FA UL TS. 
265 
the amount of the loss may be determined by electrical 
measurements, and if there be only one defective point, its 
position in the line can be fixed by the same methods. In 
submarine cables the defective insulation can be due only to 
a loss of current at one or more points in the insulating 
covering. The second condition (excessive resistance or want 
of continuity in the conductor) implies a breakage of the wire, 
either in the external line or in the internal office wires, or 
perhaps in the earth connection. In many cases it is possible 
to fix the position of the rupture of the line. This usually 
causes a loss of current to the earth, for in overhead lines the 
wire falls to the ground; and commonly when the conductor 
of a cable breaks, it is because something has divided the cable 
itself. The third accident (contact) is very frequent in over¬ 
head lines. When the contact is metallic the position of the 
fault can be fixed easily. 
The tests by which the positions of telegraph faults are 
determined are always made by means of delicate galvanometers. 
Such faults are more easily found in cables than in overhead 
lines, because the insulation of the uninjured part of the cable 
is usually better in the former. In an overhead line several 
faults may occur at the same time, and this causes the indica¬ 
tions to be more uncertain and frequently incorrect. But by 
the constant inspection of the overhead lines, visible defects are 
quickly removed, and thus the uncertainty of determination in 
the position of other faults is obviated. 
The following apparatus are used in testing for the position 
of faults ; these tests being particularly applicable to sub¬ 
marine lines and used for overhead lines in a similar manner. 
Fig. 187 shows Thomson’s mirror galvanometer, which consists 
of two superposed circular coils, each having two diamagnetic 
copper frames, so fitted to each other as to leave a groove in 
their vertical axes, and also a cylindrical opening in their 
centres. On these coils is wound a very fine wire made of an 
alloy of copper and silver and covered with silk. A very small 
and extremely light magnet is fixed to the back of a small 
circular mirror, which receives and reflects a beam of light 
through the small circular opening in the upper coil. The 
total weight of the mirror and magnet does not exceed two 
grains, These are attache^ to a light rod of aluminium, 
