=241. The simple voltaic cell= consists of a strip of copper and a strip of zinc placed in dilute sulphuric acid. (See Fig. 219.) A short time after placing the plates in the acid, bubbles of a gas (hydrogen) appear on the surface of the zinc. These bubbles increase in size and some rise to the surface of the liquid. Nothing appears upon the copper plate. If the tops of the plates are connected by a wire, an electric current is set up through the wire and the cell, and bubbles of gas also appear upon the _copper_ as well as on the zinc. In a short time the surface of the copper becomes coated with bubbles and the current becomes much _weaker_. If the plates are left in the acid for some time the zinc is found to be eaten away, having been dissolved in the acid through chemical action. The copper, however, remains practically unaffected. [Illustration: FIG. 219.--A simple voltaic cell.] =242. How the Current is Produced.=--To maintain the electric current a continual supply of energy is required. This is furnished by the _chemical action_ of the acid upon the zinc. The chemical action is in several respects like _combustion_ or _burning_, by means of which chemical energy is transformed into heat energy. In the voltaic cell the chemical action of the acid upon the zinc _transforms_ chemical energy into electrical energy. The E.M.F. or _difference of potential_ may be considered as originating at the surface of the zinc where the chemical action takes place. At this point the zinc has the lower and the liquid in contact with it the higher potential. The molecules of the acid are believed to be separated or broken up into two parts called _ions_; one ion, the SO_{4} or _sulphion_, combines with the zinc forming zinc sulphate, the other, or hydrogen (H) ion, passes over to the copper plate, and accumulates on the surface of this plate giving it a positive charge. It is therefore called the _positive ion_. The sulphion ion, or SO_{4} ion, carries a negative charge to the zinc. It is therefore called the _negative ion_. =243. The Direction of the Current.=[K]--Beginning at the surface of the zinc the _direction_ of the movement of _positive_ electricity may be traced through the liquid to the copper plate, to the wire, to the zinc plate, to the starting point, thus completing the electric circuit. When the circuit is closed it is found that the movement of electricity starts in _all_ parts of the circuit at practically _the same instant_. [K] Many scientists consider that current in a conductor consists of _negative electrons_ flowing in a direction _opposite_ to that described in Art. 243. This is called the _electron current_, as distinguished from the _electric current_ described above. [Illustration: FIG. 220.--A comparison of a voltaic cell and circuit to a water pump and connecting pipes.]