_Apparatus._ Tumbler two-thirds full of dilute sulphuric acid (258); strips of zinc, Zn (No. 60); copper strips, Cu (No. 67); iron strip, I (No. 76); lead strip, L (No. 77); carbon rod (No. 64); the galvanoscope, G V (No. 58); 2 wires with connectors (§ 226), so that the plates can be changed quickly; the wooden cross-piece, W C P (No. 70). _Arrange_ as in Fig. 90. The metal strips are all of the same size; they may be held with the hand firmly against W C P, in order to have them the same distance apart in each trial. They should be lowered to the bottom of the tumbler in each case, in order to have them acted upon by the same amount of acid. Place G V properly. [Illustration: Fig. 90.] =298. Directions.= (A) With Zn and Cu connected to G V as shown (Fig. 90), take the reading in degrees, and note in which direction (east or west) the N end of the needle is deflected. Tabulate results, as shown in Fig. 91, filling in each column of your table made out on paper. (B) In like manner try the following combinations in the order given, in each case connecting the first-mentioned plate with the left-hand binding-post, L, of G V. For (B) use zinc-iron. (C) Use zinc-lead; (D) iron-copper; (E) iron-lead; (F) lead-copper; (G) copper-carbon. [Illustration: +------+---------------+------------+-------------+------------+ | PART | PLATES. | LIQUID. | DEFLECTION. | CURRENT IN | | | | | | CELL FROM | +------+-------+-------+------------+-------------+------------+ | (A) | ZINC. |COPPER.| DIL. SULP. | 65° WEST. | CU TO ZN | | | | | ACID. | | | | (B) | | | | | | | | | | | | | | (C) | | | | | | Fig. 91.] =299. Note.= Some of the combinations produce but slight currents. In case G V is not delicate enough to show clearly which way the current passes, use the astatic galvanoscope in its place for such combinations. _=300. Discussion.=_ Exp. 116 clearly showed that different combinations of metals in the acid have different powers of pushing electricity through the galvanoscope. Although some of the pairs of metals furnished so weak a current that it was necessary to use the astatic galvanoscope to study the current, all produced _some_ current, and from the results can be formed an electromotive series (§ 301). The strength of acid, condition of plates, etc., affect the E. M. F. of a cell.