Spelter tends to run very freely when it melts, and if the brass

surface in the neighbourhood of the joint is at all clean, may run

where it is not wanted. Of course some control may be exercised by

"soiling" with fireclay or using an oxidising flame; but the erratic

behaviour of spelter in this respect is the greatest drawback to its

use in apparatus construction. The secret of success in brazing lies

in properly cleaning up the work to begin with, and in disposing the

borax so as to prevent subsequent oxidation.

§ 100. Silver Soldering.

This process resembles that last described, but instead of spelter an

alloy of silver, copper, and zinc is employed. The solder, as

prepared by jewellers to meet special cases, varies a good deal in

composition, but for the laboratory the usual proportions are:

For soft silver solder

Fine silver 2 parts

Brass wire 1 part

For hard silver solder

Sterling silver 3 parts

Brass wire 1 part

The latter is, perhaps, generally the more convenient.

Silver solders may, of course, be purchased at watchmakers' supply

shops, and as thus obtained, are generally in thin sheet. This is

snipped fine with a pair of shears preparatory to use.

As odds and ends of silver (from old anodes and silver residues)

generally accumulate in the laboratory, it is often more convenient to

make the solder one's self. In this case it must be remembered in

making hard solder by the second receipt that standard silver contains

about one-twelfth of its weight of copper--exactly 18 parts copper to

220 silver.

The silver is first melted in a plumbago crucible in a small furnace

together with a little borax; if any copper is required this is then

added, and finally the brass is introduced. When fusion is complete,

the contents of the crucible are poured into any suitable mould.

The quickest and most convenient way of preparing the alloy for use is

to convert it into filings with the assistance of a coarse file, or by

milling it, if a milling machine is available.

Equal volumes of filings and powdered glass borax are made into a thin

paste with water, and applied in an exactly similar manner to that

described under the head of "brazing." In fact all the processes

there described may be applied equally to the case under discussion,

the substitution of silver for spelter being the only variation.

The silver solder is more manageable than spelter, and does not tend

to run wild over the work: a property which makes it much more

convenient both for delicate joints and in cases where it is desired

to restrict the solder to a single point or line. Small objects are

almost invariably soldered with silver solder, and are held by forceps

or on charcoal in the pointed flame of an ordinary blow-pipe.

§ 101. On the Construction of Electrical Apparatus: Insulators.

It is not intended to deal in any way with the design of special

examples of electrical apparatus, but merely to describe a rather

miscellaneous set of materials and processes constantly required in

its construction.

It is not known whether there is such a thing as a perfect insulator,

even if we presuppose ideal circumstances. Materials as they exist

must be regarded merely as of high specific resistance, that is if we

allow ourselves to use such a term in connection with substances,

conduction through which is neither independent of electromotive force

per unit length, nor of previous history.

Even the best of these substances generally get coated with a layer of

moisture when exposed to the air, and this as a rule conducts fairly

well. Very pure crystalline sulphur and fused quartz suffer from this

defect less than any other substances with which the writer is

acquainted, but even with them the surface conductivity soon grows to

such an extent as totally to mask the internal conduction.

It is proposed to give a brief account of the properties of some

insulating substances and their application in electrical

construction, and at the same time to indicate the appliances and

methods requisite for working them.

With regard to the specific resistances which will be quoted, the

numbers must not be taken to mean too much, partly for the reason

already given. It is also in general doubtful whether sufficient care

has been taken to distinguish the body from the surface conductivity,

and consequently numerical estimates are to be regarded with

suspicion. The question of "sampling" also arises, for it must be

remembered that a change in composition amounting to, say, 1/10000 per

cent may be accompanied by a million-fold change in specific

resistance.

§ 102. Sulphur.

This element exists in several allotropic forms, which have very

different electric properties. After melting at about 125° C, and

annealing at 110° for several hours, the soluble crystalline

modification is formed. After keeping for some days--especially if

exposed to light--the crystals lose their optical properties, but

remain of the same melting-point, and are perfectly soluble in carbon

bisulphide. The change is accompanied by a change in colour, or

rather in brightness, as the transparency changes.

The "specific resistance" of sulphur in this condition is above 1028

C.G.S.E.M. units, or 1013 megohms per cubic centimetre for an electric

intensity of say 12,000 volts per centimetre. This is at ordinary

temperatures. At 75° C. the specific resistance falls to about 1025

under similar conditions as to voltage.

In all cases the conductivity appears to increase with the electric

intensity, or at all events with an increase in voltage, the thickness

of the layer of sulphur remaining the same.

The specific inductive capacity is 3.162 at ordinary temperatures, and

increases very slightly with rise of temperature. [Footnote: March

1897.--It is now the opinion of the writer that though the specific

inductive capacity of a given sample of a solid element is perfectly

definite, yet it is very difficult to obtain two samples having

exactly the same value for this constant, even in the case of a

material so well defined as sulphur.]

The total residual charge, after ten minutes' charging with an

intensity of 12,000 volts per centimetre, is not more than 4 parts in

10,000 of the original charge. In making this measurement the

discharge occupied a fraction of a second. The electric strength for

a homogeneous plate of crystalline sulphur is not less than 33,000

volts per centimetre, and probably a good deal more. If the sulphur

is contaminated with up to 3 per cent of the amorphous variety, as is

the case if it is cooled fairly quickly from a temperature of 170° C.

or over, the specific resistance falls to from 10^25 to 10^26 at

ordinary temperatures; and the specific inductive capacity increases

up to 3.75, according to the amount of insoluble sulphur present.

The residual charge under circumstances similar to those described

above, but with an intensity of about 4000 volts per centimetre is,

say, 2 per cent of the initial charge. So far as the writer is aware

sulphur is the only solid non-conductor which can be easily obtained

in a condition of approximate purity and in samples sufficiently

exactly comparable with one another; it is the only one, therefore,

that repays any detail of description.

