On Laboratory Arts Part 25

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

-- 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 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 acc.u.mulated, 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.

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 gla.s.s end-pieces by means of suitable clamps. It can then be put in a box of melted paraffin, heated up to 140 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 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.

The condenser was thoroughly packed all round in cotton-wool to a thickness of 6 inches, and its temperature was indicated more or less by a thermometer plunged through a hole in the lid of the containing box and of the condenser box, and resting on the upper surface of one set of tin-foil electrodes, from which the soft paraffin mixture had been purposely sc.r.a.ped away. The following were the results of a four hours' run at a voltage 50 per cent higher than that for which the condenser was designed--i.e. 3000 volts.

Times. Voltage Temperature Temperature Difference in Condenser. in Air.

Hrs. Min.

2 10 2750 22.8 C. 23.0 C. + 0.2

3 10 2700 23.0 C. 23.3 C. + 0.3

3 18 3200 23.1 C. 23.0 C. -0.1

4 10 3200 23.3 C. 23.7 C. + 0.4

5 10 3100 23.6 C. 23.4 C. -0.2

6 10 3000 23.8 C. 23.35 C. -0.45

An idea of the order of the amount of waste may be formed from the following additional experiment.

A condenser similar to the one described was filled with oil of a low insulating power. It was tested calorimetrically, and also by the three voltmeter method, which, however, proved to be too insensitive.

The temperature rise in the non-conducting box in air was about 0.3 C. per hour, and the loss of power was found to be less than 0.1 per cent. In the present case the actual rise was only 1 in four hours, and the integral give and take between the condenser and the air is practically nothing; consequently we may consider with safety that the rate of rise is certainly less than 1 degree per three hours. The voltage and frequency were about the same in both experiments, consequently the energy pa.s.sed is about proportional to the capacity used in the two experiments.

From this it follows that since the specific heat of both condensers was the same (nearly), the loss in the present case is a good deal less than one-tenth per cent. The residual charge is also much less than when the condenser is simply built up of paper paraffined in an unsystematic manner, and from which the air and water have been imperfectly extracted, as by baking the condenser first, and then immersing it in paraffin or oil.

It is usual to consider that the phenomena of residual charge and heating in condensers, to which alternating voltages are applied, are closely allied. This is true, but the alliance is not one between cause and effect--at all events, with regard to the greater part of the heating. The imperfect exclusion of air and moisture, particularly the latter, certainly increases the residual charge by allowing surface creeping to occur; but it also acts directly in producing heating, both by lowering the insulation of the condenser and by allowing of air discharges between the condenser plates.

Of these causes of heating, the discharges in air or water vapour are probably the more important. Long ago a theory of residual charge was given by Maxwell, based on the consideration of a laminated dielectric, the inductivity and resistance of which varied from layer to layer. It was shown that such an arrangement, and hence generally any want of h.o.m.ogeneity in a direction inclined to the lines of force leading to a change of value of the product of specific resistance and specific inductive capacity, would account for residual charge.

This possible explanation has been generally accepted as the actual explanation, and many cases of residual charge attributed to want of h.o.m.ogeneity, which are certainly to be explained in a simpler manner.

For instance, the residual charge in a silvered mica plate condenser, carefully dried, can be increased at least tenfold by an exposure of a few minutes to ordinarily damp air. The same result occurs with condensers of paraffined or sulphured paper; and these are the residual changes generally observed. The greater part must be due to creeping.

-- 112. Paraffin.

This substance has long enjoyed great popularity in the physical laboratory. Its specific resistance is given by Ayrton and Perry as more than 1025, but it is probably much higher in selected samples.

The most serviceable kind of paraffin is the hardest obtainable, melting at a temperature of not less than 52 C. It is a good plan to remelt the commercial paraffin and keep it at a temperature of, say, 120 C. for an hour, stirring it carefully with a gla.s.s rod so that it does not get overheated; this helps to get rid of traces of water vapour.

Hard paraffin, when melted, has an enormous rate of expansion with temperature, so great, indeed, that care must be taken not to overfill the vessels in which it is to be heated. Castings can only be prepared by cooling the mould slowly from the bottom, keeping the rest of the mould warm, and adding-paraffin from time to time to make up for the contraction. The cooling is gradually allowed to spread up to the free surface.

The chief use of paraffin in the laboratory is in the construction of complicated connection boards, which are easily made by drilling holes in a slab of paraffin, half filling them with mercury, and using them as mercury cups.

Since paraffin is a great collector of dust, it should be screened by paper, otherwise the blocks require to be sc.r.a.ped at frequent intervals, which, of course, electrifies them considerably. This electrification is often difficult to remove without injuring the insulating power of the paraffin. A light touch with a clean Bunsen flame is the readiest method, and does not appear to reduce the insulation so much as might be expected. The safest way, however, is to leave the key covered by a clean cloth, which, however, must not touch the surface, for a sufficient time to allow of the charges getting away.

The paraffin often becomes electrified itself by the friction of the key contacts, so that in electrometer work it is often convenient to form the cups by lining them with a little roll of copper foil twisted up at the bottom. In this case the connecting wires should, of course, be copper. Small steel staples are convenient for fastening the collecting wires upon the paraffin; or, in the case where these wires have to be often removed and changed about, drawing-pins are very handy.

With mercury cups simply bored in paraffin great trouble will often be experienced in electrometer work, owing to a potential difference appearing between the cups--at all events when the contacts are inserted and however carefully this be done. A few drops of very pure alcohol poured in above the mercury often cures this defect. The surface of paraffin is by no means exempt from the defect of losing its insulating power when exposed to damp air, but it is not so sensitive as gla.s.s, nor does the insulating power fall so far.

Two useful appliances are figured.