

HISTORY AND PROGRESS. 



193 



outer cup a is provided with a groove to which the line wire 

 is bound ; in the recess of the inner cup b, a wrought iron 

 bolt, c, is cemented, by which the insulator is attached to the 

 bracket d, on the post. A further insulation is obtained by 



Fig. 103. 



T 



Fig. 104. 



coating the stalk with vulcanite. The rim of the outer cup a 

 is rounded off inside. The purpose of this is to avoid the 

 sprinkling of the interior with rain-water, when a drop, 

 hanging upon the bottom rim, is blown off by the wind. 

 When a strong current of air separates a drop of water from 

 a sharp corner, the drop is never carried bodily off, but 

 bursts in the direction of the current. With the form 

 given to the rim by Mr. Yarley, however, when a drop 

 happens to hang on that side from which the wind comes, it 

 is driven a little way up between the two cups, and does not 

 burst. 



104. Stretching Insulators. The weight of the wire in 

 the space which it makes between two posts, assisted by the 

 occasional pressure of the wind against it, causes it, after a 

 time, to stretch and curve lower towards the earth. When 

 a single wire is suspended, this is of no importance; but 

 when several wires are supported between the same posts, by 

 stretching, they are in danger of touching each other and 

 causing interruptions of the service. To avoid this, the line 

 is provided at intervals with insulators of larger and stronger 

 make than the ordinary ones, to which the wire is made fast, 

 thus giving, at intervals of half a mile or so, fixed points to 

 the suspended Ijne, while the intermediate insulator hooks 



