Shell1

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1804

SHIELDED TRANSFORMER

SHELLAC

VARNISHING WITH SHELLAC

Applied to such parts as the windings of an inductance coil,
shellac assists insulation, and also keeps the wiring secure

in the illustration by brushing it on to
an inductance coil to keep the turns of
wire in position. Two or three coats
may be given, as desired. It may be
used for brushing over cardboard tubes
which are to be used for formers, and for
other purposes. It can also be employed
as a kind of varnish for applying to
woodwork, and may, in many cases, take
the place of other adhesives, although
it is not properly such.

The thicker varieties are often known
as stick varnish, and can be employed
either as an adhesive or insulating varnish.
The two kinds of shellac mostly employed
are those known as the white and the
brown. The former makes a practically
colourless varnish; the latter is the
form: most usually employed in wireless
work, and imparts a fairly rich colour
to the work. When lump shellac is used,
it should be dissolved in methylated
spirit, alcohol or other solvent, to the
desired consistency, and applied in the
ordinary way with a brush.

Shellac is best kept in bottles,
and to prevent the cork sticking

SHELL TRANSFORMER

In this transformer is a bobbin wound
with the primary and secondary coils.
The core is of iron wire, and is bent
over the outside of the windings
Courtesy Economic Electric Co., Ltd.

and closing the neck of the
bottle a good plan is to use
a plug made by tightly
rolled-up newspaper, oF
alternatively to place a piece
of linen around the outside
of the cork. A loose end is
left projecting, so as to give
a firm hold for removing the
cork.

SHELL TRANSFORMER.
Expression applied to some
forms of inter-valve, and
also to telephone, trans-

/ formers characterized by
the fact that a large portion
of the core is outside the
coil windings.

One example of a telephone transformer
is illustrated as made on this plan. In
essence, such a transformer is composed
of a bobbin wound with the customary
primary and secondary windings, with
the usual precautions as regards insulation.

The bobbin has a central hole, and
through this is passed a bundle of soft
iron wire rods, of such length that the
ends that project can be bent around the
outside of the windings and arranged to
meet or overlap a little. There is thus
an outer case of iron wires, and these
can be secured with a few turns of binding
wire or tape, and the whole thus com-
pleted. This type is, perhaps, the most
suited to amateur constructors, as it avoids
the necessity for punchings wherewith to
make the laminated iron closed core. See
Amplifier ; Transformer.

SHIELDED TRANSFORMER. A type
of transformer surrounded by, or incor-
porating in its construction, a metallic
exterior. Both high- and low-frequency

SHIELDED TRANSFORMER

A typical shielded transformer having an earthing terminal
situated between a pair of large terminals

Courtesy Economic Electric Co. Lid.