Several other types of insulating materials are
used in electrical work for purposes
other than those previously mentioned.
The following materials can be classified under this heading:
Oil. Certain mineral oils, which must he free form acid and alkali, posses~
excellent insulating properties. They are used in high tension switch gear to
quench the arc which forms when the switch is opened; in large transformers
they act not only as an insulating medium but also as a means of dispersing heat.
Varnislzes. lnsulating varnishes composed of resin, oil and thinners retain their
electrical properties over long periods under varying temperatures. They are used
for impregnating wound coils, thus protecting the insulation of the winding from
the effects of moisture; they also tend to bond together the individual turns and
give greater rigidity to the coil. Coils treated in this manner are usually baked
to expel moisture.
Varnislied Cambria. Cloth (usually closely—woven cotton) treated with insulating
varnish is extremely flexible and is used as tape to lap wound coils which are not
to be impregnated. It is also used in tubular Form, under the name of systoflex.
as insulating sleeving.
Presspa/in. This material consists of chemicallyireated paper and is used as
lining material For armature slots, field windings, etc. It is slightly Flexible, but
in due course of time becomes brittle: it has a tendency to warp and also to
Chattertons Compound. This is 21 mixture of Stockholm tar, resin and guttap
percha. lt has a low melting point and was originally used to seal the tops of
accumulators. More modern types ot` sealing compound have a bituminous base.
When required for use they are dissolved in petrol or carbon tetrachloride to
a workable consistency.
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