CONSTRUCTION OF AERIAL LINES. 
107 
from 3*5 to 3*8 lbs. The diameter must not vary beyond 
these limits. 
The wire must be well annealed and flexible enough to be 
closely wound on itself in several sharp turns without breaking 
or splitting. 
A piece of the 0T6 inch wire taken at random from any 
part of the roll must be able to support a weight of 10 cwt. 
without breaking. This strain is increased to 16 cwt. for the 
0*2 inch wire. 
The iron wire used in the construction of telegraph lines 
must have sufficient tenacity to resist the ordinary strains, 
and it must be flexible enough to admit of pieces being easily 
joined end to end. 
The instantaneous testing loads are so determined as to 
obtain these two conditions in a proper proportion. These 
Fig. 41 . 
loads nearly represent a strength of 2f tons for each 
square inch of section, and the strength is one which can 
be found only in wires of good quality after having been well 
annealed. 
In England, the following process is adopted for winding the 
wire in order to make sure that it is uniform throughout its 
whole length. The galvanized wire, wound on a plain drum, is 
drawn out over three, five, or seven pulleys arranged as shown 
in fig. 41. Then it passes over a large grooved pulley, and 
finally is wound upon a drum that revolves with a velocity 
about 2 per cent, greater than that of the pulley. The tension 
to which the wire is thus exposed is a sufficient test, as any 
weak part is thereby detected. 
Four different thicknesses of iron wire are used for the Post 
Office telegraphs in England, their weights per mile being 
respectively 200, 400, 600, and 800 lbs. Copper wire has of 
late been much used for telegraph and telephone lines, because 
