Page:Popular Science Monthly Volume 22.djvu/63

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though not entirely fulfilling that condition. The congress, while adopting the absolute system of the British Association, referred the final determination of the unit measure of resistance to an international committee, to be appointed by the representatives of the several governments; they decided to retain the mercury standard for reproduction and comparison, by which means the advantages of both systems are happily combined, and much valuable labor is utilized; only, instead of expressing electrical quantities directly in absolute measure, the congress has embodied a consistent system, based on the Ohm, in which the units are of a value convenient for practical measurements. In this, which we must hereafter know as the "practical system," as distinguished from the "absolute system," the units are named after leading physicists, the Ohm, Ampère, Volt, Coulomb, and Farad.

I would venture to suggest that two further units might, with advantage, be added to the system decided on by the International Congress at Paris. The first of these is the unit of magnetic quantity or pole. It is of much importance, and few will regard otherwise than with satisfaction the suggestion of Clausius that the unit should be called a "Weber," thus retaining a name most closely connected with electrical measurements, and only omitted by the congress in order to avoid the risk of confusion in the magnitude of the unit current with which his name had been formerly associated.

The other unit I should suggest adding to the list is that of power. The power conveyed by a current of an Ampère through the difference of potential of a Volt is the unit consistent with the practical system. It might be appropriately called a Watt, in honor of that master-mind in mechanical science, James Watt. He it was who first had a clear physical conception of power, and gave a rational method of measuring it. A Watt, then, expresses the rate of an Ampère multiplied by a Volt, while a horse-power is 746 Watts, and a Cheval de Vapeur 735.

The system of electro-magnetic units would then be:

1. Weber, the unit of magnetic quantity = 108 C. G. S. units,
2. Ohm, "" resistance = 109 "
3. Volt, "" electromotive force = 108 "
4. Ampère, "" current = 10-1 "
5. Coulomb, "" quantity = 10-1 "
6. Watt, "" power = 107 "
7. Farad, "" capacity = 10-9 "

Before the list can be looked upon as complete two other units may have to be added, the one expressing that of magnetic field, and the other of heat in terms of the electro-magnetic system. Sir William Thomson suggested the former at the Paris congress, and pointed out that it would be proper to attach to it the name of Gauss, who first theoretically and practically reduced observations of terrestrial magnetism to absolute measure. A Gauss will, then, be defined as the