an
what is known to be best is not honest practice in a matter which
involves life, limb, and indefinite degrees of property values.

Protectors in central-battery subscribers' equipments need no
sneak-current arresters, as the condenser reduces that hazard to a
negligible amount. Magneto subscribers' equipments usually lack
condensers in ringer circuits, though they may have them in talking
circuits on party lines. The ringer circuit is the only path through
the telephone set for about 98 per cent of the time. Sneak-current
arresters, therefore, should be a part of subscribers' station
protectors in magneto equipment, except in such rural districts as may
have no lighting or power wires. When sneak-current arresters are so
used the arrangement of the parts then is the same as in the
central-office portion of Fig. 225.

Types of Central-Office Protectors. A form of combined heat coil and
air-gap arrester, widely used by Bell companies for central-office
protection, is shown in Fig. 226. The two inner springs form the
terminals for the two limbs of the metallic-circuit line, while the
two outside springs are terminals for the continuation of the line
leading to the switchboard. The heat coils, one on each side, are
supported between the inner and outer springs. High-tension currents
jump to ground through the air-gap arrester, while sneak currents
permit the pin of the heat coil to slide within the sleeve, thus
grounding the outside line and the line to the switchboard.

[Illustration: Fig. 226. Sneak-Current and Air-Gap Arrester]

_Self-Soldering Heat Coils._ Another form designed by Kaisling and
manufactured by the American Electric Fuse Company is shown in Fig.
227. In this the pin in the heat coil projects unequally from the ends
of the coil, and under the action of a sneak current the melting of
the solder which holds it allows the outer spring to push the pin
through the coil until it presses the line spring against the ground
plate and at the same time opens the path to the switchboard. When the
heat-coil pin assumes this new position it cools off, due to the
cessation of the current, and _resolders_ itself, and need only be
turned end for end by the attendant to be reset. Many are the
variations that have been made on this self-soldering idea, and there
has been much controversy as to its desirability. It is certainly a
feature of convenience.

[Illustration: Fig. 227. Self-Soldering Heat-Coil Arrester]