THE ELECTIUC TELEGKAPH. 



D the plates are generally four to five inches wide, and three to 

 four inches deep. The thickness of the zinc plates is something 

 less than a quarter of an inch. The cells are filled with sand to 

 within an inch of the top, and the parts of the plates above the 

 sand are varnished as a protection against corrosion, and to keep 

 them clean. In general, the troughs are made either of glazed 

 earthenware or some compact wood, such as oak, or teak, made 

 water-tight by cement or marine glue. When the trough is wood 

 the partitions of the cells are slate, the width of each cell being 

 one inch and a quarter to one inch and a half. The troughs 

 contain, some twenty-four, and some twelve cells. 



Batteries of this sort, consisting of twenty-four cells, give a 

 current of sufficient force for a line of wire of 15 miles. For 

 50 miles, 48 cells, and for 75 miles, three troughs of 24 cells are 

 required. Mr. Walker considers that these batteries give super- 

 fluous force, but that it is necessary to provide against the 

 contingency of leakage by accidental defects of insulation. 



45. The durability of these batteries is increased by amalga- 

 mating the zinc plates. This is effected by first washing them in 

 acidulated water, and then immersing them in a bath of mercury 

 for one or two minutes. The mercury will combine with the zinc 

 and form a superficial coating of the amalgam of zinc. When they 

 are worn by use, they may be restored, by scouring them, and 

 submitting them to the same process, and this may be continued 

 until the zinc become too thin to hold together. 



Mr. Walker states that new batteries, when carefully put 

 together, will, with care, do duty for six or eight months, when 

 the work is not very heavy ; and by washing the sand out with a 

 fiow of water, and refilling them, they have frequently remained 

 on duty ten or twelve months, or even more, without having been 

 sent in for re-amalgamation.* 



46. Having explained, generally, the manner in which the 

 electric current is produced and maintained, I shall now proceed to 

 explain the various expedients by which it is conducted from station 

 to station, along the telegraphic line, and by which injurious 

 waste by leakage or drainage is prevented or diminished. 



The conducting wires used for telegraphic lines are of iron, 

 usually the sixth of an inch in diameter. On all European lines 

 they are submitted to a process called galvanisation, being passed 

 through a bath of liquid zinc, by which they become coated with 

 that metal. This zinc surface being easily oxydable, is soon, by 

 the action, of air and moisture, converted into the oxyde of zinc, 

 which, being insoluble by water, remains upon the wire, and 

 protects the iron from all corrosion. 



* El. Tel. Manip., p. 8. 



