The coil is now adjusted in the adjusting stand to be concentric with the axis of symmetry of the coil carrier, and the supporting pins are slipped into slot holes cut in the brackets, the brackets being swivelled as much as necessary to allow of this. When the pins are all inserted the brackets are screwed up by the screws at E. The pins are then cemented firmly to the brackets by a little plaster of Paris.

The coil carrier can now be adjusted to the galvanometer frame by means of screws at D, which pa.s.s through wide holes in the carrier and bold the latter in position by their heads. In the sectional plan the parts of the galvanometer frame are shown shaded. The front of the frame at F F is of gla.s.s, and the back of the frame is also made of gla.s.s, though this is not shown in the section.

A represents an ebonite ring into which the wire coil is cemented by means of paraffin. B B B B are quartz pins, with heads inside the ebonite ring. C C C are slotted brackets adjustable to the pins and capable of rotation by releasing the screws E E. D D are the screws holding the coil carriage to the galvanometer framework. These screws pa.s.s through large holes in the carriage so as to allow of some adjustment.

Fig. 82.

Fig. 83.

-- 104. Gla.s.s.

When gla.s.s is properly chosen and perfectly dry it has insulating properties possibly equal to those possessed by quartz or crystalline sulphur. For many purposes, however, its usefulness is seriously reduced by the persistence with which it exhibits the phenomena of residual charge, and the difficulty that is experienced in keeping it dry.

The insulating power of white flint gla.s.s is much in excess of that of soft soda gla.s.s, which is a poor insulator, and of ordinary green bottle gla.s.s. The jars of Lord Kelvin's electrometers, which insulate very well, are made of white flint gla.s.s manufactured in Glasgow, but it is found that occasionally a particular jar has to be rejected on account of its refusing to insulate, and this, if I understand aright, even when it exhibits no visible defects.

A large number of varieties of gla.s.s were tested by Dr. Hopkinson at Messrs. Chance Bros. Works, in 1875 and 1876 (Phil. Trans, 1877), and in 1887 (Proc. Roy. Soc. xli. 453), chiefly with a view to the elucidation of the laws regulating the residual charge; and incidentally some extraordinarily high insulations were noted among the flint gla.s.ses. The gla.s.s which gave the smallest residual charge was an "opal" gla.s.s; and flint gla.s.ses were found to insulate 105 times as well as soda lime gla.s.ses. The plates of Wimshurst machines are made of ordinary sheet window gla.s.s, but as the insulating property of this material appears to vary, it is generally necessary to clean, dry, and test a sheet before using it. With regard to hard Bohemian gla.s.s, this is stated by Koeller (Wien Bericht) to insulate ten times as well as the ordinary Thuringian soft soda gla.s.s.

On the whole the most satisfactory laboratory practice is to employ good white flint gla.s.s. The only point requiring attention is the preparation of the gla.s.s by cleaning and drying. Of course all grease or visible dirt must be removed as described in an earlier chapter (-- 13), but this is only a beginning. The gla.s.s after being treated as described and got into such a state as to its surface that clean water no longer tends to dry off unequally, must be subjected to a further scrub with bibulous paper and a clear solution of oleate of soda. The gla.s.s is then to be well rinsed with distilled water and allowed to drain on a sheet of filter paper.

A very common cause of failure lies in the contamination of the gla.s.s with grease from the operator's fingers. Before setting out to clean the gla.s.s the student will do well to wash his hands with soap and water, then with dilute ammonia and finally with distilled water.

In the case of an electrometer jar which has become conducting but is not perceptibly dirty, rubbing with a little oleate of soda and a silk ribbon, followed, of course, by copious washing, does very well. If there is any tin-foil on the jar, great care must be taken not to allow the gla.s.s surface to become contaminated by the sh.e.l.lac varnish or gum used to stick the tin-foil in position.

