MISCELLANEOUS PROCESSES
§ 74. Coating Glass with Aluminium and Soldering Aluminium.
A process of coating glass with aluminium has been lately discovered,
which, if I mistake not, may be of immense service in special cases
where a strongly adherent deposit is required. My attention was first
attracted to the matter by an article in the Archives des Sciences
physiques et naturelles de Genève, 1894, by M. Margot. It appears
that clean aluminium used as a pencil will leave a mark on clean damp
glass. If, instead of a pencil, a small wheel of aluminium--say as
big as a halfpenny and three times as thick--is rotated on the lathe,
and a piece of glass pressed against it, the aluminium will form an
adherent, though not very continuous coating on the glass.
Working with a disc of the size described rotating about as fast as
for brass-turning, I covered about two square inches of glass surface
in about five minutes. The deposit was of very uneven thickness, but
was nearly all thick enough to be sensibly opaque. By burnishing the
brilliance is improved (I used an agate burnisher and oil), but a
little of the aluminium is rubbed off. The fact that the burnisher
does not entirely remove it is a sign of the strength of the adherence
which exists between the aluminium and the glass. In making the
experiment, care must be taken to have the glass quite clean--or at
all events free from grease--in order to obtain the best results.
M. Margot has contributed further information to the Archives des
Sciences physiques et naturelles (February 1895). He finds that
adherence between aluminium and glass is promoted by dusting the glass
with powders, such as rouge. There is no doubt that a considerable
improvement is effected in this way; both rouge and alumina have in
my hands greatly increased the facility with which the aluminium is
deposited. M. Margot finds that zinc and magnesium resemble aluminium
in having properties of adherence to glass, and, what is more, carry
this property into their alloys with tin. Thus an alloy of zinc and
tin in the proportions of about 92 per cent tin and 8 per cent zinc
may be melted on absolutely clean glass, and will adhere strongly to
it if well rubbed by an asbestos crayon.
A happy inspiration was to try whether these alloys would, under
similar circumstances, adhere to aluminium itself, and a trial showed
that this was indeed the case, provided that both the aluminium and
alloy are scrupulously clean and free from oxide. In this way M.
Margot has solved the problem of soldering aluminium. I have satisfied
myself by trial of the perfect ease and absolute success of this
method. The alloy of zinc and tin in the proportions above mentioned
is formed at the lowest possible temperature by melting the
constituents together. It is then poured so as to form thin sticks.
The aluminium is carefully cleaned by rubbing with a cuttle bone, or
fine sand, and strong warm potash. It is then washed in water and
dried with a clean cloth. The aluminium is now held over a clean
flame and heated till it will melt the solder which is rubbed against
it. The solder sticks at once, especially if rubbed with another bit
of aluminium (an aluminium soldering bit) similarly coated. To solder
two bits of aluminium together it is only necessary to tin the bits by
this process and then sweat them together.
The same process applies perfectly to aluminium caused to adhere to
glass by the previously mentioned process, and enables strong soldered
contacts to be made to glass. In one case, while I was testing the
method, the adhesion was so strong that the solder on contracting
while cooling actually chipped the surface clean off the glass. In
order to get over this I have endeavoured to soften the solder by
mixing in a little of the fusible metal mercury amalgam; and though
this prevents the glass from being so much strained, it reduces the
adherence of the solder. It is a comfort to be able to solder
aluminium after working for so many years by way of electroplating, or
filing under solder. An alternative method of soldering aluminium
will be described when the electroplating of aluminium is discussed, §
138.
Gilding Glass. In looking over some volumes of the Journal fuer
praktische Chemie, I came across a method of gilding glass due to
Boettger (Journ. f. prakt. Chem. 103, p. 414). After many trials
I believe I am in a position to give definite instructions as to the
best way of carrying out this rather troublesome operation. The films
of gold obtained by the process are very thick, and the appearance of
the gold exceedingly fine. The difficulty lies in the exact
apportionment of the reducing solution. If too much of the reducing
solution be added, the gold deposits in a fine mud, and no coating is
obtained. If, on the other hand, too little of the reducing solution
be added, little or no gold is deposited. The secret of success turns
on exactly hitting the proper proportions.
