=247. Amalgamation.=--Local action is prevented by coating the zinc with mercury. This process is called _amalgamation_. The mercury covers the entire surface of the plate in the acid. Its action is to dissolve pure zinc and bring it to the outer surface where it is acted upon by the acid. The carbon particles are kept covered so that no local currents can be formed as long as the bits of carbon are below the surface. Amalgamation therefore prevents local action. Important Topics _The Simple Voltaic Cell_ 1. Two plates: zinc, copper; electrolyte, dilute sulphuric acid. 2. Ions: hydrogen, positive: sulphion, negative. 3. Current, where and how produced, direction, illustration. 4. Polarization: cure, local action, cure. Exercises 1. Write in your own words an account of the production of an electric current by the simple voltaic cell. Use sketches. 2. Which plate has the higher potential? How is it produced? 3. Would you expect to get an E.M.F. by forming a cell of two copper plates? Why? (3) PRACTICAL VOLTAIC CELLS =248. Advantages of Voltaic Cells.=--Many forms of voltaic cells have been devised. Several of the more common of these will be described and their electro-chemical action explained. At the present time voltaic cells are employed only where small currents are needed, such as for electric bells and induction coils. Where more than a small amount of current is required, the dynamo and the storage battery have generally taken their place as sources of electric current. The advantages of voltaic cells as electric generators are: (a) they are inexpensive, (b) they are easily taken from place to place, (c) they may be ready for instant use. The most desirable voltaic cell would be one having the following qualities: (a) High electromotive force, (b) no polarization or local action, (c) very low internal resistance, (d) small expense, both as to first cost and upkeep. [Illustration: FIG. 222.--The Leclanché cell, "wet" type.]