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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]
Instead of using a wire-wound resistance element in heat-coil
construction some manufacturers employ a mass of high-resistance
material, interposed in the path of the current. The Kellogg Company
has long employed for its sneak-current arrester a short graphite rod,
which forms the resistance element. The ends of this rod are
electroplated with copper to which the brass terminal heads are
soldered. These heads afford means for making the connection with the
proper retaining springs.
[Illustration: Fig. 228. Cook Arrester]
Another central-office protector, which uses a mass of special metal
composition for its heat producing element is that designed by Frank B.
Cook and shown in Fig. 228. In this the carbon blocks are cylindrical
in form and specially treated to make them “self-cleaning.” Instead of
employing a self-soldering feature in the sneak-current arrester of
this device, Cook provides for electrically resoldering them after
operation, a clip being designed for holding the elements in proper
position and passing a battery current through them to remelt the
solder.
In small magneto exchanges it is not uncommon to employ combined fuse
and air-gap arresters for central-office line protection, the fuses
being of the mica-mounted type already referred to. A group of such
arresters, as manufactured by the Dean Electric Company, is shown in
Fig. 229.
[Illustration: Fig. 229. Mica Fuse and Air-Gap Arresters]
Types of Subscribers Station Protectors. Figs. 230 and 231 show types
of subscribers station protectors adapted to the requirements of
central-battery and magneto systems. These, as has been said, should be
mounted at or near the point of entrance of the subscribers line into
the premises, if the line is exposed outside of the premises. It is
possible to arrange the fuses so that they will be safe and suitable
for their purposes if they are mounted out-of-doors near the point of
entrance to the premises. The sneak-current arrester, if one exists,
and the carbon arrester also, must be mounted inside of the premises or
in a protecting case, if outside, on account of the necessity of
shielding both of these devices from the weather. Speaking generally,
the wider practice is to put all the elements of the subscribers
station protector inside of the house. It is nearer to the ideal
arrangement of conditions if the protector be placed immediately at the
point of entrance of the outside wires into the building.
[Illustration: Fig. 230. Western Electric Station Arrester]
[Illustration: Fig. 231. Cook Arrester for Magneto Stations]
_Ribbon Fuses_. A point of interest with relation to tubular fuses is
that in some of the best types of such fuses, the resistance material
is not in the form of a round wire but in the form of a flat ribbon.
This arrangement disposes the necessary amount of fusible metal in a
form to give the greatest amount of surface while a round wire offers
the least surface for a given weight of metal–a circle encloses its
area with less periphery than any other figure. The reason for giving
the fuse the largest possible surface area is to decrease the
likelihood of the fuse being ruptured by lightning. The fact that such
fuses do withstand lightning discharges much more thoroughly than
round fuses of the same rating is an interesting proof of the
oscillating nature of lightning discharges, for the density of the
current of those discharges is greater on and near the surface of the
conductor than within the metal and, therefore, flattening the fuse
increases its carrying capacity for high-frequency currents, without
appreciably changing its carrying capacity for direct currents. The
reason its capacity for direct currents is increased at all by
flattening it, is that the surface for the radiation of heat is
increased. However, when enclosed in a tube, radiation of heat is
limited, so that for direct currents the carrying capacity of fuses
varies closely with the area of cross-section.

In order that the armature and cores may be normally polarized, a
permanent magnet _6_ is secured to the center of the yoke piece _1_.
This bends around back of the electromagnets and comes into close
proximity to the armature _5_. By this means one end of each of the
electromagnet cores is given one polarity–say north–while the
armature is given the other polarity–say south. The two coils of the
electromagnet are connected together in series in such a way that
current in a given direction will act to produce a north pole in one
of the free poles and a south pole in the other. If it be assumed that
the permanent magnet maintains the armature normally of south polarity
and that the current through the coils is of such direction as to make
the left-hand core north and the right-hand core south, then it is
evident that the left-hand end of the armature will be attracted and
the right-hand end repelled. This will throw the tapper rod to the
right and sound the right-hand bell. A reversal in current will
obviously produce the opposite effect and cause the tapper to strike
the left-hand bell.
An important feature in polarized bells is the adjustment between the
armature and the pole pieces. This is secured in the Western Electric
bell by means of the nuts _7_, by which the yoke _4_ is secured to the
standards _3_. By moving these nuts up or down on the standards the
armature may be brought closer _to_ or farther _from_ the poles, and
the device affords ready means for clamping the parts into any
position to which they may have been adjusted.
[Illustration: Fig. 79. Polarized Bell]
_Kellogg Ringer._ Another typical ringer is that of the Kellogg
Switchboard and Supply Company, shown in Fig. 80. This differs from
that of the Western Electric Company mainly in the details by which
the armature adjustment is obtained. The armature supporting yoke _1_
is attached directly to the cores of the magnets, no supporting side
rods being employed. Instead of providing means whereby the armature
may be adjusted toward or from the poles, the reverse practice is
employed, that is, of making the poles themselves extensible. This is
done by means of the iron screws _2_ which form extensions of the
cores and which may be made to approach or recede from the armature by
turning them in such direction as to screw them in or out of the core
ends.
[Illustration: Fig. 80. Polarized Bell]
[Illustration: Fig. 81. Biased Bell]
_Biased Bell._ The pulsating-current generator has already been
discussed and its principle of operation pointed out in connection
with Fig. 77. The companion piece to this generator is the so-called
biased ringer. This is really nothing but a common alternating-current
polarized ringer with a light spring so arranged as to hold the
armature normally in one of its extreme positions so that the tapper
will rest against one of the gongs. Such a ringer is shown in Fig. 81
and needs no further explanation. It is obvious that if a current
flows in the coils of such a ringer in a direction tending to move the
tapper toward the left, then no sound will result because the tapper
is already moved as far as it can be in that direction. If, however,
currents in the opposite direction are caused to flow through the
windings, then the electromagnetic attraction on the armature will
overcome the pull of the spring and the tapper will move over and
strike the right-hand gong. A cessation of the current will allow the
spring to exert itself and throw the tapper back into engagement with
the left-hand gong. A series of such pulsations in the proper
direction will, therefore, cause the tapper to play between the two
gongs and ring the bell as usual. A series of currents in a wrong
direction will, however, produce no effect.