The reducing solution consists of a mixture of aldehyde and glucose,
and the difficulty I have had in following Boettger's instructions
arose from his specifying "commercial aldehyde" of a certain specific
gravity which it was impossible to reproduce. I did not wish to
specify pure aldehyde, which is not very easily got or stored, and
consequently I have had to determine a criterion as to when the
proportion of reducing solution is properly adjusted.
The aldehyde is best made as required. I employed the ordinary
process as described in Thorpe's Dictionary of Applied Chemistry, by
distilling alcohol, water, sulphuric acid, and manganese dioxide
together. The crude product is mixed with a large quantity of calcium
chloride (dry--not fused), and is rectified once. The process is
stopped when the specific gravity of the product reaches 0.832 at 60°
F. The specific gravity of pure aldehyde is 0.79 nearly.
The following is a modification of Boettger's formula:-
Solution I
1 gram of pure gold is converted into chloride--got acid free--i.e.
to the state represented by AuCl3, and dissolved in 120 cc. of water.
This solution is the equivalent of one containing 6.5 grains of
trichloride to the ounce of water.
Solution II.
6 grams sodium hydrate.
100 grams water.
Solution III.
0.2 grams glucose (bought as pure).
12.6 cubic centimetres 95 per cent alcohol.
12.6 cubic centimetres water.
2.0 cubic centimetres aldehyde, sp. gr. 0.832.
To gild glass these solutions are used in the following proportions by
volume:-
16 parts of No. I.
4 parts of No. II.
0.8 parts of No. III.
The glass is first cleaned well with acid and washed with water: it
is then rinsed with Solution No. III. If it is desired to gild the
inside of a glass vessel, Solution No. III. may be placed in the
vessel first, and the walls of the vessel rinsed round carefully.
Solutions I. and II. are mixed separately and then added
to III.--after about two minutes the whole is well shaken up.
If it be desired to gild a mirror of glass, the glass-plate is
suspended face downwards in a dish of the mixed solutions--care being
taken to rinse the glass with Solution III. first.
If the mixture darkens in from 7' to 10' in diffuse daylight and at
60°F. it will gild well, and it generally pays to make a few trials
in a test tube to arrive at this. If too much reducing solution is
present the liquid will get dark more rapidly, and vice versa. The
gilding will require several hours--as much as twelve hours may be
needed.
The reaction is one of great chemical interest, being one of that
class of reactions which is greatly affected by capillarity. Thus it
occasionally happens that when the reducing solution is not in the
right proportion, gold will be deposited at the surface of the liquid
(so as to form a gilt ring on the inside of a test tube), the
remainder of the gold going down as mud. The gold deposited is at
first transparent to transmitted light and is deeply blue. I thought
this might be due to a trace of copper or silver, but on carefully
purifying the gold no change of colour was noted. If the reducing
solution is present in slightly greater proportions than that given in
the formula, the gold comes down with a richer colour, and has a
tendency to form a mat surface and to separate from the glass. The
gold which is deposited more slowly has a less rich colour but a
brighter surface. The operation should be interrupted when a
sufficient deposit has been obtained, because it is found that the
thicker the deposit, the more lightly is it held to the glass surface.
§ 75. The Use of the Diamond-cutting Wheel.
A matter which is not very well known outside geological circles is
the manipulation of the diamond-cutting wheel, and as this is often of
great use in the physical laboratory, the following notes may not be
out of place. I first became acquainted with the art in connection
with the necessity which arose for me to make galvanometer mirrors out
of fused quartz, and it was then that I discovered with surprise how
difficult it is to obtain information on the point. I desire to
express my indebtedness to my colleagues, Professor David and Mr.
Smeeth, for the instruction they have given me. In what follows I
propose to describe their practice rather than my own, which has been
of a makeshift description. I will therefore select the process of
cutting a slice of rock for microscopical investigation.
§ 76. Arming a Wheel.
Fig. 63.
A convenient wheel is made out of tin-plate, i.e. mild steel sheet,
about one-thirtieth of an inch thick and seven inches in diameter.
This wheel must be quite flat and true, as well as round; too much
pains cannot be taken in securing these qualities. After the wheel is
mounted, it is better to turn it quite true by means of a
watch-maker's "graver" or other suitable tool. The general design of
a rock-cutting machine will be clear from the illustration (Fig. 63).
