190 



THE ELECTRIC TELEGRAPH. 



of a porcelain bell, Fig. 99, provided at the top with a notch 

 for the reception of the line-wire, which is secured by a pin 

 in a hole at right angles to it. One end of a bolt is cemented 

 into a hole in the under-side of the insulator-cap, and by this 

 it is secured to a cross-bar of wood screwed on to the post. 



Fig. 99. 



When a post has to carry a number of wires, the cross-bars 

 are of different lengths, the longest being at the top, and each 

 succeeding one shorter than the one above it, that, if a wire 

 should break, it would fall clear of the insulator- caps 

 beneath it. 



In 1852, Siemens and Halske invented their bell-insulator, 

 which is the strongest and one of 

 the best- construe ted supports. It 

 consists of a cast-iron bell, a a, 

 Fig. 100, with a flange, b b, by 

 which it is screwed against the 

 post. Inside the bell is cemented 

 a porcelain cup, c c, ribbed inside 

 and out to give a good hold to the 

 cement. The cup, c c, in turn, 

 carries the stalk or hook, d d, which 

 supports the line- wire. The parts 

 are put together, while hot, with a cement composed of 

 sulphur and oxide of iron. As a further mode of insulation, 

 the iron stalks or hooks are covered with vulcanite before 



