Abstract
On the Absolute Measurement of Electric Currents, by Prof. Lord Rayleigh.—The absolute measurement of current is more difficult than that of resistance. All the methods hitherto employed require either accurate measurements of the horizontal intensity of the earth's magnetism or of coils of small radius and many turns. This latter is difficult to evaluate, as it is impossible to measure the length of the wire wound, as the tension necesjary to make the wire lie evenly, stretches it very considerably, whilst it is most important to determine the mean radius accurately, as an error therein doubles itself in the final result. The method of Kohlrausch is free from this objection, but it requires a knowledge of the moment of inertia, a quantity not easy accurately to determine. When the electromagnetic action is a simple force, it can be determined directly. In Mascart's recent determination, a large solenoid is suspended vertically in a balance, and is acted on by a ilat co-axal coil of much larger radius. This is simple to think about, but not calculated to secure precise results. The appearance of accuracy is illusory, unless it can be assumed that the distribution of wire is absolutely uniform. It would appear that all the turns of the suspended coil should operate as much as possible, that is, that the suspended coil should be compact, and should be placed in the position of maximum effect. There is a further incidental advantage in this arrangement. The expression for the attraction involves as factors the product of the number of turns, the square of the current, and a function of the mean radii of the two coils, and of the distance between their mean planes. This function is of no dimen-ions. When the position is such that the function for two given coils is a maximum, the result is practically dependent only on the two mean radii, and the function being of no dimensions, can involve these mean radii only in the form of a ratio. This can be obtained electrically with full precision by dividing a current between them in such a way that no effect is produced on a small magnet at their common centre. In practice it will be desirable to duplicate the fixed coil, placing the suspended coil midway between two similar fixed ones, through which the current passes in opposite directions.
Article PDF
Rights and permissions
About this article
Cite this article
The British Association: Section A—Mathematical and Physical. Nature 26, 465–466 (1882). https://doi.org/10.1038/026465a0
Issue Date:
DOI: https://doi.org/10.1038/026465a0