Oerstedt's Discovery.
Oerstedt discovered in 1820 that a magnetic needle tended to place
itself at right angles to a current of electricity. This fundamental
experiment is the basis of the galvanometer.
Fig. 251. OERSTEDT'S DISCOVERY.
Ohm.
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.
Ohmage.
The Resistance of a circuit expressed in ohms.
Ohm, B. A.
The British Association unit of resistance; the resistance of a column
of mercury 1 square millimeter in cross sectional area and 104.9
centimeters long; the B. A. Unit of Resistance.
Ohm, Board of Trade.
The approximate ohm as recommended by the British Board of Trade on the
advice of a committee (Sir W. Thomson, Dr. J. Hopkinson, Lord Rayleigh
and others). It is the resistance of a mercury column one square
millimeter in section, and 106.3 centimeters long at 0º C. (32º F.)
Synonym--New Ohm.