. The elements of physiological physics: an outline of the elementary facts, principles, and methods of physics; and their applications in physiology. Biophysics. 138 PHYSIOLOGICAL PHYSICS. [Chap. xin. offered by a conductor is dependent on the tempera- ture. Increased temperature diminishes resistance, that is, increases conductivity. To measure resistances the simple rheostat may be used. A Daniell's element is placed in the circuit of a galvanometer; an extremely sensitive one is not necessary. When the current is permitted to pass through the galvanometer the needle is deflected. If, now,
. The elements of physiological physics: an outline of the elementary facts, principles, and methods of physics; and their applications in physiology. Biophysics. 138 PHYSIOLOGICAL PHYSICS. [Chap. xin. offered by a conductor is dependent on the tempera- ture. Increased temperature diminishes resistance, that is, increases conductivity. To measure resistances the simple rheostat may be used. A Daniell's element is placed in the circuit of a galvanometer; an extremely sensitive one is not necessary. When the current is permitted to pass through the galvanometer the needle is deflected. If, now, the body whose resistance is to be measured be placed in the same circuit, the current from the Daniell, encountering the resistance of the body, will not be able to deflect the needle to the same extent. Note the amount of deflection. Remove the body, and place, instead, the rheostat in circuit. Send the cur- rent again through the galvanometer, and then wind the rheostat wire on to the wooden cylinder ; this adds resistance, and the needle will be brought slowly back from its maximum deflection caused by the Daniell. Wind on the wire till the resistance it offers causes the needle to be brought back to the position it occupied when the body to be measured was in circuit. At that position the amount of wire wound on to the wooden cylinder will be the measure of the resistance offered by the body. A method of greater value is the method by WHEATSTONE'S BRIDGE. It shown in diagram in 71. At e is a Daniell's ele- ment; and AB is the platinum e wire of a long compensator, Fig 71.—Wheatstoue's Bridge. * i • r • xi v J of which s is the slider. (See page 121.) From e the positive electrode goes to A, the negative to B. From A a wire passes to r, the body whose resistance is to measured, from which again a wire is carried to one binding screw of Re, a resistance box, from whose other binding screw a wire. Please note that these images are extracted from scanned page images that
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