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 |
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° C. for, say, half an hour, and the sheets are then floated on the top of paraffin, kept melted at 140° 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 must be made in the shape of a cool bottle between the air pump and the box. If this precaution be omitted, and if any paraffin splashes on to the hot surface of the box, it volatilises with decomposition and the products go to stop up the pump. Paraffin with a melting-point of 50° C. or upwards does well.
The bath should be allowed to cool to about 60° C. before the papers are removed, so that enough paraffin may be carried out to thoroughly coat the paper and prevent the entrance of air.
Fig.
85.