Pure metal resistance standards




Thomas, James Louis, 1894-

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In the United States the value of the ohm is based upon the resistance of a group of one-ohm manganin standards kept at the National Bureau of Standards. Values were assigned to these coils in 1910 on the basis of mercury ohm determinations. Since that time, it has been assumed that the average value of the best ten of these standards has remained constant. Since the relative values of these coils change but little, it is assumed that the group as a whole is changing even less. There is no way, however, of knowing that the coils do not rise and fall together in resistance. They might rise and fall seasonally by 20 parts in a million or more, but no test of this change can be made, as sufficient accuracy is probably not obtainable to do so by either mercury ohm or absolute ohm determinations. This uncertainty is rather disturbing since measurements can be made to a precision of one part in 1,000,000, and a primary standard is desired whose value is known to the same accuracy. The change in material for resistance standards has been first from pure metals to two-component precious metal alloys and then to three-component base metal alloys. It would seem, a priori, however, that these changes have proceeded in the wrong direction. The most important consideration in the choice of a material for primary standards is undoubtedly that of stability. It is to be expected that alloys will be less stable in composition, and hence in resistance, than pure metals, as they are liable to changes in phase in addition to any changes that might take place in pure metals. Also the base metals are much more subject to surface action than are the noble metals. There is, moreover, a little direct experimental evidence in favor of the use of pure metals. When Matthiessen, in 1863, constructed standards of precious metal alloys for the maintenance of the B.A. unit, he included two coils of platinum made from unannealed commercial wire. These have remained in the possession of the British Association for the Advancement of Science and have been compared with one another and with the other B.A. coils by a number of observers. While the precision with which the platinum coils have been measured has been low, on account of their large temperature coefficients, all these observers have found that their relative values have not changed. They have used these coils as a basis for discussing the stability of the remaining coils, made of alloys. The fact that there has been no measurable relative change in these standards during a period of over 50 years would indicate that the rate of change, if any, is quite small. On the basis of the performance of these platinum coils of the British Association, and of the other considerations mentioned above, it was decided to attempt to make pure metal standards which would be suitable for present day primary resistance standards. It was hoped in this way to improve considerably the stability of primary standards, or to obtain additional data as to the stability of manganin standards