Very pure sulphur can be bought by the ton if necessary from the

United Alkali Company of Newcastle-on-Tyne. It is recovered from

sulphur waste by the Chance process, which consists in converting the

sulphur into hydrogen sulphide, and burning the latter with

insufficient air for complete combustion. The sulphur is thrown out

of combination, and forms a crystalline mass on the walls and floor of

the chamber.

The sulphur which comes into the market consists of this mass broken

up into convenient fragments. In order to purify it sufficiently for

use as an insulator, the sulphur may be melted at a temperature of

120° to 140° C, and filtered through a plug of glass wool in a zinc

funnel; as thus prepared it is an excellent insulator. To obtain the

results mentioned in the table it is, however, necessary to conduct a

further purification (chiefly from water) by distillation in a glass

retort.

The sulphur thus obtained may be cast of any desired form in zinc

moulds, the castings and moulds being immediately removed to an

annealing oven at a temperature of from 100° to 110° C, where they

are left for several hours. If the sulphur is kept melted for some

time at 125° C. the annealing is not so important.

The castings may be removed from the mould by slightly heating the

latter, but many breakages result. Insulators made on this plan are

much less affected by the condensation of moisture from the air than

anything except fused quartz. They are, however, very weak

mechanically, and apt to crack by exposure to such changes of

temperature as go on from day to day. It is clear, however, that in

spite of this their magnificent electrical properties fit them for

many important uses.

If the sulphur be cooled rapidly from 170° C. or over, a mixture of

the crystalline and amorphous varieties of sulphur is obtained. This

mixture is very much stronger and tougher than the purely crystalline

substance, and may be worked with ordinary hardwood tools into fairly

permanent plates, rods, etc. Sheets of pure thick filter paper may

also be dipped into sulphur at 170° C, at which temperature air and

moisture are mostly expelled, and such sheets show a very considerable

insulating power. The sulphur does not penetrate the paper, which

therefore merely forms a nucleus.

Cakes of the crystalline or mixed varieties may be made by grinding up

some purified sulphur, moistening it with redistilled carbon

bisulphide, or toluene, or even benzene (C6H6), and pressing it in a

suitable mould under the hydraulic press. The plates thus formed are

porous, but are splendid insulators, especially if made from the

crystalline variety of sulphur, and they appear to keep their shape

very well, and do not crack with ordinary temperature changes.

The metals which resist the action of sulphur best are gold and

aluminium; while platinum and zinc are practically unacted upon at

temperatures below a red heat--in the former case,--and below the

boiling-point of sulphur in the latter.

A very convenient test of the purity of sulphur is the colour assumed

by it when suddenly cooled from the temperature at which it is

viscous. Quite pure sulphur remains of a pale lemon yellow under this

treatment, but the slightest trace of impurity, such as arises from

dust containing organic matter, stains the sulphur, and renders it

darker in colour.

§ 103. Fused Quartz.

This is on the whole the most reliable and most perfect insulator for

general purposes. No exact numerical data have been obtained, but the

resistivity must certainly be of the same order as that of pure

sulphur at its best. The influence of the moisture of the air also

reaches its minimum in the case of quartz, as was originally observed

by Boys.

As yet, however, the material can only be obtained in the form of rods

or threads. For most purposes rods of about one-eighth of an inch in

diameter are the most convenient. These rods may be used as

insulating supports, and succeed perfectly even if they interpose less

than an inch of their length to electrical conduction. The sketch

(Figs. 81 and 81A) shows (to a scale of about one-quarter full size)

a complete outfit for elementary electrostatic experiments, such as

has been in use in the writer's laboratory for five years. With these

appliances all the fundamental experiments may be performed, and the

apparatus is always ready at a moment's notice.

Fig. 81.

Though quartz does not condense moisture or gas to form a conducting

layer of anything like the same conductivity as in the case of glass

or ebonite, still it is well to heat it if the best results are to be

obtained. For this purpose a small pointed blow-pipe flame may be

used, and the rods may be got red-hot without the slightest danger of

breaking them. They then remain perfectly good and satisfactory for

several hours at least, even when exposed to damp and dusty air.

The rods are conveniently held in position by small brass ferrules,

into which they are fastened by a little plaster of Paris. Sealing-wax

must be avoided, on account of the inconvenience it causes when the

heating of the rods is being carried out.

One useful application of fused quartz is to the insulation of

galvanometer coils (Fig. 82), another to the manufacture of highly

insulating keys (Fig. 83); while as an insulating suspension it has

all the virtues. If it is desired to render the threads conducting

they may be lightly silvered, and will be found to conduct well enough

for electrometer work before the silver coating is thick enough to

sensibly impair their elastic properties.

Fig. 81A.

Fig. 82 is a full-size working drawing of a particular form of

mounting for galvanometer coils. The objects sought to be attained

are:

(1) high insulation of the coils,

(2) easy adjustment of the coils to the suspended system.

The first object is attained as follows. The ebonite ring A is bored

with four radial holes, through which are slipped from the inside the

fused quartz bolt-headed pins B. The coil already soaked in hard

paraffin is placed concentrically in the ring A by means of a special

temporary centering stand. The space between the coil and the ring is

filled up with hard paraffin, and this holds the quartz pins in

position. The system of ebonite ring, coil, and pins is then fastened

into the gun-metal coil carrier, which is cut away entirely, except

near the edges, where it carries the pin brackets C. These brackets

can swivel about the lower fastening at E before the latter is

tightened up.

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 pass 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 glass, and the back of the frame is also made of

glass, 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

pass through large holes in the carriage so as to allow of some

adjustment.

Fig. 82.

Fig. 83.

§ 104. Glass.

