116 A. L. Day and B. B. Sosman — 



mate evaluation. Accordingly, after every exposure of suffi- 

 cient length to endanger the thermoelectric readings, all the 

 thermoelements were removed from the furnace and their wires 

 tested for homogeneity. Where contamination was found, 

 the contaminated portion of the wire was at once cut off This 

 is the only absolutely safe method of avoiding errors from this 

 cause, for it amounts to the use of new thermoelements exclu- 

 sively in all the determinations of temperature distribution 

 within the furnace as well as for establishing the absolute 

 temperature of the metal melting points. 



A very simple method of testing the wires for contamina- 

 tion has been developed which consists in connecting the junc- 

 tion end of the w 7 ire to be tested, together with an uncontami- 

 nated wire, to the potentiometer and moving the free end of 

 the standard wire along the wire to be tested, while heating 

 the contact point of the two with a blast lamp.* The varia- 

 tion of the E.M.F. produced at this junction indicates the 

 degree of contamination of the wire ; in the uncontaminated 

 portion this E.M.F. is small and constant within 3 mv. The 

 temperature obtained by the blast lamp flame is sufficiently 

 constant for the purpose and lies between 1460° and 1500°. 



The wires could be relied upon to give a constant E.M.F. 

 within 2 mv. at 1000° over a length of at least 50 cm , so that 

 redeterminations of the fixed points were not necessary after 

 cutting off each small portion of contaminated wire. Each 

 test for contamination was continued over the 50 cm of wire 

 adjacent to the hot junction and so served as a test for the 

 homogeneity of the new wire which replaced the portion cut 

 off. In two cases a sudden change of E.M.F. along the unused 

 wire amounting to about 10 mv. showed the probable presence 

 of a junction point in the original sample from which the wire 

 was drawn. Such a junction point was of course not intro- 

 duced into the heated portion of the furnace. 



In this connection, it should be pointed out that the relative 

 weight to be given to the element inside the bulb, as compared 

 with the outside elements, is greater at temperatures above 

 1100° than at temperatures below, for two reasons : (1) The 

 temperature at the middle of the bulb is not so much influenced 

 above 1100° by the temperature of the lower part of the fur- 

 nace, as it is below 1100° ; (2) the outside elements are much 

 more subject to contamination than the inside element by 

 reason of the protection afforded by the intervening bulb walls 

 against contaminating material from the heating coils. This 

 is well shown by the data in Table VIII on the melting points 

 of diopside, nickel and cobalt. In the first measurements of 

 these temperatures, the elements were left on the bulb through 



*W. P. White, loc. cit. 



