345. Directions. (A) Be sure that 2-F C gives a constant current, shown by the uniform deflection at G V, when arranged as in [Fig. 106]. Do not use a cell that quickly polarizes. The coil, X, forms a part of the circuit; it is joined to wires, 1 and 2, by means of metal plates, so that it may be quickly removed without disturbing either G V or 2-F C. Carefully read the deflection at G V.
(B) Remove X from the circuit, and join the free end of wire, 2, to binding-post, X, and the free end of wire, 1, to a small piece of sheet copper, which can be firmly pressed upon the G-s wire to make a contact. Move this along on the G-s wire until the deflection produced equals that of part (A), remembering that the longer the G-s wire in the circuit the less the deflection. Make two or three trials, as one or two cm. difference in length make but a little difference in the deflection. Note the number of cm. of G-s wire used, the resistance of which must equal that of the coil, X.
(C) Find the resistance of X by multiplying the length just found by the resistance of each cm., and compare the result with the value found by using the bridge method directly.
EXPERIMENT 140. To measure the E. M. F. of a cell by comparison with the two-fluid cell.
Apparatus. Rheostat ([§ 344]); the two-fluid cell, 2-F C ([Exp. 113]), the E. M. F. of which may be taken as 1 volt; dry cell, D C; galvanoscope, G V. Arrange first as in [Fig. 107].
346. Directions. (A) Be sure that 2-F C gives a constant current. Take the reading of G V without the rheostat in the circuit; that is, with wires, 2 and 1, joined directly. The deflection should be 50 or 60 degrees at least, and be constant.
(B) Attach a small piece of copper to the end of 1, and firmly rub it along upon the G-s wire, thus introducing resistance into the circuit, until the deflection is, say, 60° (50 or 55 degrees will do). Note the length of G-s wire used and call it (B).