When glass 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 glass is much in excess of that of

soft soda glass, which is a poor insulator, and of ordinary green

bottle glass. The jars of Lord Kelvin's electrometers, which insulate

very well, are made of white flint glass 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 glass 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 glasses. The glass which gave the smallest residual charge

was an "opal" glass; and flint glasses were found to insulate 105

times as well as soda lime glasses. The plates of Wimshurst machines

are made of ordinary sheet window glass, 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 glass, this is stated by Koeller (Wien Bericht) to insulate

ten times as well as the ordinary Thuringian soft soda glass.

On the whole the most satisfactory laboratory practice is to employ

good white flint glass. The only point requiring attention is the

preparation of the glass 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 glass 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

glass 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 glass

with grease from the operator's fingers. Before setting out to clean

the glass 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 glass surface to become contaminated by the shellac varnish

or gum used to stick the tin-foil in position.

Finally, the glass 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 glass is preferably wiped with

a clean bit of filter paper; any hairs which may be left upon the

glass will brush off easily when the glass is dry.

In order to obtain satisfactory results the glass 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 glass, so that

unvarnished glass surfaces must be kept for apparatus which is

practically air-tight.

For outside or imperfectly protected uses the glass does better when

varnished. It is a fact, however, that varnished glass is rarely if

ever so good as unvarnished glass at its best. Too much care cannot

be taken over the preparation of the varnish; French polish, or

carelessly made shellac varnish, is likely to do more harm than good.

The best orange shellac 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 shellac. In making varnish, time and

trouble are saved by making a good deal at one operation--a

Winchester full is a reasonable quantity. The bottle may be filled

three-quarters full of the shellac 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 shellac

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 shellac and alcohol, which may be added at once for future use.

The glass 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 glass is left to dry long enough for the shellac 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 shellac softens

and begins to yield its fragrant characteristic smell.

If the coating is too heavy, or if the heating is commenced before the

shellac is sufficiently dry, the latter will draw up into "tears,"

which are unsightly and difficult to dry properly. On no account must

the shellac be allowed to get overheated. If the varnish is not quite

hard when cold it may be assumed to be doing more harm than good.

In varnishing glass 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 pass, so as

to prevent alcohol distilling from one part of the tube to another.

The tube is finally heated to the softening point of shellac, 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 brass, with ordinary

brass-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 brass, 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 scraping off the

outer layer of ebonite so as to expose a fresh surface. For this

purpose a bit of sheet glass broken so as to leave a curved edge is

very useful, and the ebonite is then scraped 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 scraped up. Rods or tubes are, of course, most quickly treated on

the lathe with rough glass 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. xxxvi. 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 glass," 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 (Thèse 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 illustration 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 lumière se

propageant dans le vide, mettrait plus de 3000 ans A se transmettre

d'une extrémité à 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 circumstances.

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.

This source of residual charge was carefully guarded against by

Rowland and Nichols (Phil. Mag. 1881) in their work on quartz, and is

referred to by M. Bouty, who adduces some experiments to show that his

own results are not vitiated by it. On the other hand, M. Bouty shows

that a small rise in temperature enormously affects the state of a

mica surface, and that the surface gets changed in such a way as to

become very fairly conducting at 300° C. Also anybody can easily try

for himself whether exposing a mica condenser plate which has been

examined in presence of phosphorus pentoxide to ordinary air for five

minutes will not enormously increase the residual charge, as has

always been the case in the writer's experience, and if so, it is open

to him to suggest some cause other than surface creeping as an

explanation.

M. Bouty, using less perfectly dried mica, did not get so good a

result as to smallness of residual charge as the one above quoted.

The chief use of mica for laboratory purposes depends on the ease with

which it can be split, and also upon the fact that it may be

considerably crumpled and bent without breaking. It therefore makes

an excellent dielectric in so far as convenience of construction is

concerned in the preparation of condensers, and lends itself freely to

the construction of insulating washers or separators of any kind. Its

success as a fair insulator at moderate temperatures has led to its

use in resistance thermometers, where it appears to have given

satisfaction up to, at all events, 400° C.

It is worth a note that according to Werner Siemens, who had immense

experience (Wied. Ann. vol. clix.), soapstone is the only reliable

insulator at a red heat, but, no doubt, a good deal depends on the

particular specimen investigated.

§ 107. Use of Mica in Condensers.

If good results are desired it is essential to select the mica very

carefully. Pieces appreciably stained,--particularly if the stain is

not uniformly distributed,--cracked pieces, and pieces tending to

flake off in patches should be rejected. The best samples of mica

that have come under the writer's observation are those sheets sold

for the purpose of giving to silver photographic prints that hideous

glazed surface which some years ago was so popular.

Sheets of mica about 0.1 to 0.2 mm. thick form good serviceable

condenser plates, and will certainly stand a pressure of 300 volts,

and most likely a good deal more. The general practice in England

seems to have been to build up condensers of alternate sheets of

varnished or paraffined-mica and tin-foil.

This practice is open to several objections. In the first place, the

capacity of a condenser made in this way varies with the pressure

binding the plates together. In the second place, the amount of mica

and tin-foil required is often excessive in consequence of the

imperfect contact of these substances. Again, the inevitable air film

between the mica and tin-foil renders condensers so made unsuitable

for use with alternating currents, owing to the heating set up through

air discharges, and which is generally, though often (if not always)

wrongly, attributed to dielectric hysteresis.

These imperfections are to a great extent got over by M. Carpentier's

method of construction, which is, however, rather more costly both in

material and labour. On the other hand, wonderful capacities are

obtained with quite small amounts of mica. M. Bouty mentions a

condenser of one microfarad capacity weighing 1500 grms. and

contained in a square box measuring 12 centimetres on the side, and

about 3 centimetres thick.

The relation between the capacity and surface of doubly-coated plates

is in electro-static units:

Capacity = (sp. ind. capacity X area of one surface)/(4pi X thickness)

This may be reduced to electro-magnetic units by dividing by 9x10^20,

and to microfarads by further multiplying by 10^15.

