INSULATING SUPPORTS. 



Tig. 21. 



the pole (3), and is covered with paint or varnish. The con- 

 ducting wire is fastened to the top of the cap by projecting 

 iron points, and the whole of the iron cap is thus in the circuit, as 

 the wire is of iron and not insulated. To 

 prevent the deposit of moisture, the glass is 

 covered by a varnish of gum-lac dissolved 

 in alcohol, and the ring-like form of the 

 glass is to cause any moisture to be 

 carried to the edge and there drop off.* 



The wires on the American lines are 

 not usually galvanised. 



57. One of the forms of insulating sup- 

 port used on the German lines is repre- 

 sented in fig. 22, and consists of an 

 insulating cap placed on the tapering end 

 of a post T. The post terminates in a 

 point c, an inch and a half in length 

 and about six lines in diameter; this 

 pole is covered with a porcelain cap d d, 

 a sort of reversed cup ; on its summit e, 

 with lead, in which the conducting wire 



b b enters ; this insulator is then covered 

 with a roof. 



58. It may be asked what prevents 

 the escape of the electric fluid from 

 the surface of the wire between post 

 and post ? In general when wires are 

 used on a smaller scale for the trans- 

 mission of electric currents, the escape 

 of the fluid is prevented by wrapping 

 them with silk or cotton thread, which 

 thus forms a non-conducting cover 

 upon them, but on the scale on which 

 they are used on telegraphic lines 

 the expense of this, independently 



of the difficulty of protecting such covering from destruction by 

 weather, would render it inadmissible. 



59. The atmosphere, when dry, is a good non-conductor ; but 

 this quality is impaired when it is moist. In ordinary weather, 

 however, the air being a sufficiently good non-conductor, a 

 metallic wire will, without any other insulating envelope except 

 the air itself, conduct the stream of electricity to the necessary 

 distances. It is true that a coated wire, such as we have 



* Turnbull on the Electric Telegraph, p. 176. 



Philadelphia, 1853. 

 141 



