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.