=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. [Illustration: FIG. 221.--Local action.]