M. Carpentier begins, of course, by having his mica scrupulously clean

and well selected. It is then silvered by one of the silvering

processes (§ 65) on both sides, for which purpose the sheets may be

suspended in a paraffined wood rack, so as to lie horizontally in the

silvering solution, a space of about half an inch being allowed

between the sheets. The silvering being finished, the sheets are

dipped along two parallel edges in 75 per cent nitric acid. With

regard to the third and fourth edges of the sheet, the silver is

removed on one side only, using a spun glass brush; if we agree to

call the two surfaces of the mica A and B respectively, and the two

edges in question C and D, then the silver is removed from the A side

along edge C, and from the B side along edge D. The silvered part is

shown shaded in Fig. 84. By this arrangement the silver and mica

plates may be built up together so as to form the same mutual

arrangement of contacts as in an ordinary mica tin-foil condenser.

Fig. 84.

It need hardly be said that the sheets require very complete washing

after treatment with nitric acid, followed by a varnishing of the

edges as already described in the case of glass, and baking at a

temperature of 140° C. in air free from flame gases, till the shellac

begins to emit its characteristic odour and is absolutely hard when

cold.

The plates are then built up so as to connect the sheets which require

to be connected, and to insulate the other set. General contact is,

if necessary, secured by means of a little silver leaf looped across

from plate to plate--a part of the construction which requires

particular attention and clean hands, for it is by no means so easy to

make an unimpeachable contact as might at first appear.

The condenser, having been built up, may be clamped solid and placed

in its case; the capacity will not depend appreciably on the

tightness of the clamp screws--a great feature of the construction.

Such a condenser will not give its best results unless absolutely dry.

I have kept one very conveniently in a vacuum desiccator over

phosphorus pentoxide, but if of any size, the condenser deserves a box

to itself, and this must be air-tight and provided with a drying

reagent, so arranged that it can be removed through a manhole of some

sort.

Contact to the brass-work on the lid may be made by pressing spring

contacts tightly down upon the ends of the silver foils and carrying

the connections through the lid. This also serves to secure the

condenser in position.

§ 108. Micanite.

This substance, though probably comparing somewhat unfavourably with

the insulators already enumerated, and being subject to the

uncertainties of manufacture, has during the last few years achieved a

considerable success in American electrical engineering construction.

It is composed of scrap mica and shellac varnish worked under pressure

to the desired shape, and may be obtained in sheets, plates, and rods,

or in any of the forms for which a die happens to have been

constructed.

Of course, in special cases it would be worth while to prepare a die,

and then the attainable forms would be limited by moulding

considerations only. The writer's experience is very limited in this

matter, but Dr. Kennelly, with whom he communicated on the subject,

was good enough to reply in favour of micanite for engineering work.

§ 109. Celluloid.

This material is composed of nitrocellulose and camphor.

It has fair insulating properties, and may be obtained in a variety of

forms, but has now been generally abandoned for electrical work on

account of its inflammability.

§ 110. Paper.

Pure white filter paper, perfectly dry, is probably a very fair

insulator; the misfortune is that in practice it cannot be kept dry.

Under the most favourable circumstances its specific resistance may

approach 1024 E.M. units. It must therefore be considered rather as a

partial conductor than as an insulator. The only case of the use of

dry paper as an insulator in machine construction which has come under

the writer's notice is in building up the commutators of the small

motors which used to drive the Edison phonographs.

Its advantages in this connection are to be traced to the fact that a

commutator so built up is durable and keeps a clean surface. Of

course, the use of paper as an insulator for telephone wires is well

known, but its success in this direction depends less upon its

insulating properties than upon the fact that it can be arranged in

such a way as to allow of the wires being partially air insulated, an

arrangement tending to reduce the electrostatic capacity of the wire

system.

At one time it was the custom of instrument makers to employ ordinary

printed paper in the shape of leaves torn from books or the folios of

old ledgers to form the dielectric of the condensers used in

connection with the contact breakers of induction coils. This

practice has nothing but economy to recommend it, for cases often

occur in which the paper, by gradual absorption of moisture from the

air, comes 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

§ 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 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.

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° 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 scraped 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 passed 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 homogeneity 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

homogeneity, 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 glass 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 scraped 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 glass, nor does the insulating power fall so far.

Two useful appliances are figured.

Fig. 86. Fig. 87.

One, in which paraffin appears as a cement, is an insulating stand

made out of a bit of glass or ebonite tube cemented into an Erlenmeyer

flask, having its neck protected from dust when out of use by a rubber

washer, the other a "petticoat" insulator made by cementing a flint

glass bottle into a glass dish with paraffin. In course of time the

paraffin will be found to have separated from the glass. When this

occurs the apparatus may be melted together again by placing it on

the water bath for a few minutes.

§ 113. Vaseline, Vaseline Oil, and Kerosene Oil.

These, when dry, insulate almost, but not quite as well as solid

paraffin. H. Koeller (Wien Berichte, 98, ii. 201, 1889; Beibl. Wied.

Ann. 1890, p. 186), working with very small voltages, places the

final(?) specific resistance of commercial petroleum, ether, and

vaseline oil at about 2 X 1027 C.G.S. This is ten times higher than

the value assigned to commercial benzene (C6H6), and a hundred times

higher than the value for commercial toluene.

All these numbers mean little or nothing, but the petroleum and

vaseline oil were the best fluid insulators examined by Koeller. By

mixing vaseline with paraffin a soft wax may be made of any desired

degree of softness, and by dissolving vaseline in kerosene an

insulating liquid of any degree of viscidity may be obtained.

Hard paraffin may be softened somewhat by the addition of kerosene,

and an apparently homogeneous mass cast from the mixture. It will be

found, however, that in course of time the kerosene oozes out, unless

present in very small quantity. Koeller has found (loc. cit.) that

some samples of vaseline oil conducted "vollstaendig gut," but I have

not come across such samples. Vaseline oil, however, is sold at a

price much above its value for insulating purposes.

