Volume 3

Elementary treatise on natural philosophy / by A. Privat Deschanel ; translated and edited, with extensive additions, by J.D. Everett.

  • Everett J. D. (Joseph David), 1831-1904.
Date:
1870-1872
Catalogue details

Licence: Public Domain Mark

Credit: Elementary treatise on natural philosophy / by A. Privat Deschanel ; translated and edited, with extensive additions, by J.D. Everett. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    GfiO 534. Tangent Galvanometer.—The tangent galvanometer, wljich is simpler in its construction and use, and is much more frequently employed, consists of a declination needle mounted in the centre of a vertical circle whose plane always coincides with the magnetic meridian, the length of the needle being small in comparison with the radius of the circle. Let 0 (Fig. 460) be tlie centre of suspen- sion, a b the initial position of the needle, and a b' its deflected position. Tlie force F exerted on either pole by the current is sen- sibly the same at a' as at a on account of the smallness of the needle, and it acts in the direction Ik, while the horizontal force of the earth upon the pole acts along a'm, Fig. 460.—Principle of Tangent Galvanpmeter. and these two forces give a resultant alonjr oa'. Hence, taking the triangle ola as the triangle of forces/ the force exerted by the current is to the hori- zontal force exerted by the earth as la to ol, or as tan a to unity; that is, the current is proportional to the tangent of the deflection. In order to permit the deviations of the short needle to be accur- ately read, a long pointer is attached to it, usually at right angles, the two ends of which move along a fixed horizontal circle. 535. Multiplier.—The idea of carrying a wire several times round a needle in a vertical plane is due to Schweiger. The form of apparatus de- signed by him, called Schweiger's mul- tiplier, is represented in Fig. 461. The difference between the rectangular and the circular form is merely a matter of detail. The name multiplier is derived from tlie fact that, if the current is not sensibly diminished by increasing the number of convolutions of wire through wliich it has to pass, the force exerted on since each convolution exerts its own force on the needle indepen- dent of the rest. Cases, however, frequently occur in wliich the increased resistance introduced by increasing the number of convolu- * The parallelogram of forces is divided by its diagonal into two triangles, either of whicli may be called the triangle of forces. Fig. 461.—Schweiger's MuUipliGr.