and not in the alloy. The pure platinum sometimes seems to 

 absorb enough volatile or other contact products, when unprotected in 

 a furnace at very high temperatures, to alter both its resistance and 

 its thermo-electric potential, f Changes of this kind are not serious 

 when a number of control elements are constantly available, and they 



Fig. i. Thermoelement (standard 



form) in position. 

 Fig. 2. A new form of thermo-ele- 



ment. 



Day. 



f Holborn& Day, Am. Journ. Sci. (4), 10, p. 171, 1900. 