Finally, the gla.s.s should be dried by radiant heat and raised to a temperature of 100 C. at least, and kept at it for at least half an hour. Before drying it is of course advisable to allow the water to drain away as far as possible, and if the water is only the ordinary distilled water of the laboratory, the gla.s.s is preferably wiped with a clean bit of filter paper; any hairs which may be left upon the gla.s.s will brush off easily when the gla.s.s is dry.

In order to obtain satisfactory results the gla.s.s must be placed in dry air before it has appreciably cooled. This is easily done in the case of electrometer jars, and so long as the air remains perfectly dry through the action of sulphuric acid or phosphorus pentoxide, the jar will insulate. The slightest whiff of ordinarily damp air will, however, enormously reduce the insulating power of the gla.s.s, so that unvarnished gla.s.s surfaces must be kept for apparatus which is practically air-tight.

For outside or imperfectly protected uses the gla.s.s does better when varnished. It is a fact, however, that varnished gla.s.s is rarely if ever so good as unvarnished gla.s.s at its best. Too much care cannot be taken over the preparation of the varnish; French polish, or carelessly made sh.e.l.lac varnish, is likely to do more harm than good.

The best orange sh.e.l.lac must be dissolved in good cold alcohol by shaking the materials together in a bottle. The alcohol is made sufficiently pure by starting with rectified spirit and digesting it in a tin flask over quick-lime for several days, a reversed condenser being attached. A large excess of lime must be employed, and this leads to a considerable loss of alcohol, a misfortune which must be put up with.

After, say, thirty hours' digestion, the alcohol may be distilled off and employed to act on the sh.e.l.lac. In making varnish, time and trouble are saved by making a good deal at one operation--a Winchester full is a reasonable quant.i.ty. The bottle may be filled three-quarters full of the sh.e.l.lac flakes and then filled up with alcohol; this gives a solution of a convenient strength.

The solution, however, is by no means perfect, for the sh.e.l.lac contains insoluble matter, and this must be got rid off.'' One way of doing this is to filter the solution through the thick filtering paper made by Schleicher and Schuell for the purpose, but the filtering is a slow process, and hence requires to be conducted by a filter paper held in a clip (not a funnel) under a bell jar to avoid evaporation.

Another and generally more convenient way in the laboratory is to allow the muddy varnish to settle--a process requiring at least a month--and to decant the clear solution off into another bottle, where it is kept for use. The muddy residue works up with the next lot of sh.e.l.lac and alcohol, which may be added at once for future use.

The gla.s.s to be varnished is warmed to a temperature of, say, 50 C, and the varnish put on with a lacquering brush; a thin uniform coat is required. The gla.s.s is left to dry long enough for the sh.e.l.lac to get nearly hard and to allow most of the alcohol to evaporate. It is then heated before a fire, or even over a Bunsen, till the sh.e.l.lac softens and begins to yield its fragrant characteristic smell.

If the coating is too heavy, or if the heating is commenced before the sh.e.l.lac is sufficiently dry, the latter will draw up into "tears,"

which are unsightly and difficult to dry properly. On no account must the sh.e.l.lac be allowed to get overheated. If the varnish is not quite hard when cold it may be a.s.sumed to be doing more harm than good.

In varnishing gla.s.s tubes for insulating purposes it must be remembered that the inside of the tube is seldom closed perfectly as against the external air, and consequently it also requires to be varnished. This is best done by pouring in a little varnish considerably more dilute than that described, and allowing it to drain away as far as possible, after seeing that it has flooded every part of the tube.

During this part of the process the upper end of the tube must be closed, or evaporation will go on so fast that moisture will be deposited from the air upon the varnished surface. Afterwards the tube may be gently warmed and a current of air allowed to pa.s.s, so as to prevent alcohol distilling from one part of the tube to another.

The tube is finally heated to the softening point of sh.e.l.lac, and if possible closed as far as is practicable at once.

-- 105. Ebonite or Hard Rubber.

