The practical unit of resistance; 1E9 C. G. S. electro-magnetic units. The legal ohm is the resistance of a mercury column 1 square millimeter in cross-sectional area and 106 centimeters in length. There has been considerable confusion, owing to inaccuracy in early determinations, in the valuation of the ohm. In this work the legal ohm is used. The different ohms will be found defined in their place. Resistance units of various names may also be consulted.

The following table gives the relative values of the different ohms.
                   Length of
                   Mercury                         Board of
                   Column in    True     B.A.      Trade      Legal
                   Centimetre.  Ohm.     Ohm.      Ohm.       Ohm.
True Ohm,          106.24       1.       1.0128    .9994      1.0022
B. A.Ohm,          104.9        .9874    1.        .9868      .9889
Board of Trade Ohm 106.3        1.00050  1.0133    1.         1.0028
Legal Ohm,         106.0        .9977    1.0112    .9971      1.