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I N connection with our measurements of the heat capacity of niobium in the normal and superconducting state, it was desirable to find a secondary thermometer for use in the range 2° to 20°K with the following requirements: reproducibility after cycling to room temperature high sensitivity, negligible change of calibration in the presence of a magnetic field. Such a thermometer has been the subject of a wide search by investigators in low temperature physics for many years. The most commonly used secondary thermometer, leaded phosphor-bronze, is reproducible but has sensitivity only below 7.2°K owing to the superconductivity of the lead. However, in addition to being insensitive above 7.2°K, it is also insensitive below 7.2°K in the presence of a magnetic field.The fact that carbon possesses desirable resistance-temperature properties in the liquid helium temperature range is well known, and considerable use has been made of carbon thermometers in various forms, such as india ink, Aqua Dag, etc. None of these carbon thermometers showed reproducibility from day to day, and in some cases the sensitivity was not sufficient. At the Low Temperature Symposium held at the National Bureau of Standards in March, 1951, J. R. Clement of the Naval Research Laboratory pointed out that a very convenient and sensitive carbon thermometer was available commercially in the form of carbon radio resistors, and that those rated at one watt manufactured by the Allen-Bradley Company showed high sensitivity in the helium range.Because a one-watt resistor was larger than desirable for mounting in our specimen of niobium, we chose a half-watt resistor at random from the stock on hand at Columbia University. The outer plastic covering was ground away so as to expose the carbon. The resistor was then covered with a 0.001-inch layer of clear glyptal lacquer and baked. It was then cemented into a cylindrical hole bored in the niobium. The total weight of the resistor and leads was then less than 0.2 gram. During the period April through August, 1951, the specimen and thermometer were cooled from room temperature to about 2°K seven times. Using a constant measuring current of 10 microamperes and a Wenner potentiometer, a careful resistance-temperature calibration was made each time using the vapor pressure of liquid helium. In this temperature range about 50 different calibration points were obtained. These points were found to lie on a smooth curve with over 90 percent of the points differing from the curve by less than 0.002 degrees.

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The heat capacities of three samples of lanthanum have been measured in the temperature range 1.6 to to 6.5°K. A four-constant formula was found which represented to high precision the resistance-temperature relation of the carbon composition resistance thermometer from 1.6 to 7.2°K. Two superconducting transitions were found in each sample: one at 4.8°K and the other at 5.9°K. These are associated respectively with the hexagonal close-packed and face-centered cubic modifications of the metal. Below 2.5°K, a magnetic field of 10 000 gauss was found insufficient to quench completely the superconducting phase. The values of the normal heat capacity constants for the purest sample, averaged over the two crystal structures present, were determined by a thermodynamic analysis of the data to be y = (24.1 ±0.6) X10~4 cal/mole (°K) 2 , 0=142±3°K. The data are further analyzed for evidence of a law of corresponding states among superconductors.