es to insulate so badly that it practically short circuits the
spark gap, and so stops the action of the coil. Three separate cases
have come within the writer's experience.

Some measurements of the resistance of paper have been made by F.
Uppenborn (Centralblatt fuer Electrotechnik, Vol. xi. p. 215,
1889). There is an abstract of the paper also in Wiedemann's
Beiblaetter (1889, vol. xiii. P. 711). Uppenborn examined the
samples of paper under normal conditions as to moisture and obtained
the following results:-

Description of Paper

I

Pressure Intensity

II.

Specific Resistance corresponding to pressures as in Column I. Ohms.

III

Pressure Intensity.

IV.

Specific Resistance corresponding to Column III. Ohms.

Common cardboard 2.3 mm. thick

0.05 kilo. per 6000 sq. mm.

4.85 x 1015

20 kg. per 6000 sq. mm.

4.7 x 1014

Gray paper, 0.26 mm. thick

0.05 kilo. per 5000 sq. mm.

3.1 x 10^15

20 kg. per 5000 sq. mm.

8 x 1014

Yellow parchment paper-09 mm. thick

0.05 kilo. per 5300 sq. mm.

3.05 x 1016

20 kg. per 5300 sq. mm.

8 x 1016

Linen tracing cloth

0.05 kilo. per 6000 sq. mm.

1.35 x 1016

20 kg. per 33,000 sq. mm.

1.86 x 10^15

Sec. 111. Paraffined Paper.

Like wood and other semiconductors, paper can be vastly improved as an
insulator by saturating it with melted paraffin. To get the best
results a pure paper free from size must be employed--gray Swedish
filter paper does well. This is dried at a temperature above 100 deg. C.
for, say, half an hour, and the sheets are then floated on the top of
paraffin, kept melted at 140 deg. C. or thereabout in a baking dish. As
soon as the paper is placed upon the melted paraffin the latter begins
to soak through, in virtue of capillary action, and drives before it
the air and moisture, causing a strongly marked effervescence.

After about one minute the paper may be thrust below the paraffin to
soak. When a sufficient number of papers have accumulated, and when
no more gas comes off, the tray may be placed in a vacuum box (Fig.
85), and the pressure reduced by the filter pump. As the removal of
the air takes time, provision must be made for keeping the bath hot.

A vacuum may be maintained for about an hour, and air then readmitted.
Repeated exhaustions and readmissions of air, which appear to improve
wood, do not give anything like such a good result with paper. In
using a vacuum box provision mu