tive application of the newly discovered law to the prediction of new phenomena. The first stage is generally the longest, the second the most contradictory and difficult, while the third is the most fruitful and may perhaps be regarded as the most interesting.
The branch of electrical science to which Ohm's law belongs is now so well advanced into the third stage, into which it may be regarded as having been ushered by Clerk Maxwell, that it is difficult to look back to stages one or two. Yet eighty-five years ago the theory of electricity was in the first stage, and the 3^ears from 1825 to 1860, in some respects, mark the limits of the second. This period includes Ohm's work and that of his immediate successors.
Ohm's work was made possible by the discoveries of Galvani, Volta, Oersted, Ampère, Seebeck and others, whose researches had so broadened the knowledge of phenomena connected with the galvanic circuit as to show the probability of a connecting theory. The formulation of such a connecting theory is the task to which Ohm set himself, approaching the problem from the experimental side. The workers of the early nineteenth century had already in a measure identified and separated the factors at play in the galvanic circuit. Examples of such factors are: the "contact force" at the terminals of the circuit, the "flow of current" along the wire, the "electroscopic force" between any two points of a circuit, the tendency of electricity to escape into the air, and the polarization of the electrodes. There was, however, a lack of definiteness of ideas, as well as of methods of quantitative measurement.
It is no small task to select the necessary from among the incidental factors and to express their relation in concise form. Upon this achievement rests Ohm's chief claim to fame. This is not, however, his only claim to consideration, for besides establishing the law which bears his name, he devised mathematical methods for determining the distribution of electricity in a complex system of conductors, for both steady and variable currents. He did much in clearing up the conception of such terms as electromotive-force, current and resistance. In fact, he did for Volta what Maxwell later did for Faraday. The contributions of Dr. Ohm to the theory of electricity were therefore many sided. They were accomplished because he was a trained mathematician, a skilled experimenter and a keen, logical thinker. A less trained man could not have completed his work; one less honest would have been misled ere the end was reached. Finally, it is interesting to note that recognition of the value of his labors and of the importance of his law came tardily. Like many others, his work was misunderstood and only late in life did appreciation and the ambition of his youth—a university appointment—fall to his lot.
A complete statement of the law discovered by Dr. Ohm involves two independent propositions as follows:
