Nickel-chromium-aluminium-copper resistance wire

A review is given of the principal materials used for the construction of resistance standards. The difficulty of producing manganin and constantan commercially with the requisite small value of temperature coefficient at room temperature makes attractive the newer alloys, whose temperature coefficients can be controlled by simple heat treatment. One of these alloys, having the additional advantage of a resistivity three times that of manganin, has been studied at the National Physical Laboratory. It is composed of nickel, chromium, aluminium and copper and is known commercially as Evanohm. When the temperature coefficient, at a given temperature, has been reduced to zero by heat treatment, the curvature of the resistance/temperature characteristic is only one-tenth that of manganin. The stability of resistors constructed of this material has been investigated and has been generally found to be of the order of a few parts in 105 per year. The investigations are continuing and it is hoped that better figures may be obtained for well-aged standards. The stability is not adversely affected¿and may be improved¿by operation at temperatures up to 120°C. Above 140°C there is usually an increase of resistance, but even at 400°C this increase is not rapid. Operation above 400°C is not recommended even for low-accuracy resistors. Further advantages of the material are a low thermal e.m.f. to copper, high mechanical strength and high ductility. The disadvantages are the necessity of hard soldering and the susceptibility of the resistance to change due to cold working, including vibration. The resistance standards under investigation for long-term stability are five 1-ohm standards, and one standard of each of the following values: 1 000 ohms, 100 000 ohms, 1 megohm and 10 megohms. An additional 100 000-ohm standard of an alternative design has recently been constructed.