Method of Measurement

Specimens were in the form of ⅛ inch diameter drawn rod, and were suspended vertically in a split tube resistance furnace. Two telemicroscopes, capable of measuring within limits of ±0.0001 inch, were used to determine the changes in length between reference marks on the specimens originally 3 inches apart. The experiments were carried out in air.

The rods used for these tests were in the hard drawn condition when inserted into the cold furnace; it was found that annealing in situ induced permanent decreases in length. Some typical effects are illustrated in Fig. 1, which relates to the 20 per cent rhodium alloy. The first heating curve was fairly smooth up to 1360°C, after which decreases in length at constant temperature were observed. After one hour at 1500°C the cooling curve was found to be appreciably lower than the first heating curve. Measurements at room temperature showed a permanent decrease in length of approximately 0.003 inch. Subsequent heating and cooling did not cause further appreciable changes in length.

Experimental Results

All the alloys studied exhibited this shrinkage effect to some extent, and the expansion curves plotted in Fig. 2 are those obtained during cooling after approximately one hour at 1500°C. All the curves are similar in character and agree fairly well with data for rhodium published by Ebert in 1938 (1) and with data for pure platinum published by Holborn, Scheel and Henning in 1919 (2). Although the expansion coefficients appear to increase slightly with rhodium content the effect is not pronounced, and all the alloys tested expanded 16 to 18 × 10^–3 inch per inch over the range o to 1500ºC. The 30 per cent rhodium alloy expands somewhat less than its composition might suggest. The proportional changes in length of the alloys tested over the temperature range o to 1500°C are summarised in the table on the preceeding page.