| . | (5) |
The experiments by which these results were established constitute the most important of Ohm's work and they well repay careful study. After some preliminary experimentation his apparatus was reduced to the following essential parts: a thermo-couple for generating a steady current and a specially constructed magnetic torsion balance for measuring the current strength. The apparatus and scheme of connections is shown in Fig. 3.
During the earlier part of his experimental work one thermojuncture was surrounded by a steam-jacket and the other by an icejacket; during the latter part of the work the hot juncture was left at room temperature. The procedure for the experiment was as follows: The steam and ice jackets were first brought to their respective temperatures, the length of the test wire was then adjusted to the required length and lastly the torsion head turned until the magnetic needle was brought back to its zero position. The reading of the torsion head was then recorded and the experiment repeated with a new length of wire.
With the completion of these experiments Dr. Ohm had established both parts of his law, and may be said to have solved the problem to which he had set himself. As in the case of Sir Isaac Newton and the law of gravity, Ohm now found himself in the possession of a key to many doors closed to previous workers, and he proceeded at once to use it, as is shown by his theoretical paper of 1836 and his book of 1827.
V. This paper was published in the Poggendorf Annalen, is ten pages long and is entitled "Versuch einen Theorie der durch galvanische Kräfte hervorgebrachten elektroskopischen Erscheinungen." It is a purely theoretical paper and foreshadows the book which he wrote the following year. In this paper Ohm enunciates his complete law, contrary to the widely accepted statement that the law was first given in the book of 1827 (e. g., Reed and Guthe, "College Physics," 1911). It also contained the correct formula based on this law, for the change of terminal potential difference due to a change in the external resistance. Finally the application of the law to many practical problems is discussed.
VI. The year 1827 furnishes the final paper of the series upon the galvanic circuit, followed by the appearance of the book elaborating the newly discovered relations. This paper appeared in Schweigger's Journal and was entitled "Einige elektrische Versuche." It is a paper of eight pages and contains the results of two experimental investigations confirmatory of the work of the previous year, as follows: (1) A verification of the conclusion as to the uniformity of the distribution of current over the cross-section of the conductor; (2) a verification of the formulæ for the combination of resistances in parallel. This may be
