378 Holborn and Day — fflcpa?isio?i of Certain 



Germany. Some 5 mm from the end of each bar plane surfaces 

 were filed and polished upon it in the plane of the axis and 

 upon these five fine divisions were ruled at millimeter intervals. 

 The bar of platinum, which was 5 mm in thickness, was measured 

 in both the oven tubes described. 



By way of example, a full series of actual readings of the 

 microscope is contained in Table la. The micrometer readings 

 are represented by F and F' for the fixed wires, I, II, ... Y 

 and I', II 7 , . . Y 7 for the settings of the movable wires upon 

 the ruled divisions on the two ends of the bar respectively. 

 The temperatures in degrees or microvolts are contained under t. 



At each temperature (20°, 250°, 500°, 750°, 1000° and then 

 19° after the oven had cooled down) two sets of readings were 

 made each containing settings upon each division of the bar and 

 the fixed microscope cross-wire in their proper succession — the 

 first in the forward, the second in the backward direction of 

 the micrometer screw. This symmetry served to minimize the 

 effect of a slight creeping of the bar such as was often notice- 

 able at the higher temperatures. The series also included five 

 observations of the temperature at proper intervals. 



lilf 



From the observations the displacement of the various divis- 

 ions during the change of temperature from t to t' was calcu- 

 lated and is entered for each separately in Table lb. A 

 positive sign indicating that the displacement has taken place 

 toward the end being read. The columns M and M ' contain 

 the intervals expressed in turns of the micrometer screw (r and 

 r f ) and the corresponding values reduced to millimeters, the 

 sums being then brought together under 2. For the remain- 

 ing observations only the values of M, M' and I are given. 

 Table Ic contains these values for the observations made 

 upon the platinum bar which was heated in the first oven tube 

 on the first two days and in the second on the remaining. It will 

 be seen that the corresponding values of I agree extremely well. 



The i values are then corrected slightly to correspond to the 

 round numbers 0°, 250°, etc., by the use of an approximate 

 formula for the expansion X s of the bar from which the re- 

 quired values of — r- s were obtained. 

 at 



The values of t observed at the middle point of the bar 

 require to be corrected for the fall in temperature toward the 

 ends as described above. These are contained in Table Id, in 

 which only 2 17 , the mean of each two corresponding values of 2, 

 is given. 



