Lessons in elementary physics / by Balfour Stewart.

  • Balfour Stewart
Date:
1895
Catalogue details

Licence: Public Domain Mark

Credit: Lessons in elementary physics / by Balfour Stewart. Source: Wellcome Collection.

    433/502 page 413
    i.i'ss;. XI.VI ELECTRICITY IN MOTION 4'3 * a very delicate instrument for indicating' the existence of such ^ currents, and for measuring their intensity. It is called a galvanometer. Let us begin by supposing that a current passes in the plane of the magnetic meridian, vertically above .and below a deli- cately suspended needle, as in Fig-. i2gr, the direction of the current being denoted bv the arrow-heads. The current abov'e the needle will, by Case 2, cause the needle to turn so as to place its marked pole a above the plane of the paper, and bv Case 3 the current below the needle will have a similar action. Tims the action of the two currents will supple- ment each other. Now if the current be coiled many times round about the needle in the .same direction, as in the figure, before it is brought back to the battery, and if each of these turns be insulated by inclosing the wire in a non-conductor, then will tlie various turns of the coil supplement each other, and their united action upon the needle will become very powerful. An instrument constructed in this way is called a Simple Galvanometer, of which there are two types, according as the needle moves, as in Fig. in a horizontal or in a vertical plane. The latter is the least delicate of the two, but it is more convenient for use in telegraphy and for general u.se for detecting currents. 397. Astatic Galvanometer.—In this arrangement the struggle is between the directive force of the earth tending to keep the needle parallel to the wires, and the influence of the current tend- ing to bring it into a position at right angles to the wires, .and the apparatus will be rendered much more sensitive if we can overcome tlie directive force of tlie e.artli. This is done by having two magnetic needles of the .same strength, suspended as in Fig. 130, the one needle being wholly above the current, and having its poles opposed to the
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