432 A. L. Day and J. K. Clement — Gas Thermometer. 



arranged it, and there was therefore nothing to do but to make 

 the time of the exposure as short as possible, and by the use 

 of two elements fastened together and extending out of the 

 furnace at opposite ends, to so arrange the conditions that any 

 contamination, if sufficient to affect the temperature, would 

 become immediately apparent. As W. P. White of this, 

 laboratory has shown in a recent paper,* the most critical 

 portion of a thermoelement is not the portion along which the 

 temperature is constant, but the region where the element 

 passes from one temperature to another. In our furnace, for 

 example, the region of exposure to constant temperature could 

 give rise to no error of reading however much the element 

 might be contaminated in that region, but if a contaminated 

 portion of the element w r ere at any time to come into the 

 region lying between the end of the bar and the outside of the 

 furnace an immediate difference in its reading should become 

 evident. It was therefore arranged that the junctions of two 

 elements should be bound together so as to record the temper- 

 ature of the same point within the furnace and that whenever 

 this combination of two elements was moved toward one end 

 of the bar or the other, that a greater length of one of the 

 elements should be exposed within the furnace than of the 

 other. If there is contamination a difference in reading 

 between the two elements will be immediately conspicuous. In 

 the earlier observations comprising this investigation, only one 

 element was used, and by way of control at the close of a long 

 series of- observations a second element was introduced in the 

 manner indicated above. It then became immediately evident 

 that the first element had become contaminated and that the 

 observations made with it were affected to a degree which 

 could not be established after the observations themselves were 

 over, and which therefore necessitated the rejection of several 

 entire series. This misfortune may serve to emphasize the 

 necessity of using more than one thermoelement in all cases 

 where it is possible to do so. 



Three other difficulties were met with which proved to be 

 sources of considerable inconvenience, and which serve in 

 greater or less degree to place limits upon the accuracy attain- 

 able in this particular apparatus. The first was the temper- 

 ature gradient along the bar, of which mention has already 

 been made. Earlier observers have sometimes been content 

 in similar cases to heat a bar with the electric furnace and to 

 make their measurements upon cold projecting ends, that is, 

 under conditions such that the actual temperature along the 

 bar varies from the temperature of the room to a maximum 

 near the middle of the bar. The resulting temperature to 

 * Walter P. White, Phys. Rev., xxvi, p. 535, 1908. 



