In the light of Ohm's later work it is easy to see that this formula is absurd, a conclusion indeed soon reached by Ohm himself. It however marks one of the steps in the discovery of the law, and an examination of the experimental data shows that it represented these data very closely. Ohm was not however to be long misguided by an equation which could be easily checked by increasing the range of the experiment. He saw that, no matter how precise equation (3) might prove as an approximate formula, it could hardly represent a law of nature. He therefore prepared to test an external resistance fifteen hundred feet long. He does not seem to have published the result of this experiment, but he must have seen his mistake, for he promises to develop a new and correct equation. This he did a year later.
II. Two more papers were published by Ohm in the year 1825, as follows: "Über Leitungs-fähigkeit der Metalle für Elektricität," containing a preliminary announcement of his studies on the relative conductivities of different metals. The results were published the following year and are considered under paper IV.
III. "Ueber Electricitätsleiter." This was a discussion of the discrepancy between the results of Barlow and Becquerel, together with the acknowledgment of the inaccuracy of his own formula for the "loss of force" and an intimation of his intention to revise the formula. This closes the work for the year 1825. During these experiments he used for the source of current a cell and measured the potential difference by means of a Coulomb torsion balance. The progress of the work is marked by a clearing of the way, by a grasp of the problem and a development of method rather than by positive achievement.
IV. Two papers appeared during the year 1826: the first of these was by far the most important and was a long one of twenty-nine pages, entitled "Bestimmung des Gesetzes, nach welehem Metalle die Contact-Electricität leiten, nebst einem Entwurfe zu einer Theorie des Voltaischen Apparates und des Schweigger'schen Multiplicators." While not so well known as his book on the mathematical theory of the circuit, puplished a year later, it is in reality his most important work and contains the following results.
1. The data on relative conductivities promised in paper II. Ohm measured the relative conductivities of copper, gold, silver, zinc, brass, iron, platinum, tin and lead. Without going into the details of these experiments it is important to note that Ohm had gained a thorough appreciation of the significance of the specific conductivity of a conductor,
2. Experiments showing that the resistance of two conductors is the same when they have the same ratio of length to cross-section. Ohm's first experiments on cross-section proved the proposition which had already been proven by Davy and Becquerel, namely, that the resistance
