SOURCES OF ELECTRIC CURRENTS. To maintain a steady electric current some method must he used which will produce a constant difference of potential. Four may be mentioned: (i I If an ordinary electrostatic machine i- used it will be found to give a current of small intensity. the energy for the current coining from the work done in producing the charges by means of the machine. 2I If two wires of different material, e.g. bras, and iron. arc joined end to end, fo•m ing a single circuit, there will be two junetions of brass with iron: and. if these two junctions are kept at difb rent temperature,, there will be (in general) an electric eurrent round the cir cuit. The energy for this current comes almo4 entirely from the source of heat at the hot junc tion: but it in; y conic ill rils from the fall of temper:min...1 one of the wires. The E. :NI, F. of the current is. as a rule, extremely small. ( See TnERMO- ELF( TRI( ITV.) ( 3) If two different solid e.g. rolls of copper and zinc, dip into a dilute solution of sulphu•ie acid in water, the two metal, will be found to be at a ditterenee of potential. The same is true of any two metal- dipping into any liquid—other than a fused inetal—whielt is an electric eondoetor. Such an ment is called a 'cell' or 'ele ment.' The one mentioned has many disadvan tages. some of whirls may be obviated by using two liquids s'1 rated by a porous partition. (For
a description of different of cells. Tue ('ELI. or. Il•rrERY.) If a cell. as described above, consisting of a zine and copper rod (lipping in dilute sulphuric acid i- used to produce a eu• rent in a wire joining the projecting end- of the two rod-, it is observed that the current in the wire is in the direction copper to zinc; that the zinc' rod away, and the loss in mass of the zinc is proportional to the current-strength and to the time: that bubble, of hydrogen gas form at the copper rod. It is known from chemi cal eNperiments that, when zinc is di-solved in a vessel containing dilute sulphuric acid, hydrogen gas is formed and the temperature rises, shorting in the reaction energy is given out. In the simple cell just described the zinc dissolves, hydrogen gas is formed, and the energy liberated is used in producing the electric current. A similar explanation of the supply of energy may be given for all evils, it being noted, however, that in some cases energy is taken front the liquid of the cell itself—apart from chemical action—as is shown by its tall in temperature, and in others energy is given to the cell, as is shown by a rise in temperature. (-II Electric currents may be produced by dynamos (see DYNA MO-ELECTRIC 1\ IACIIINERY). the principle of which will be examined under induced Curreats.