File:EB1911 Telegraph - Terminal Insulator.jpg

English: Technical drawing of a terminal insulator for a telegraph wire. At the termination of a line a large insulator, mounted on a strong steel bolt having a broad base flange, is employed. Connexion is made into the office (or to the underground system, as is often the case) from the aerial wire by means of a copper conductor, insulated with gutta-percha, which passes through a “leading in” cup, whereby leakage is prevented between the wire and the pole. The insulators are planted on creosoted oak arms, 2½ in. sq. and varying in length from 24 to 48 ins., the 24 and 33 in. arms taking two, and the 48 in. four, insulators. The unequal lengths of the 24 and 33 in. arms are adopted for the purpose of allowing one wire to fall clear of that beneath it, in the case of an insulator breaking or the securing binder giving way. The poles are of red fir, creosoted, this method of preservation being the only one now used for this purpose in the United Kingdom. The number of poles varies from about 15 to 22 per m. of line; they are planted to a depth of from 2 to 4 ft. in the ground. For protection from lightning each pole has an “earth wire” running from the top, down to the base.