Ohm's Law and the Ohm.--The various tests here described depend for their truth upon the definite relation existing between the electric current, its pressure, and the resistance which the circuit offers to its flow. This relation was fully investigated by Ohm in 1827. Using the same conductor, he proved not only that the current varies with the pressure, but that it varies in direct proportion. Ohm's law has already been discussed in a previous chapter and the several ways of expressing it are repeated here for convenience: volts 1. Amperes = -------; ohms 2. Volts = amperes × ohms; volts 3. Ohms = ---------. amperes Various values have been assigned, from time to time, to the ohm or unit of resistance, the unit in use at the present time being known as the _International ohm_. This was recommended at the meeting of the British Association in 1892, was adopted by the International Electrical Congress held in Chicago in 1893, and was legalized for use in the United States by act of Congress in 1894. The International ohm in graphically defined in fig. 548. The previous values given to the ohm which were more or less generally accepted are as follows: The Siemens' Ohm.--A resistance due to a column of mercury 100 cm. long and 1 sq. mm. in cross section at 0° C. B. A. (British Association) Ohm.--A resistance due to a column of mercury approximately 104.9 cm. long and 1 sq. mm. in cross section at 0° C. Legal Ohm.--A resistance due to a column of mercury 106 cm. long and 1 sq. mm. in cross section at 0° C. This unit was adopted by the Paris conference of 1884. OHM TABLE[A] | | Inter-| | | | | Date |national| Legal|B. A. |Siemens'| | | Ohm| Ohm|Ohm | Ohm| -------------------------+------+--------+--------+--------+--------| International Ohm |1893-4| 1.0000| 1.0028| 1.0136| 1.0630| Legal Ohm | 1884 | .9972| 1.0000| 1.0107| 1.0600| B. A. Ohm | 1864 | .9866| .9894| 1.0000| 1.0488| Siemens' Ohm | | .9407| .9434| .9535| 1.0000| [A] NOTE.--In the above table to reduce, for instance, British Association ohms to International ohms, multiply by .9866, or divide by 1.0136; to reduce legal ohms to International ohms, multiply by .9972, or divide by 1.0028, etc. [Illustration: Fig. 547.--Double contact key. It is of especial value in connection with a Wheatstone bridge. When used with the latter it forms a combination battery and galvanometer key. The battery is wired to the top leaves of the key and the galvanometer to the lower leaves. Hence, when operated, the battery circuit will be closed before the galvanometer circuit, as it is desirable to avoid undue disturbance of the needle.]