=324. The Simple Wheatstone's Bridge= (Fig. 103) consists of a wooden base, W, at the ends of which are fastened two aluminum conductors, 1 and 3. At one side of W is fastened another conductor, 2. In Fig. 104 are side views of the conductors. These are used merely for convenience in making connections, and take the place of the metal plates used in previous experiments. A German-silver wire, G-s W, is stretched between 1 and 3, and under this is a scale, S, divided into 100 small parts, these being tenths of the larger divisions. The ends of G-s W are held between eyelets, as shown at E, Fig. 104.
=Reading the Scale.= The value of part A can be read directly from the scale, using the lower row of figures. The point marked P, for example, would be read 3.7 (three and seven-tenths large divisions); B would be 6.3, found by subtracting 3.7 from 10. The sum of A and B must always equal 10. The 6.3 may also be read directly by using the upper row of figures for the whole numbers, counting the tenths to the left. Try to divide the smallest divisions into halves, at least; that is, if A = 3.75, B = 6.25. Take the readings carefully.
=325. Directions.= (A) Touch the free end of wire, 3, to the point, C, which has a higher potential than M P. Press down K for an instant only. Some current should pass through A G, as a shunt. Should it pass from C to M P or the reverse? Note in which direction the right-hand end of the astatic needle is deflected.
(B) Swing the end of 3 around and touch it to the point, Z, which has a lower potential than M P. Press K for an instant, watch the needle, and compare with the results in (A).
(C) Move the free end of 3 along on G-s W, touching K at intervals, until a point is found at which the needle of A G is not deflected. How does the potential of this point compare with that of M P?
_=326. Discussion; Equipotential Points.=_ Since one end of the G-s W has a higher, and its other end has a lower potential than M P, there must be, somewhere on it, a point at which the potential is the same as at M P. This place is quickly found by sliding the free end of wire, 3, along, pressing K occasionally, until A G shows that no current tends to pass through it in either direction, when the current passes from C to Z through the two branches of the divided circuit. This point and M P are called _equipotential points_.