ELECTRIC TELEGRAPHS. 



+3 



Fig. 1. 



the same inventors have been much more extensively used, the former 

 requiring only one wire, and the latter two. 



All public telegraphs have now for many years been worked by 

 voltaic currents ; the magneto-electric system, which was tried on 

 some lines, having been found to involve a needless expenditure of 

 labor. 



According to Mr. Culley, engineer-in-chief to the post-office, the 

 battery which had been adopted by the authorities of that department 

 is a modified Daniell's, consisting of 

 a teak trough, divided into cells by 

 plates of glass or slate, and well 

 coated with marine glue, each cell 

 being divided into two by a slab of 

 porous porcelain. The zinc plates 

 measure four inches by two, and 

 the copper plates, which are very 

 thin, are four inches square. The 

 zinc hangs at the upper part of its 

 cell, which is filled with dilute solu- 

 tion of sulphate of zinc. The cop- 

 per cell is filled with a saturated 

 solution of sulphate of copper, and 

 crystals of this salt are placed at 

 the bottom. The expenditure in 

 sulphate of copper is about a pound 

 and a half for each cell per annum. 



The wires for land -telegraphs 

 are commonly of what is called gal- 

 vanized iron, that is, iron coated 

 with zinc, supported on posts by 



means of glass or porcelain insulators, so contrived that some part of 

 the porcelain surface is sheltered from rain, and insulates the wires 

 from the posts, even in wet weather. Wires thus suspended are called 

 air-lines. 



Underground wires are, however, sometimes employed. They are 

 insulated by a coating of gutta-percha, and are usually laid in pipes, 

 an arrangement which admits of their being repaired or renewed with- 

 out opening the ground except at the drawing-in boxes. There is less 

 leakage of electricity from subterranean than from air lines, but their 

 cost is greater, and they are less suited for rapid signalling on account 

 of the retardation caused by the inductive action between the wire and 

 the conducting earth, which is similar to that between the two coat- 

 ings of a Ley den jar. 



The early inventors of electric telegraphs supposed that a current 

 could not be sent from one station to another without a return-wire to 

 complete the circuit. Steinheil, while conducting experiments on a 



Insulators. 



