ratus
and the earlier protective systems depended on such an arrangement. The
difficulty with such delicate fuses is that they are not robust enough
to be reliable, and, worse still, they change their carrying capacity
with age and are not uniform in operation in different surroundings and
at different temperatures. They are also sensitive to lightning
discharges, which they have no power to stop or to divert.

Protection Against Sneak Currents. For these reasons, a system
containing fuses and air-gap arresters only, does not protect against
abnormal currents which are continuous and small, though large enough
to injure apparatus _because_ continuous. These currents have come to
be known as sneak currents, a term more descriptive than elegant.
Sneak currents though small, may, when allowed to flow for a long time
through the winding of an electromagnet for instance, develop enough
heat to char or injure the insulation. They are the more dangerous
because insidious.

[Illustration: Fig. 222. Tubular Fuse with Asbestos Filling]

_Sneak-Current Arresters._ As typical of sneak-current arresters,
Fig. 223 shows the principle, though not the exact form, of an
arrester once widely used in telephone and signal lines. The normal
path from the line to the apparatus is through a small coil of fine
wire imbedded in sealing wax. A spring forms a branch path from the
line and has a tension which would cause it to bear against the ground
contact if it were allowed to do so. It is prevented from touching
that contact normally by a string between itself and a rigid support.
The string is cut at its middle and the knotted ends as thus cut are
imbedded in the sealing wax which contains the coil.

[Illustration: Fig. 223. Principle of Sneak-Current Arrester]

A small current through the little coil will warm the wax enough to
allow the string to part. The spring then will ground the line. Even
so simple an apparatus as this operates with considerable accuracy.
All currents below a certain critical amount may flow through the
heating coil indefinitely, the heat being radiated rapidly enough to
keep the wax from softening and the string from parting. All currents
above this critical amount will operate the arrester; the larger the
current, the shorter the time of operating. It will be remembered that
the law of these heating effects is that the heat generated =
_C^{2}Rt_, so that if a certain current operates the arrester in, say
40 se