This exceedingly useful substance can be bought of a perfectly useless quality. Much of the ebonite formerly used to cover induction coils for instance, deteriorates so rapidly when exposed to the air that it requires to have its surface renewed every few weeks.

The very best quality of ebonite obtainable should be solely employed in constructing electric works. It is possible to purchase good ebonite from the Silvertown Rubber Company (and probably from other firms), but the price is necessarily high, about four shillings per pound or over.

At ordinary temperatures ebonite is hard and brittle and breaks with a well-marked conchoidal fracture. At the temperature of boiling water the ebonite becomes somewhat softened, so that it is readily bent into any desired shape; on cooling it resumes its original hardness.

This property of softening at the temperature of boiling water is a very valuable one. The ebonite to be bent or flattened is merely boiled for half an hour or so in water, taken out, brought to the required shape as quickly as possible, and left to cool clamped in position.

The sheet ebonite as it comes from the makers is generally far from flat. It is often necessary to flatten a sheet of ebonite, and of course this is the more easily accomplished the smaller the sheet.

Consequently a bit of ebonite of about the required size is first cut from the stock sheet by a hack-saw such as is generally used for metals. This piece is then boiled and pressed between two planed iron plates previously warmed to near 100 C.

With pieces of ebonite such as are used for the tops of resistance boxes, measuring, say, 20 X 8 X 11 inches, very little trouble is experienced. The sheets when cold are found to retain the flatness which has been forced upon them perfectly well. It is otherwise with large thin sheets such as are used for Holtz machines. I have succeeded fairly, but only fairly, by pressing them in a "gluing press," consisting of heavy planed iron slabs previously heated to 100 C.

I do not know exactly how best to flatten very thin and large sheets.

It is easy to make large tubes out of sheet ebonite by taking advantage of the softening which ebonite undergoes in boiling water.

A wooden mandrel is prepared of the proper size and shape. The ebonite is softened and bent round it; this may require two or three operations, for the ebonite gets stiff very quickly after it is taken out of the water. Finally the tube is bound round the mandrel with sufficient force to bring it to the proper shape and boiled in water, mandrel and all. The bath and its contents are allowed to cool together, so that the ebonite cools uniformly.

Tubes made in this way are of course subject to the drawback of having an unwelded seam, but they do well enough to wind wire upon if very great accuracy of form is not required. If very accurate spools are needed the mandrel is better made of iron or slate and the spool is turned up afterwards. The seam may be strapped inside or at the ends by bits of ebonite acting as bridges, and the seam itself may be caulked with melted paraffin or anthracene.

Working Ebonite.

Ebonite is best worked as if it were bra.s.s, with ordinary bra.s.s-turning or planing tools. These tools should be as hard as possible, for the edges are apt to suffer severely, and blunt tools leave a very undesirable woolly surface on the ebonite. In turning or shaping ebonite sheets it is as well to begin by taking the skin off one side first, and then reversing the sheet, finishing the second side, and then returning to the first. This is on account of the fact that ebonite sometimes springs a little out of shape when the skin is removed.

Turned work in ebonite, if well done, requires no sand-papering, but may be sufficiently polished by a handful of its own shavings and a little vaseline. The advantage of using a polished ebonite surface is that such a surface deteriorates more slowly under the influence of light and air than a surface left rough from the tool. If very highly polished surfaces are required, the ebonite after tooling is worked with fine pumice dust and water, applied on felt, or where possible by means of a felt buff on the lathe, and finally polished with rouge and water, applied on felt or cloth.

Ebonite works particularly well under a spiral milling cutter, and sufficiently well under an ordinary rounded planing tool, with cutting angle the same as for bra.s.s, and hardened to the lightest straw colour.

It is not possible, on the other hand, to use the carpenter's plane with success, for the angle of the tool is too acute and causes the ebonite to chip.

