To obtain some of the information necessary for redefining a practical temperature scale in terms of resistance thermometry up to the gold point, we have intercompared nine high temperature platinum resistance thermometers and eight standard thermocouples a t 50 degree intervals over the range 630.74 to 1064.43 "C. Values of temperature were derived from "before and after" calibrations of the instruments a t the freezing points of gold, silver, antimony, zinc, and tin, and a t the triple point of water. -Analysis of the data showed that the standard deviation of temperatures measured with thermometers was about 4mK, while with thermocouples it ranged from 25 to 30mK. When systematic errors were accounted for, we found that values of temperature on IPTS-68 could be determined with a single thermometer, using an interpolation formula analogous to that used below 630.74 "C, with about one-third the uncertainty usually attributed to a single thermocouple determination ( h200 mK). The data also revealed that there is a lack of "smoothness" in the scale a t 630.74 "Cfirst derivatives with respect to t,, are discontinuous by about 0.1% a t this temperature. -We conclude from the results of this work that if a practical temperature scale were redefined in terms of presently available high temperature platinum resistance thermometers, it could be realized with significantly less uncertainty ( h12 mK for a single determination) than IFTS-68, as it is now realized with thermocouples ( k200 mK for a single determination). We recommend, however, that the adoption of such a scale be deferred until indicated improvements in thermometers have been made, and until current researches in gas thermometry have provided new values of thermodynamic temperatures for calibrating fixed points.

Two sets ofhigh~temperature platinum resistance thermometers of different design have been tested in the temperature range 0 to 1100 GC. One set was constructed at the National Institute of Metrology. in the People's Republic of China, and the other at the National Bureau of Standards. The results of the tests provide information on long-and shorHime thermometer stability, and on other characteristics such as temperature coefficient, immersion, self-heating effect. electrical leakage, and durability. The results also show that the behavior of the two sets is similar enough to allow them to be considered as a single set of thermometers, and that the sets perform as well as, or better than, other sets of thermometers tested earlier. It is expected that this information will aid in the evaluation of the high-temperature platinum resistance thermometer as an interpolating instrument for a practical temperature scale up to the gold point.

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The construction of a high-temperature platinum resistance thermometer having a resistance at°C of 2.5 ohm, suitable for use as a defining instrument up to the freezing point of gold (1064.43°C, the "gold point"), is described. The thermometer is made with high-purity fused silica supports and insulators. The resistor is reference-grade platinum wire wound in a single-layer, bif ilar-helix. The necessity of using the highest purity materials and of scrupulous cleaning procedures of all component parts, subassemblies and assemblies is stressed. To minimize the effect of electrical leakage, a fifth or guarding lead is incorporated. The stability and other characteristics of this type of thermometer have been evaluated at temperatures up to 1064.43°C.