In arranging a telegraphic circuit, the voltaic batteries and the instruments are introduced at breaks in the telegraph wire. The course of the electric current is from the copper end of the battery through the transmitting instrument, then along the wire to the receiving instrument; from that it passes to the earth and is thus returned to the transmitting station, where it completes the circuit by being conducted from the earth-plate to the zinc end of the voltaic battery. The arrangement for completing the circuit will be more clearly understood by reference to the accompanying diagram. [Illustration] The wire from _C_, which is the copper pole of the voltaic battery, is connected with the instrument _A_; the electric current is then transmitted along the wire _D_ to the receiving instrument _B_; thence it is transferred to the earth-plate _E_, passes through the earth to the corresponding plate _E´_, which is connected with _Z_, the zinc pole of the battery. When a communication is returned from _B_ to _A_, a similar arrangement is made; the wires connected with the instruments being so arranged as to bring into action a voltaic battery at _B_, and to throw out of circuit the one at _A_; for the connection with the battery is only made when the transmitting instrument is worked. Since all the electric telegraphs in different parts of the world are connected with the earth, as one portion of the circuit, it might be supposed that the various currents would mingle, and occasion a confusion of messages; but it must be borne in mind that no electric current is formed until a communication be made from one pole of a voltaic battery to the other, and as such communication can only be completed through the insulated wire, the earth-currents cannot mingle, but each one passes to the proper terminus of its respective battery. The accompanying diagram and explanation may serve to remove the difficulty of understanding why the two circuits are maintained quite distinct. [Illustration]