EXPERIMENT 125. To study the construction and use of a simple "Wheatstone's Bridge."
Apparatus. [Fig. 102]. A Wheatstone's bridge, W B (No. 80), ([§ 324]); astatic galvanoscope, A G (No. 59); dry cell, D C (No. 51); key, K (No. 55); 7 wires with spring connectors, two of which, R and X, are equal in length; metal plate, M P, for connecting wires.
Arrange as in [Fig. 102]. The carbon of D C is joined to K, and this to the point, C, of the bridge. The zinc of D C connects with the point Z on W B. The A G is placed between the branches for clearness. Wire 3 is joined to the left-hand binding-post of A G, and wire 4 joins M P with the right-hand one. When the end of wire 3 does not touch G-s W, it is evident that as soon as K is pressed, the current divides at C on its way to Z, where the branches unite again. K is used so that D C will not be polarized by steady use.
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.