TILE EARTII-EARTII CURRENTS. Mention has been made of the 'earth' in the preceding descrip tion. This is the technical term used in relation to the tact, discovered by Steinheil in 1833, that the earth itself serves the purpose of completing the circuit, and renders the employment of a sec ond or return wire unnecessary. The `earth' may consist of a buried plate of metal connected with the battery or line wire, and of sufficient surface to afford the necessary contact, with sufficient earth to make a good connection, it being under stood that a small quantity of earth does not connect well. Gas o• water pipes form excellent 'earths,' care being taken that the connection is made with the main pipe itself, and not with a branch, where a badly made point might spoil the connection. Where similar `earths' are in use, as, for instance, a copper plate at one end and an iron pipe at the other, a quasi battery is created, and minute currents pass along the line; these, however, are too weak to be of consequence. The
earth, being of great size. offers no sensible resist ance to the passage of the current, in the same way that a large wire offers less resistance than a thin one. While this quality of the earth is one of the most valuable aids to telegraphy (re ducing so materially the cost of wire erection), it presents at times those embarrassing inter ruptions known as earth currents. These cur rents, at all times unwelcome visitors to a tele graph office, are very variable, changing rapidly from positive to negative, altering their direction with the hour of the day, and leaving one circuit to appear on another in a manner not explain able.