We may now proceed to look at the Wheatstone needle telegraph and see the method of working it. We know already that when a pair of metallic plates are immersed in a fluid which acts chemically more rapidly on the one than the other, and a wire connects the upper parts of these plates, this wonderful agency is set in motion, and circulates from the one plate to the other (fig. 242). This arrangement may be best shown by using one plate of zinc and the other of copper, and a dilute solution of sulphuric acid for the liquid; this, however, produces by far too little of the agent to be used on a telegraphic line, there are therefore combinations of such pairs of plates, so arranged that the power of one pair shall be added to the next in such a way that at the end of the series (called a “battery”) there shall be a great increase of the power accumulated; this arrangement is shown in fig. 244. Now (if the power be sufficient) it does not signify what length of wire there may be between the two ends of this arrangement or “battery”; whether they be connected by a few feet or many hundred miles, the electricity passes instantaneously from one end to the other; and furthermore, it has been found in practice, that this electrical influence can be transmitted through the earth in one direction if sent by a wire in the other; for instance, if a wire from one end of the battery be carried on from London to Liverpool, instead of having another from Liverpool to London, to connect the two ends of the battery, it is found to answer the same purpose if the end of the wire at Liverpool be fastened to a plate of metal buried beneath the surface of the earth, and the other end of the battery at London furnished with a similar plate, also buried. In this arrangement, the electricity will pass beneath the surface of the earth from Liverpool to London, and through the wire from London to Liverpool, thus completing the circuit. The end from which the electricity passes is called the “positive electrode,” that to which it returns, the “negative electrode.” Fig. 245 will show this arrangement. [Illustration: Fig. 243.—The Needle Telegraph.] [Illustration: Fig. 244.—The Battery.] [Illustration: Fig. 245.—The circuit.] [Illustration: Fig. 246. Fig. 247. Fig. 248. Magnetic needles.]