Kerosene oil is best obtained dry by drawing it directly from a new

tin and exposing it to air as little as possible. Of course, it may

be dried by chemical means and distillation, but this is usually (or

always) unnecessary.

Fig 88.

There is some danger of kerosene containing minute traces of sulphuric

acid, and it and other oils may be conveniently tested for insulation

in the following manner. The quartz electroscope is taken, and the

insulating rod heated in the blow-pipe. The electroscope will now

insulate well enough to show no appreciable collapse of the leaves in

one or two hours' time. Upon the plate of the electroscope is put a

platinum or copper cylinder, and this is filled with kerosene (say) up

to a fixed mark.

The electroscope is placed on a surface plate, or, at all events, on a

sheet of plate glass, and a "scribing block" is placed along side it

and the scriber adjusted to dip into the kerosene to any required

depth. This is done by twisting a bit of wire round the scribing

point and allowing it to project downwards. The point itself serves

to give an idea of the height to which the vessel may be filled. The

liquid is adjusted till its surface is in contact with the end of the

scribing point, and the wire then projects into the liquid and forms

an electrode of constant area of surface. The scribing block is put

to earth. A charge is given to the electroscope, and the time

required for a given degree of collapse of the leaves noted.

The kerosene is then removed and its place taken by vaseline or

paraffin, known to insulate well as a standard for comparison. The

experiment is then repeated, and the time noted for the same degree of

collapse. This test, though of course rough, is generally quite

sufficient for workshop purposes, and is easily applied. Moreover, it

does not require correction for electrometer leakage, as generally

happens when more elaborate appliances are used.

The actual resistance of insulating oils depends so much on the

electrical intensity, on the duration of that intensity, and on the

previous history of the oil as to the direction of the voltage to

which it has been subjected--to say nothing of the effects of traces

of moisture--that quantitative experiments are of no value unless

they are extremely elaborate. I shall therefore only append the

following numbers due to Bouty, Ann. de Chemie et de Physique (6),

vol. xxvii. p. 62, 1892, in which the effect of the conductivity on

the determination of the specific inductive capacity was properly

allowed for:-

Carbon

Bisulphide.

Turpentine.

Benzene (C6H6) at 20° C.

Benzene at

60° C.

Specific inductive capacity

2.715

2.314

2.21

2.22

Specific resistance in ohms per cubic centimetre

1.5 x 1013,

1.75 x 1012

1.56 x 1011

7.9 x 1011

[Footnote: Professor J. J. Thomson, and Newall (Phil. Proc. 1886)

consider that carbon bisulphide showed traces of a "residual charge"

effect; hence, until this point is cleared up, we must regard Bouty's

value as corresponding only to a very short, but not indefinitely

short, period of charge. On this point the paper must be consulted.

March 1897--The writer has investigated this point by an independent

method, but found no traces of "residual charge."]

Information as to the specific inductive capacity of a large number of

oils may be found in a paper by Hopkinson, Phil. Proc. 1887, and in a

paper by Quincke in Wiedemann's Annalen, 1883.

§ 114. Imperfect Conductors.

Under this heading may be grouped such things as wood, slate, marble,

etc--in fact, materials generally used for switchboard insulation.

An examination of the insulating power of these substances has

recently been made by B. O. Peirce (Electrical Review, 11th January

1895) with quite sufficient accuracy, having in view the impossibility

of being certain beforehand as to the character of any particular

sample. The tests were made by means of holes drilled in slabs of the

material to be examined. These holes were three-eighths of an inch in

diameter, and from five-eighths to three-quarters of an inch deep, and

one inch apart, centre to centre. A voltage of about 15 volts was

employed. The following general results were arrived at:-

(1) Heating in a paraffin bath always increases the resistance of

wood, though only slightly if the wood be hard and dense.

(2) Frequent exhaustion and readmission of air above the surface of

the paraffin always has a good effect in increasing the resistance of

wood.

(3) When wood is once dry, impregnating it with paraffin tends to keep

it dry.

(4) Red vulcanised fibre, like wood, absorbs paraffin, but it cannot

be entirely waterproofed in this way.

(5) The resistance of wood with stream lines along the grain is twenty

to fifty per cent lower than when the stream lines cross the grain.

(6) The "contact" resistance between slabs of wood pressed together is

always very high.

The following table will sufficiently illustrate the results obtained.

The stone was dried in the sun for three weeks in the summer (United

States), and the wood is described as having been well seasoned:-

CURRENT WITH THE GRAIN

Lowest Resistance Highest Resistance Lowest Specific Highest Specific

between two Cups between two Cups Resistance in Resistance in

in Megohms. in Megohms. Megohms. Megohms.

Ash.

550 920 380 700

Cherry

1100 4000 2800 6000

Mahogany

430 730 310 610

Oak

220 420 1050 2200

Pine.

330 630 360 1470

Hard pine.

10 48 17 1050

Black walnut

1100 3000 320 2100

Red fibre

2 4 3 60

Slate

184 280

Soapstone.

330 500

White marble

2000 8800

§ 115. As to working the materials very little need be said.

Fibre is worked like wood, but has the disadvantage of rapidly taking

the edge off the tools. In turning it, therefore, brass-turning

tools, though leaving not quite such a perfect finish as wood-turning

tools, last much longer, and really do well enough. Fibre will not

bear heating much above 100°C--at all events in paraffin. At 140°

C. it becomes perfectly brittle. Its chief merit lies in its great

strength. So far as insulation is concerned, Mr. Peirce's experiments

show that it is far below most kinds of wood.

Slate. This is a vastly more useful substance than it is generally

credited with being. It is very easily worked at a slow speed, either

on the shaping machine or on the lathe, with tools adjusted for

cutting brass, and it keeps its figure, which is more than can be said

for most materials. It forms a splendid base for instruments,

especially when ground with a little emery by iron or glass grinders,

fined with its own dust, and French polished in the ordinary way.

