with
the insulators already enumerated, and being subject to the
uncertainties of manufacture, has during the last few years achieved a
considerable success in American electrical engineering construction.
It is composed of scrap mica and shellac varnish worked under pressure
to the desired shape, and may be obtained in sheets, plates, and rods,
or in any of the forms for which a die happens to have been
constructed.

Of course, in special cases it would be worth while to prepare a die,
and then the attainable forms would be limited by moulding
considerations only. The writer's experience is very limited in this
matter, but Dr. Kennelly, with whom he communicated on the subject,
was good enough to reply in favour of micanite for engineering work.

Sec. 109. Celluloid.

This material is composed of nitrocellulose and camphor.

It has fair insulating properties, and may be obtained in a variety of
forms, but has now been generally abandoned for electrical work on
account of its inflammability.

Sec. 110. Paper.

Pure white filter paper, perfectly dry, is probably a very fair
insulator; the misfortune is that in practice it cannot be kept dry.
Under the most favourable circumstances its specific resistance may
approach 1024 E.M. units. It must therefore be considered rather as a
partial conductor than as an insulator. The only case of the use of
dry paper as an insulator in machine construction which has come under
the writer's notice is in building up the commutators of the small
motors which used to drive the Edison phonographs.

Its advantages in this connection are to be traced to the fact that a
commutator so built up is durable and keeps a clean surface. Of
course, the use of paper as an insulator for telephone wires is well
known, but its success in this direction depends less upon its
insulating properties than upon the fact that it can be arranged in
such a way as to allow of the wires being partially air insulated, an
arrangement tending to reduce the electrostatic capacity of the wire
system.

At one time it was the custom of instrument makers to employ ordinary
printed paper in the shape of leaves torn from books or the folios of
old ledgers to form the dielectric of the condensers used in
connection with the contact breakers of induction coils. This
practice has nothing but economy to recommend it, for cases often
occur in which the paper, by gradual absorption of moisture from the
air, com