. Ann. vol. clix.), soapstone is the only reliable
insulator at a red heat, but, no doubt, a good deal depends on the
particular specimen investigated.

Sec. 107. Use of Mica in Condensers.

If good results are desired it is essential to select the mica very
carefully. Pieces appreciably stained,--particularly if the stain is
not uniformly distributed,--cracked pieces, and pieces tending to
flake off in patches should be rejected. The best samples of mica
that have come under the writer's observation are those sheets sold
for the purpose of giving to silver photographic prints that hideous
glazed surface which some years ago was so popular.

Sheets of mica about 0.1 to 0.2 mm. thick form good serviceable
condenser plates, and will certainly stand a pressure of 300 volts,
and most likely a good deal more. The general practice in England
seems to have been to build up condensers of alternate sheets of
varnished or paraffined-mica and tin-foil.

This practice is open to several objections. In the first place, the
capacity of a condenser made in this way varies with the pressure
binding the plates together. In the second place, the amount of mica
and tin-foil required is often excessive in consequence of the
imperfect contact of these substances. Again, the inevitable air film
between the mica and tin-foil renders condensers so made unsuitable
for use with alternating currents, owing to the heating set up through
air discharges, and which is generally, though often (if not always)
wrongly, attributed to dielectric hysteresis.

These imperfections are to a great extent got over by M. Carpentier's
method of construction, which is, however, rather more costly both in
material and labour. On the other hand, wonderful capacities are
obtained with quite small amounts of mica. M. Bouty mentions a
condenser of one microfarad capacity weighing 1500 grms. and
contained in a square box measuring 12 centimetres on the side, and
about 3 centimetres thick.

The relation between the capacity and surface of doubly-coated plates
is in electro-static units:

Capacity = (sp. ind. capacity X area of one surface)/(4pi X thickness)

This may be reduced to electro-magnetic units by dividing by 9x10^20,
and to microfarads by further multiplying by 10^15.

M. Carpentier begins, of course, by having his mica scrupulously clean
and well selected. It is then silvered by one of the silvering
processes (Sec. 65) on both sides, for