Page:Electricity (1912) Kapp.djvu/136

132 strength and the "volt" for e.m.f., in which case the unit of resistance is called the "ohm." The following table gives the specific resistance reduced to a standard wire one metre in length and one square millimetre in cross section at ordinary room temperature—

1em Silver Copper Aluminium Iron Mercury Platinum

0.0158 0.0165 0.0287 0.125 0.953 0.094

The fact that a column of mercury one metre long and one square millimetre in section has a resistance of nearly an ohm, has led to suggestions to adopt mercury as a standard of resistance, and indeed, before the true value of the ohm had been determined by electrodynamic investigation, the mercury column was taken as approximately representing an ohm. It might be thought that such a standard would be acceptable to physicists, because it would enable each investigator to reproduce the standard at any time for himself, and thus render him independent of others. It is, however, not at all easy to produce such a standard. There is not only the