Sheath for Magnet Coils.
In 1867 C. E. Varley proposed the use of a copper sheath surrounding a
magnet core to diminish self-induction. It has since been used by Brush
and others. Sometimes metallic foil is laid between the successive coils
of wire.
Synonym--Mutual Induction Protector.
Sheath for Transformers.
A protective sheath of copper, interposed between the primary and
secondary circuits of an alternating current transformer. It is
connected to the earth. If the primary coil loses its insulation before
it can leak to the secondary it is grounded. This protects the secondary
circuit from the high electro-motive force of the primary circuit.
Shellac.
A resin; produced as an exudation upon the branches of certain Asiatic
trees, such as the banyan (Ficus religiosa). It is due to punctures in
the bark of the trees in question, which punctures are made by the
female of the insect coccus ficus or c. lacca.
Commercial shellac contains about 90 per cent. of resinous material, the
rest is made up of wax, gluten, coloring matter and other substances.
Shellac is soluble in alcohol, and in aqueous solutions of ammonium
chloride, of borax and in strong ammonia solution. Long standing is
required in the case of the last named solvent. Dilute hydrochloric and
acetic acids dissolve it readily; nitric acid slowly; strong sulphuric
acid is without action on it. Alkalies dissolve it.
In electric work it is used as an insulator and dielectric. Its
alcoholic solution is used to varnish glass plates of influence
machines, for the coils of induction coils and similar purposes.
Resistance in ohms per centimeter cube at 28° C. (82.4 F.)--(Ayrton),
9.0E15