Humboldt State University ® Department of Chemistry

Robert A. Paselk Scientific Instrument Museum

From : Kimball, Arthur Lalane, A College Text-Book of Physics, 3rd ed., H. Holt and Co., New York (1923) pp.436-7.
© Copyright 1998 R. Paselk

The resistivities of alloys cannot in general be calculated from those of their constituents, but are often much greater than would be expected. The temperature coefficients of German silver, platinoid, and manganin are much less than those of pure metals; for this reason as well as for their large specific resistances these substances have been used extensively in making resistance coils.

 Temperature Coefficients
 German-silver (Cu 50, Ni 26, Zn 24)

 Platinoid (Cu 60, Ni 14, Zn 24, Tg 2)

 Manganin (Cu 84, Ni 12, Mn 4)

646. Standard Resistance. - Standard resistances are made of wire having a small temperature coefficient and not otherwise subject to change. The best coils are made of manganin. The
coil is provided with heavy copper terminals of almost negligible resistance, and is so mounted that it will quickly take the temperature of the oil bath in which it is immersed, and by which its temperature is maintained constant.
647. Resistance Boxes. - Boxes of coils having different resistances are made so as to be conveniently used in measurements, as shown in figure 363. On the hard-rubber top of the box are mounted a number of blocks of brass which can be connected by brass plugs fitting between them. Within the box are the resistance coils wound on spools, one end of a coil being soldered to one block and the other end to the next one so that
one coil bridges each gap. The external circuit is connected at the terminal binding screws, and when all the plugs are in, the only resistance is that of the brass blocks and plugs themselves. But if a plug is pulled out the current must then flow through the coil joining the blocks, and accordingly that resistance is introduced.

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© R. Paselk
Last modified 22 July 2000