Spools for coils of considerable radial dimension may be most

conveniently made of slate instead of wood or brass, both because it

keeps its shape, and because it insulates sufficiently well to stop

eddy currents--at all events, sufficiently for ordinary practice. An

appreciable advantage is that slate may be purchased at a reasonable

rate in large slabs of any desired thickness. It is generally cut in

the laboratory by means of an old cross-cut saw, but it does not do

much damage to a hard hack saw such as is used for iron or brass.

Marble. According to Holtzapffell, marble may be easily turned by

means of simple pointed tools of good steel tempered to a straw

colour. The cutting point is ground on both edges like a wood-turning

tool, and held up to the work "at an angle of twenty or thirty

degrees" (?with the horizontal). The marble is cut wet to save the

tool. As soon as the point gets, by grinding, to be about one-eighth

of an inch broad it must either be drawn down or reground; a flat

tool will not turn marble at all.

A convenient saw for marble is easily made on the principle of the

frame saw. A bit of hoop iron forms a convenient blade, and is

sharpened by being hammered into notches along one edge, using the

sharp end of a hammer head. The saw is liberally supplied with sand

and water--or emery and water, where economy of time is an object.

The sawing of marble is thus really a grinding process, but it goes on

rapidly. Marble is ground very easily with sand and water; it is

fined with emery and polished with putty powder, which, I understand,

is best used with water on cloth or felt. As grinding processes have

already been fully described, there is no need to go into them here.

I have no personal knowledge of polishing marble.

§ 116. Conductors.

The properties of conductors, more particularly of metals, have been

so frequently examined, that the literature of the subject is

appallingly heavy. In what follows I have endeavoured to keep clear

of what might properly appear in a treatise on electricity on the one

hand, and in a wiring table on the other. The most important work on

the subject of the experimental resistance properties of metals has

been done by Matthieson, Phil. Trans. 1860 and 1862, and British

Association Reports (1864); Callender, Phil. Trans. vol. clxxiii;

Callender and Griffiths, Phil. Trans. vol. clxxxii; The Committee

of the British Association on Electrical Standards from 1862 to

Present Time; Dewar and Fleming, Phil. Mag. vol. xxxvi. (1893);

Klemencic, Wiener Sitzungsberichte (Denkschrift), 1888, vol. xcvii. p.

838; Feussner and St. Lindeck, Zeitsch. fuer Inst. 'Kunde, ix. 1889,

p. 233, and B. A. Reports, 1892, p. 139. Of these, Matthieson, and

Dewar and Fleming treat of resistance generally, the latter

particularly at low temperatures.

[Footnote: The following is a list of Dr. Matthieson's chief papers on

the subject of the electrical resistance of metals and alloys: Phil.

Mag. xvi. 1858, pp. 219-223; Phil. Trans. 1858, pp. 383-388 Phil.

Trans. 1860, pp. 161-176; Phil. Trans. 1862, pp. 1-27 Phil. Mag. xxi.

(1861), pp. 107-115; Phil. Mag. xxiii. (1862), pp. 171-179;

Electrician, iv. 1863, pp. 285-296; British Association Reports,

1863, p. 351.]

Matthieson, and Matthieson and Hockin, Klemencic, Feussner, and St.

Lindeck deal with the choice of metals for resistance standards.

Callender's, and Callender and Griffiths' work is devoted to the study

of platinum for thermometric purposes.

The bibliography referring to special points will be given later. The

simplest way of exhibiting the relative resistances of metals is by

means of a diagram published by Dewar and Fleming (loc. cit.), which

is reproduced on a suitable scale on the opposite page. For very

accurate work, in which corrections for the volumes occupied by the

metals at different temperatures are of importance, the reader is

referred to the discussion in the original paper, which will be found

most pleasant reading. From this diagram both the specific resistance

and the temperature coefficient may be deduced with sufficient

accuracy for workshop purposes. In interpreting the diagram the

following notes will be of assistance. The diagram is drawn to a

scale of so-called "platinum temperatures"--that is to say, let R0,

R100, Rt be the resistances of pure platinum at 0°, 100°, and t° C.

respectively, then the platinum temperature pt is defined as

pt = 100 X (Rt-R0)/(R100-R0)

This amounts to making the temperature scale such that the temperature

at any point is proportional to the resistance of platinum at that

point. Consequently on a resistance temperature diagram the straight

line showing the relation between platinum resistance and platinum

temperature will "run out" at the platinum absolute zero, which

coincides more or less with the thermodynamic absolute zero, and also

with the "perfect gas" absolute zero. Platinum temperatures may be

taken for workshop purposes over ordinary ranges as almost coinciding

with air thermometer temperatures. The metals used by Professors

Dewar and Fleming were, with some exceptions, not absolutely pure, but

in general represent the best that can be got by the most refined

process of practical metallurgy. We may note further that the

specific resistance is only correct for a temperature of about 15° C,

since no correction for the expansion or contraction of material has

been applied.

The following notes on alloys suitable for resistance coils will

probably be found sufficient.

§ 117. Platinoid.

This substance, discovered by Martino and described by Bottomley

(Phil. Proc. Roy. Soc. 1885), is an alloy of nickel, zinc, copper, and

1 per cent to 2 per cent of tungsten, but I have not been able to

obtain an analysis of its exact composition. It appears to be

difficult to get the tungsten to alloy, and it has to be added to part

of the copper as phosphide of tungsten, in considerably greater

quantity than is finally required. The nickel is added to part of the

copper and the phosphide of tungsten, then the zinc, and then the rest

of the copper. The alloy requires to be remelted several times, and a

good deal of tungsten is lost by oxidation.

