Nitrogen Thermometer from Zinc to Palladium. 117 



several runs, in consequence of which the temperatures derived 

 by the outside elements steadily increase through the series 

 (i. e., the readings of the outside elements on the bulb steadily 

 decreased), whereas the temperatures derived from the inside 

 element are fairly constant. Its contamination was found to 

 be less in amount and distributed over a region of more con- 

 stant temperature. 



For insulating the thermoelement wires from the bulb and 

 furnace, capillary tubes, both of Marquardt porcelain and of 

 silica glass, were employed. The Marquardt tubes are open 

 to the objection that they are very porous and offer little pro- 

 tection against contamination. The silica glass capillaries pro- 

 tected the wires very much better, but at 1100° and above 

 they devitrified rapidly and at the end of a measurement at 

 1400° or over fell from the wires in small fragments, so that 

 the wires had to be taken off and reinsulated after a single run. 

 „ For the convenience of others who may confront similar 

 problems, it may be added that such extreme precautions as 

 cutting off the elements at the first sign of contamination are 

 excessive for most purposes. The region of highest tempera- 

 ture, and therefore of most rapid contamination in a good 

 furnace, is also a region of constant temperature. Contamina- 

 tion would therefore produce little effect upon the reading of 

 the thermoelement until it had crept out into the colder parts 

 of the furnace, which it will do slowly during long exposures. 

 The distribution of the contamination in an aggravated case is 

 shown in the accompanying table, which is arranged in such 

 a way that not only the magnitude of the contamination but 

 also its distribution with respect to the bulb is roughly shown. 

 The electromotive forces are determined, as has been 

 explained, by bringing successive points of the contaminated 



Before After 



Heating. Heating. 



Microvolts Microvolts 





40 cm 



— 4 



— 4 





35 



-6 



-8 



Outside of furnace 



30 



— 8 



— v 





25 



— 9 



* _ G 



Bend of stem 



20 



-6 



— 10 





15 



-5 



-3 





12 



— 5 



+ 2 





10 



— 5 



+ 9 



Shoulder of bulb 



8 



— 5 



+ 83 





6 



— 5 



+ 83 





4 



— 6 



+ 41 





2 



— 6 



+ 55 



Middle of bulb 







— 8 





