Fig. 220.—A comparison of a voltaic cell and circuit to a water pump and connecting pipes.
244. The production of the current may be illustrated by describing a device for producing a continuous circulation of water. Thus let Cu and Zn represent two pipes connected by two horizontal tubes, one at V provided with a valve and one at P with a rotary Pump. (See Fig. 220.) Suppose the pipes filled to the level of V and the pump started. The pump will force water from Zn to Cu, through P, the level falling in Zn and rising in Cu. If the valve V is open the water will flow back through V as long as the pump is working. If V is closed, the level in Cu will rise as high as the driving force of the pump can send it. If now V is opened, the pump will maintain the water in circulation from Cu to Zn through V. In the illustration, the tubes Cu and Zn correspond to the conducting plates of copper and zinc of a voltaic cell. The pump P represents the chemical action which produces the electrical pressure. The upper pipe represents the part of the circuit outside of the cell, the valve V corresponds to an electric key or switch which is used to open and close the electric circuit.
245. Polarization.—In the simple voltaic cell, after the circuit is closed, bubbles of hydrogen collect upon the copper plate. This accumulation of hydrogen gas is called polarization. It acts as a non-conducting layer upon the surface of the plate and seriously interferes with the movement of electricity from the liquid to the copper plate not only in the simple voltaic cell but in many others as well. Some voltaic cells are made entirely free from this defect, either (a) by the removal of the hydrogen as fast as it is formed, or (b) by the use of such chemicals that no hydrogen is produced.
246. Local Action.—It is noticed that when a strip of zinc is placed in dilute acid that bubbles appear upon the surface of the zinc. The appearance of these bubbles indicates that some of the hydrogen ions carrying positive electricity have moved to the zinc plate. Careful examination of the plate after it has been in acid shows numerous black spots upon it. These are bits of carbon. They are always found in ordinary zinc. Small electric currents are set up which run from molecules of pure zinc into the liquid and back to the carbon particles, thus forming small closed circuits. (See Fig. 221.) The formation of these circuits from and to the zinc is called local action. This action is a defect in voltaic cells since a part of the current is thus kept from passing through the main outside circuit, and the zinc may be consumed even when no outside current is flowing.
Fig. 221.—Local action.
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.