Fig. 85 is a section of a vacuum vessel which has been found very convenient. It measures about two feet in diameter at the top. It is round, because it is much easier to turn one circular surface than to plane up four surfaces, which has to be done if the box is square. Both the rim of the vessel and the approximating part of the cover require to be truly turned and smoothly finished. A very good packing is made of solid indiarubber core about half an inch thick. This is carefully spliced--cemented by means of a solution of rubber in naphtha, and the splice sewed by thick thread. The lid ought to have a rim fitting inside the vessel, for this keeps the rubber packing in place; the rim has been accidentally omitted in Fig. 85. The bolts should not be more than five inches apart, and should lie at least half an inch in diameter, and the rim and lid should be each half an inch thick. Condensers may now be built up of sheets of this prepared paper interleaved with tin-foil in the ordinary way. If good results are required, the condenser when finished is compressed between wooden or glass end-pieces by means of suitable clamps. It can then be put in a box of melted paraffin, heated up to 140 deg. C, and exhausted by means of the water pump for several hours. In this process the air rushes out from between the paper and foils with such vehemence that the paraffin is generally thrown entirely out of the box. To guard against this the box must be provided with a loosely fitting and temporary lid, pierced with several holes. The real test as to when exhaustion is complete would be the cessation of any yield of air or water. Since it is not generally convenient to make the vacuum box so air-tight that there are absolutely no leaks at all, and as the paraffin itself is, I think, inclined to "crack" slightly at the temperature of 140 deg. C, this test or criterion cannot be conveniently applied. Two exhaustions, each of about two hours' duration, have, however, in my experience succeeded very well, provided, of course, that the dielectric is prepared as suggested. At the end of the exhaustion process the clamping screws are tightened as far as possible, the condenser remaining in its bath until the paraffin is pasty. Condensers made in this way resist the application of alternating currents perfectly, as the following tests will show. The dielectric consisted of about equal parts of hard paraffin and vaseline. A condenser of about 0.123 microfarads capacity and an insulation resistance of 2000 megohms, [Footnote: As tested by a small voltage.] having a tin-foil area of 4.23 square metres (about), and double papers each about 0.2 mm. thick, designed to run at 2000 volts with a frequency of 63 complete periods, was tested at this frequency.