As the zinc dissolves it parts with its latent energy, and this energy forces the electric current through the circuit. While the hydrogen of the decomposed acid makes its way towards the - plate, the SO4 part of it travels towards the Zn plate, where the ZnSO4 is formed. (See [§ 270].)
EXPERIMENT 111. To see what is meant by "local currents" in the cell.
Apparatus. Tumbler of dilute sulphuric acid. ([§ 258]); strip of unamalgamated zinc; crystal of copper sulphate (blue vitriol) (No. 86); a galvanized iron nail (No. 69), this being iron covered with zinc.
272. Directions. (A) Hold the nail in the acid for a few seconds, and note result.
(B) Rub the copper sulphate upon the zinc simply in one spot, then place the zinc in the acid, noting the result at the spot.
273. Local action; Local Currents. Ordinary commercial zinc contains such impurities as carbon, iron, lead, etc., in small quantities. It was seen, [Exp. 109],[108] that when different metals were in contact with the zinc, the zinc was rapidly dissolved by the acid. The impurities in the zinc act like the copper plate in the simple cell, thus producing local currents in the zinc, which rapidly destroy it without doing any good. These currents pass from zinc to impurities, and back to the zinc, without going out into the main wire. This local action takes place even when the main circuit is open.
274. Reasons for Amalgamating Zinc Plates. Pure zinc is not affected by dilute sulphuric acid, but it is too expensive to use in cells; so amalgamated zinc is used instead, because it is cheaper, and acts the same as pure zinc. The impurities are removed from the surface of the zinc, as the zinc alone is dissolved by the mercury. There is, then, a liquid layer of pure zinc with mercury upon the surface of the amalgamated plate. This is not acted upon by the acid when the circuit is open. A stronger and more regular current is produced with amalgamated zinc than with the impure unamalgamated zinc.
EXPERIMENT 112. To study the "single-fluid" Galvanic cell.
Apparatus. The galvanoscope G V (No. 58), (See [§ 240], etc.); the simple cell arranged as described in [§ 275], the zinc being amalgamated.
275. The Simple Cell should be arranged so that the plates will be held firmly in position. The zinc, Zn (No. 60), and copper, Cu (No. 67), should be fastened to the wooden cross-piece, W C P (No. 70), as shown in [Fig. 85]. Care should be taken not to use longer screws than those provided for (No. 72). If the screws touch each other they will short circuit the cell. Partly fill the tumbler (No. 65) with dilute sulphuric acid ([§ 258]), join wires with connectors to the plates. The free ends of the wires are then ready to join to apparatus. The ends of wires may be fastened under the screw-heads instead of using connectors on the plates. Do not put the plates into the acid until you read the "directions."