The alloy is of a fine white colour, and is very little affected by

air--in fact, it is to some extent untarnishable. The specific

resistance will be seen to be about one and a half times greater than

that of German silver, and the temperature coefficient is about 0.021

per cent per degree C. (i.e. about nineteen times less than copper,

and half that of German silver). To all intents and purposes it may

be regarded as German silver with 1 per cent to 2 per cent of

tungsten. It does not appear to have been particularly examined for

secular changes of resistance.

118. German Silver. This material has been exhaustively examined of

late years by Klemencic and by Feussner and St. Lindeck. Everybody

agrees that German silver, as ordinarily used for resistances, and

composed of copper four parts, zinc two parts, nickel one part, is

very ill-fitted for the purpose of making resistance standards. This

is due

(1) to its experiencing a considerable increase in resistance on

winding. Feussner and St. Lindeck found an increase of 1 per cent

when German silver was wound on a core of ten wire diameters.

(2) To the fact that the change goes on, though with gradually

decreasing rate, for months or years;

(3) to the fact that the resistance is permanently changed (increased)

by heating to 40° C. or over. By "artificially ageing" coils of

German silver by heating to 150° C, say for five or six hours, its

permanency is greatly improved, and it becomes fit for ordinary

resistance coils where changes of, say, 1/5000 do not matter.

It is a remarkable property of all nickel alloys containing zinc that

their specific resistance is permanently increased by heating, whereas

alloys which do not contain zinc suffer a change in the opposite

direction. The manufacturers of German silver appear to take very

little care as to the uniformity of the product put on the market;

some so-called German silver is distinctly yellow, while other samples

are bright and white.

It is noted by Price (Measurements of Electrical Resistance, p. 24)

that German silver wire is apt to exhibit great differences of

resistance within quite short lengths. This has been my own

experience as well, and is a great drawback to the use of German

silver in the laboratory, for it makes it useless to measure off

definite lengths of wire with a view to obtaining an approximate

resistance. In England German silver coils are generally soaked in

melted hard paraffin. In Germany, at all events at the Charlottenburg

Institute, according to St. Lindeck--coils are shellac-varnished and

baked. In any case it appears to be essential to thoroughly protect

the metal against atmospheric influence.

§ 119. Platinum Silver.

In the opinion of Matthieson and of Klemencic the 10 per cent silver,

90 per cent platinum alloy is the one most suitable for resistance

standards. At all events, it has stood the test of time, for, with

the following exceptions, all the British Association coils

constructed of it from 1867 to the present day have continued to agree

well together. The exceptions were three one-ohm coils, which

permanently increased between 1888 and 1890, probably through some

straining when immersed in ice. One coil changed by 0.0006 in 1

between the years 1867 and 1891. According to Klemencic, absolute

permanency is not to be expected even from this alloy.

Its recommendation as a standard depends on its chemical inertness,

its small temperature coefficient (0.00027 per degree), and its small

thermo-voltage against copper, as the following table (taken from

Klemencic) will show:-

Thermo-voltages in Micro-volts per degree against Copper

over the Range 0° to 17° C.

Platinum iridium 7.14 micro-volts per degree C.

Platinum silver 6.62 micro-volts per degree C.

Nickelin 28.5 micro-volts per degree C.

German silver 10.43 micro-volts per degree C.

Manganin (St. Lindeck) 1.5 micro-volts per degree C.

Mechanically, the platinum silver is weak, and is greatly affected as

to its resistance by mechanical strains--in fact, Klemencic considers

it the worst substance he examined from this point of view--a

conclusion rather borne out by Mr. Glazebrook's experience with the

British Association standards already referred to (B. A. Reports, 1891

and 1892).

Taking everything into account, it will probably be well to construct

standards either with oil insulation only, or to bake the coils in

shellac before testing, instead of soaking in paraffin. Fig. 89

illustrates a form of an oil immersed standard which is in use in my

laboratory, and through which a considerable current may be passed.

The oil is stirred by means of a screw propeller.

Fig. 89.

Fig. 89 represents a standard resistance for making Clerk cell

comparisons by the silver voltameter method. The framework on which

the coils are wound consists of a base and top of slate. The pillars

are of flint glass tube surrounding brass bolts, and cemented to the

latter by raw shellac. Grooves are cut in the glass sleeves to hold

the wires well apart. These grooves were cut by means of a file

working with kerosene lubrication. A screw stirrer is provided, and

the whole apparatus is immersed in kerosene in the glass box of a

storage cell. The apparatus is aged to begin with by heating to a

temperature a good deal higher than any temperature it is expected to

reach in actual work. After this the rigidity of the frame is

intended to prevent any further straining of the wire. The apparatus

as figured is not intended to be cooled to 0° C, so that it is put in

as large a box as possible to gain the advantage of having a large

volume of liquid. The top and bottom slates measure seven inches by

seven inches, and the distance between them is seven inches. The

inner coil is wound on first, and the loop which constitutes the end

of the winding is brought up to a suitable position for adjustment.

The insulation of the heavy copper connectors is by means of ebonite.

§ 120. Platinum Iridium.

Platinum 90 per cent, iridium 10 per cent. This material was prepared

in some quantity at the cost of the French Government, and distributed

for test about 1886. Klemencic got some of it as representing

Austria, and found it behaved very like the platinum silver alloy just

discussed. The temperature coefficient is, however, higher than for

platinum silver (0.00126 as against 0.00027). The mechanical

properties of the alloy are, however, much better than those of the

silver alloy; and in view of the experience with B. A. standards

above quoted, it remains an open question whether, on the whole, it

would not be the better material for standards, in spite of its

higher price. Improvements in absolute measurements of resistance,

however, may render primary standards superfluous.

§ 121. Manganin.

