116 
THE TELEGRAPH . 
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made sufficiently secure without its being prevented from 
turning round to allow of passing into the hook a wire already 
stretched. When this form of in¬ 
sulator is used in curves of large 
radius a little wooden support is 
nailed to the post, and on this rests 
the expanded part of the bell. 
This plan of fixing the large 
insulators has all the advantages of 
firmness. The proper penetration 
of the screw has the effect of com¬ 
pressing the insulator and the 
beech-wood support against the post, 
and of causing a slight indentation 
of the latter by the two extremities 
of the stirrup. From this results 
a suitable degree of elasticity, while 
the support is able to withstand a 
great pulling strain. This arrange¬ 
ment may theretore be advantage¬ 
ously adapted for thick wires, and 
for curves of small radius. 
The small form of insulator is 
used :—1st, for wire of 0T2 inch 
diameter : 2nd, for wire of 0T6 inch 
diameter in a straight line, and on 
curves of large radius. 
The larger form is used for sup¬ 
porting wires of 0*20 inch dia¬ 
meter, and wire of 0T6 diameter in 
curves of small radius. 
The advantages possessed by these 
insulators on account of their sha{e 
are the following :— 
1st. The porcelain bell, having 
perfectly symmetrical sections about 
its axis, can be turned on the pot- 
ter’s wheel. The thickness of the 
1 f porcelain is nearly the same in every 
part, and this renders the firing more easy. These properties, 
-JL, 
