Note. In practice it is most convenient to make connections as shown in [Fig. 105] when measuring resistances ([Exp. 126]). The arrangement given in [Fig. 102] is simply for explanation. It will be seen that the smaller A is, compared with B, the larger the unknown resistance compared with your standard.
EXPERIMENT 126. To measure the resistance of a wire by means of Wheatstone's Bridge; the "bridge method."
Apparatus. Same as in [Exp. 125]; the two-ohm resistance coil, R C (No. 79); a coil of wire, X, as, for example, the 15-turn coil on the galvanoscope, G V (No. 58).
Arrange as in [Fig. 105]. You will observe that the central conductor of the bridge (2, [Fig. 104]) takes the place of M P in previous explanations. We still have the same kind of a divided circuit as explained in [Exp. 125], A G being connected with points of equal potential. It will be found convenient to have D C at the right, and A G facing you at the left, the key being in front. (See [Exp. 107] in regard to adjusting A G.)
Notice that you have a standard resistance (2 ohms) in place of R, [Fig. 102], and an unknown resistance (galvanoscope coil) in place of X. (See [§ 330].)
329. Directions. (A) Touch the free end of wire, 3, to the left-hand side of the bridge-wire, press the key for an instant, only, and note the direction taken by the right-hand end of the needle. Move the end of wire, 3, to the right-hand side of the bridge-wire, touch key, watching needle. Does the needle move more or less than before? In the same or opposite direction? If the deflections are opposite, the point that has the same potential as binding-post, 2, must be between the two points touched.
(B) Be sure that all connections are good. Find the point on G-s W, at which there is no deflection, as directed in [Exp. 125] (C). Note the readings on the scale, as explained in [§ 324].