The zinc sulphate produced weakens the effect of the acid; in fact, the acid has to be renewed occasionally, as it is changed to the sulphate which remains in solution. =271. Action in Cell Using Impure Zinc.= The above action takes place in the cell when impure zinc is used, even when no current passes, heat being produced by the reaction instead of useful electricity. (See Local Currents.) =Action in Cell Using Pure Zinc.= When pure zinc (or impure zinc that has been properly amalgamated) is used in the cell, it dissolves, or is eaten away, only when the current passes. It should be noted that the bubbles of hydrogen do not even then appear at the zinc; they are not seen throughout the body of the liquid. There seems to be an unseen transfer of hydrogen from the zinc, through the liquid, to the copper (or other - plate used), and it appears there in the shape of bubbles. The larger the quantity of pure zinc dissolved, the stronger the current, and the larger the amount of hydrogen produced. 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 SO_4 part of it travels towards the Zn plate, where the ZNSO_4 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, 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.