In boring ebonite the drill should be withdrawn from the hole pretty often and well lubricated, for if the borings jam, as they are apt to do, the heat developed is very great and the temper of the drill gets spoiled. Ebonite will spoil a drill by heating as quickly as anything known; on the other hand, it may be drilled very fast if proper precaution is taken.

It is advisable to expose ebonite to the light as little as possible, especially if the surface is unpolished, for under the combined action of light and air the sulphur at the surface of the ebonite rapidly oxidises, and the ebonite becomes covered with a thin but highly conducting layer of sulphurous or sulphuric acid or their compounds.

When this happens the ebonite may be improved by scrubbing with hot water, or washing freely with alcohol rubbed on with cotton waste in the case of apparatus that cannot be dismounted.

A complete cure, however, can only be effected by sc.r.a.ping off the outer layer of ebonite so as to expose a fresh surface. For this purpose a bit of sheet gla.s.s broken so as to leave a curved edge is very useful, and the ebonite is then sc.r.a.ped like a cricket bat. In designing apparatus for laboratory use it is as well to bear in mind that sooner or later the ebonite parts will require to be taken down and sc.r.a.ped up. Rods or tubes are, of course, most quickly treated on the lathe with rough gla.s.s cloth, and may be finished with fine sandpaper, then pumice dust and water, applied on felt. After cleaning the pumice off by means of water and a rag, the final touch may be given by means of vaseline, applied on cloth or on ebonite shavings.

-- 106. Mica.

A great variety of minerals go under this name. Speaking generally, the Russian micas coming into commerce are potash micas, and mica purchased in England may be taken to be potash mica, especially if it is in large sheets.

At ordinary temperatures "mica" of the kind found in commerce is an excellent insulator. Schultze (Wied. Ann. vol. x.x.xvi. p. 655) comes to the conclusion that both at high and at low temperatures mica (of all kinds?) is a better insulator than white "mirror gla.s.s," the composition of which is not stated. The experiments of the author referred to were apparently left unfinished, and altogether too much stress must not be laid on the results obtained, one of which was that mica conducts electrolytically to some extent at high temperatures.

Bouty (Journal de Physique, 1890 [9], 288) and J. Curie (These de Doctorat, Paris, 1888) agree in making the final conductivity of the mica used in Carpentier's condensers exceedingly small--at all events at ordinary temperatures. Bearing in mind that for such substances the term specific resistance has no very definite meaning, M. Bouty considers it is not less than 3.19 x 1028 E.M. units at ordinary temperatures. M. Bouty gives a note or ill.u.s.tration of what such numbers mean--a precaution not superfluous in cases where magnitudes are denoted logarithmically. Referring to the value quoted, viz.

3.19 x 1028, M. Bouty says, "Ce serait la resistance d'une colonne de mercure de 1mmq de section et de longueur telle que la lumiere se propageant dans le vide, mettrait plus de 3000 ans A se transmettre d'une extremite a I'autre de la colonne."

M. Bouty returns to the study of mica (muscovite) in the Journal de Physique for 1892, p. 5, and there deals with the specific inductive capacity, which for a very small period of charge he finds has the value 8--an enormous value for such a good insulator, and one that it would be desirable to verify by some totally distinct method. This remark is enforced by the fact that M. Klemencic finds the number 6 for the same constant. The temperature coefficient of this constant was too small for M. Bouty to determine. The electric intensity was of the order of 100 volts per centimetre, and the experiments seem to indicate that the specific inductive capacity would be only slightly less if referred to a period of charge indefinitely short.

I have found that the residual charge in a mica condenser, made according to Carpentier's method (to be described below), is about 1 per cent of the original charge under the following circ.u.mstances.

Voltage 300 volts on a plate 0.2 mm. thick, duration of charge ten minutes, temperature about 20 C. To get this result the mica must be most carefully dried. This and other facts indicate that the so-called residual charge on ordinary condensers is, to a very large extent, due to the creeping of the charge from the armatures over the more or less conducting varnished surfaces of the mica, and its slow return on discharge.