THE ELECTRIC TELEGRAPH. 



described, would be subject to less waste of the electric fluid en 

 route ; but it is more economical to provide batteries sufficiently 

 powerful to bear this waste, than to cover such extensive lengths 

 of wire with any envelope. 



60. Atmospheric electricity having been found to be occasion- 

 ally attracted to the wires, and to pass along them, so as to 

 disturb the indications of the telegraphic instruments, and 

 sometimes even to be attended with no inconsiderable danger 

 to those employed in working the apparatus ; various expedients 

 have been contrived for removing the inconvenience and averting 

 the danger. The current produced by this atmospheric electricity 

 is often so intense as to render some of the finer wires used in 

 certain parts of the apparatus at the stations, red hot, and 

 sometimes even to fuse them. It also produces very injurious 

 effects by demagnetising the needles, or imparting permanent 

 magnetism to certain bars of iron included in the apparatus, 

 which thus become unfit for use. 



61. One of the expedients used for the prevention of these 

 inconvenient and injurious effects is to place common lightning 

 conductors on the posts. The points of these are shown upon the 

 posts in figs. 10, 11, and 12. 



62. Mr. Walker of the South Eastern Company and M. Bre- 

 guet of Paris, have each invented an instrument for the better 

 protection of telegraphic stations from atmospheric electric dis- 

 charges. Both these contrivances have been found in practice to 

 be efficacious, and though differing altogether in form they are 

 similar in principle. In both, a much finer wire than any which 

 lies in the regular route of the current is interposed between the 

 line wire and the station, so that an intense and dangerous 

 atmospheric current must first pass this fine wire before reaching 

 the station. Now it is the property of such a current to raise the 

 temperature of the conductor over which it passes to a higher 

 and higher point in proportion to the resistance which such con- 

 ductor offers to its passage. But the resistance offered by the 

 wire is greater in the same proportion as its section is smaller. 

 The safety wire interposed in these contrivances is, therefore, of 

 such thinness that it must be fused by a current of dangerous 

 intensity. The wire being thus destroyed all electric communi- 

 cation with the station is cut off, and the extent of the incon- 

 venience is the temporary suspension of the business of the line 

 until the breach has been repaired. 



Expedients are used on the American lines to divert the 



atmospheric electricity from the wires, consisting merely of a 



number of fine points projecting from a piece of metal connected 



with the earth by a rod of metal. These points are presented to 



142 