Discovered by Weston--at all events as to its application to

resistance coils. A glance at the diagram will exhibit its unique

properties, on account of which it has been adopted by the

Physikalisch Technischen Reichsanstalt for resistance standards. The

composition of the alloy is copper 84 per cent, manganese 12 per cent,

nickel 4 per cent, and it is described as of a steel-gray colour.

Unfortunately it is apt to oxidise in the air, or rather the manganese

it contains does so, so that it wants a very perfect protection

against the atmosphere.

Like German silver, manganin changes in resistance on winding, and

coils made of it require to be artificially aged by heating to 150°

for five hours before final adjustment. The annealing cannot be

carried out in air, owing to the tendency to oxidation. The method

adopted by St. Lindeck (at all events up to 1892) is to treat the

coil with thick alcoholic shellac varnish till the insulation is

thoroughly saturated, and then to bake the coil as described. The

baking not only anneals the wire, but reduces the shellac to a hard

and highly insulating mass.

Whether stresses of sufficient magnitude to produce serious mechanical

effects can be set up by unequal expansion of wire and shellac during

heating and cooling is not yet known, but so far as tested (and it

must be presumed that the Reichsanstalt tests are thorough) no

difficulty seems to have been met with. In course of time, however,

probably the best shellac coating will crack, and then adieu to the

permanency of the coil! This might, of course, be obviated by keeping

the coil in kerosene, which has no action on shellac, but which

decomposes somewhat itself.

The method of treatment above described suffices to render coils of

manganin constant for at least a year (in 1892 the tests had only been

made for this time) within a few thousands per cent. Manganin can be

obtained in sheets, and from this material standards of 10-2, 10-3,

and 10-4 ohms are made by soldering strips between stout copper bars,

and these are adjusted by gradually increasing their resistance by

boring small holes through them. The solder employed is said to be

"silver."

Mr. Griffiths (Phil. Trans. vol. clxxxiv. [1893], A, p. 390) has

had some experience with manganin carrying comparatively heavy

currents, under which circumstances its resistance when immersed in

water was found to rise in spite of the varnish which coated it.

Other experiments in which the manganin wire was immersed in paraffin

oil did not exhibit this effect, though stronger currents were passed.

On the whole, manganin appears to be the best material for coil boxes

and "secondary" resistance standards. Whether it is fit to rank with

the platinum alloys as regards permanency must be treated as an open

question.

§ 122. Other Alloys.

The following tables, taken from the work of Feussner and St.

Lindeck, Zeitschrift fuer Instrumenten Kunde, 1889, vol. ix. p.

233, together with the following notes, will suffice.

§ 123. Nickelin.

This is only German silver with a little less zinc, a little more

nickel, and traces of cobalt and manganese. It behaves like German

silver, but is an improvement on the latter in that all the faults of

German silver appear upon a reduced scale in nickelin.

§ 124. Patent Nickel.

Practically a copper nickel alloy, used to some extent by Siemens

and Halske. It stands pretty well in the same relation to nickelin as

the latter does to German silver. After annealing as for manganin it

can be made into serviceable standards which do not change more than a

few thousandths per cent. I have not come across a statement of its

thermo-voltage against copper.

§ 125. Constantin.

Another nickel copper alloy containing 50 per cent of each

constituent. It appears to be a serviceable substance, having a

temperature coefficient of 0.003 per cent per degree only, but an

exceedingly high thermo-voltage, viz. 40 micro-volts per degree

against copper.

1 2 3 4 5 6 7 8

German Nickelin made Rheo- Patent Nickel Manga- Nickel

Silver by Obermaier tane nese Manga-

Dia- Dia- Dia- Dia- Copper nese

meter meter meter meter Copper

1.0mm 0.1mm 0.6mm 1.0mm

Copper 60.16 61.63 54.57 53.28 74.41 74.71 70 73

Zinc 25.37 19.67 20.44 16.89 0.23 0.52 ... ...

Tin ... ... ... ... trace ... ...

Nickel 14.03 18.46 24.48 25.31 25.10 24.14 ... 3

Iron 0.30 0.24 0.64 4.46 0.42 0.70 ... ...

Cobalt trace 0.19 ... ... trace trace ... ...

Mang- trace 0.18 0.27 0.37 0.13 0.17 30 24

anese.

99.86 100.37 100.40 100.31 100.24 100.24 ... ...

Specific

resistance

30.0 33.2 44.8 52.5 34.2 32.8 100.6 47.7

Temperature

coefficient

0.00036 0.00030 0.00033 0.00041 0.00019 0.00021 0.00004 0.00003

The specific resistance is in microhms, i.e. 10-6 ohms per cubic

centimetre, and the temperature coefficient in degrees centigrade.

126. Nickel Manganese Copper.

I can find no other reference with regard to this alloy mentioned by

Lindeck. Nicholls, however (Silliman's Journal [3], 39, 171, 1890),

gives some particulars of alloys of copper and ferromanganese. The

following table is taken from Wiedemann's Beiblatter (abstract of

Nicholl's paper, 1890, p. 811). All these alloys appear to require

annealing at a red heat before their resistances are anything like

constant.

Let x be percentage of copper, then 100--x is percentage of

"ferromanganese."

Values of x. 100 99.26 91 .88 86.98 80.4 70.65

Specific

resistance

with respect

to copper

(? pure) 1 1.19 11.28 20.4 27.5 45.1

Temperature

coefficient

per degree

x 10^6(hard) 3202 2167 138 16 22 -24

Ditto (soft) ... ... 184 80 66 21

If nickel is added, alloys of much the same character are obtained,

some with negative temperature coefficients--for instance, one

containing 52.51 per cent copper, 31.27 per cent ferromanganese, and

16.22 nickel.

A detailed account of several alloys will be found in a paper by

Griffiths (Phil. Trans. 1894, p. 390), but as the constants were

determined to a higher order of accuracy than the composition of the

material--or, at all events, to a higher degree of accuracy than

that to which the materials can be reproduced--there is no advantage

in quoting them here.