164> M. Langberg on the Determination of the. Temperature 



holes in the centres of which the rod passed, the battery and 

 that part of the rod to be examined were effectually protected 

 from the radiant heat of the boiling vessel. The support of 

 thebattery was screwed to a sledge which could be moved along 

 the edge of the horizontal divided board in a direction parallel 

 with the metallic rod, by which means the relative distances 

 of the points to be examined on the rod and their tempera- 

 tures could be easily ascertained. In order that the battery 

 should press with equal force each time against the rod, a 

 spiral spring was so placed in the support of the battery as 

 to force it upwards against the under side of the rod ; or it 

 could be brought into contact with the rod in a direction at 

 right angles to it. 



The observations were made in the following manner : — 

 "When the rod had attained a constant temperature, which 

 seldom occurred until about 2^ to 3 hours after the com- 

 mencement of the experiment, the sledge bearing the battery 

 was so placed that the upper end of the battery bore perpen- 

 dicularly upon the under side of the rod. The spiral spring 

 was then allowed to force the end against that part of the rod 

 the temperature of which was to be examined : the needle 

 of the multiplier diverged immediately. I waited generally 

 about two minutes to allow the needle to come to rest, and 

 having noted the divergence, removed the battery. After each 

 observation I allowed four minutes to elapse before the battery 

 was again placed in contact with the rod, partly that the needle 

 of the multiplier might return to 0°, and partly that the equi- 

 librium of temperature in the rod, which might possibly have 

 been disturbed by its contact with the battery, might again be 

 restored. This latter precaution was however needless, as 

 observations made upon the same part of the rod immediately 

 the one after the other, were found to give the same deviations 

 in the needle as when a space of time was allowed to elapse 

 between each observation. 



Thus far we have given very nearly the author's own words ; 

 but as our space will not allow us to follow him through each 

 experiment, we shall hei'e briefly add some of the precautions 

 which were taken to avoid error, and then give the results to 

 which the experiments have led. 



To ascertain whether the battery itself, after being for some 

 time in contact with the rod, might by becoming warm no 

 longer indicate with correctness the difference of temperature 

 between the rod and the surrounding air, it was left in con- 

 nexion with the heated rod for three quarters of an hour, but 

 during the whole of that time the divergence of the needle of the 

 multiplier scarcely changed. The circumstance that the needle 



