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.
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.