The wheel being set up correctly, the next step is to arm it with
diamond dust. For this purpose it is before all things necessary that
real diamond dust should be obtained. The best plan is to procure a
bit of "bort" which has been used in a diamond drill, and whose
properties have therefore been tested to some extent. This is ground
in a diamond mortar--or rather hammered in one--and passed through a
sieve having at least 80 threads to the inch. The dust may be
conveniently kept in oil.
To arm the wheel, a little dust and oil is taken on the finger, and
laid on round the periphery of the wheel. A bit of flint or agate is
then held firmly against the edge of the wheel and the latter is
rotated two or three times by hand. The rotation must be quite
slow--say one turn in half a minute--and the flint must be held firmly
and steadily against the wheel. Some operators prefer to hammer the
diamond dust into the wheel with a lump of flint, or agate, but there
is a risk of deforming the wheel in the process. When a new wheel is
set up, it may be necessary to repeat the above process once every
half hour or so till the cutting is satisfactory, but when once a
wheel is well armed it will work for a long time without further
attention.
§ 77. Cutting a Section.
A wheel 7 inches in diameter may be rotated about 500 times per
minute, and will give good results at that speed. The work, as will
be seen from the diagram, is pressed against the edge of the wheel by
a force, which in the case quoted was about the weight of eleven
ounces. This was distributed along a cutting arc of three-quarters of
an inch.
A convenient cutting lubricant is a solution of Castile soap in water,
and this must be freely supplied; if the wheel gets dry it is almost
immediately spoiled owing to the diamond dust being scraped off. In
the figure the lubricant is supplied by a wick running into the
reservoir. I have used both clock oil and ordinary gas-engine oil as
lubricants, with equally satisfactory results. As to the speed of
cutting, in the experiment quoted a bit of rather friable "gabbro,"
measuring three-quarters of an inch on the face by five-eighths of an
inch thick, was cut clean through in six minutes, or by 3000 turns of
the wheel. The travel of the edge was thus between 5000 and 6000
feet, or say 9000 feet, nearly 2 miles, per inch cut.
A good solid rock, like basalt, can be cut into slices of about 3/32
inch thick. A very loose rock is best boiled in Canada balsam, hard
enough to set, before it is put against the wheel.
Instead of a grinding machine a lathe may be employed. The disc is,
of course, mounted on the mandrel, and the work on the slide-rest.
The latter must be disconnected from its feed screws, and a weight
arranged over a pulley so as to keep the work pressed against the
wheel by a constant force.
It may, perhaps, occur to the reader to inquire whether any clearance
in the cut is necessary. The answer is that in all probability, and
in spite of every care, the wheel will wobble enough to give
clearance. If it does not, a little diamond dust rubbed into the side
of the wheel, as well as the edge, will do all that is required. The
edge also, after two or three armings, "burrs" a little, and thus
provides a clearance naturally. It is not unlikely that in the near
future the electric furnace will furnish us with a number of products
capable of replacing the diamond as abrading agents. The cost of the
small amount of diamond dust; required in a laboratory is so small,
however, that it; is doubtful whether any appreciable economy will be,
effected.
§ 78. Grinding Rock Sections, or Thin Slips of any Hard Material.
A note on this is, perhaps, worth making, for the same reasons as were
given for note, § 75, which it naturally follows. Just as
trout-fishing; is described by Mr. Francis as the "art of fine and far
off," [Footnote: In the Badminton Library, volume on Fishing.]
section grinding may be called "the art of Canada balsam cooking," as
follows. A section of rock having been cut from the lump as just
described, it becomes; necessary to grind it down for purposes of
microscopical investigation. For this purpose it is placed on a slip
of glass, and cemented in position by Canada, balsam. Success in the
operation of grinding the mounted section depends almost entirely on
the way in which the mounting is done, and this in its turn depends on
the condition to which the Canada has been brought.
To illustrate the operations, I will describe a specific case, viz.
that of grinding the section of "gabbro"' above described, for
microscopical purposes. One side of the section is probably
sufficiently smooth and plane from the operation of the diamond wheel;
if not, it must be ground by the finger on a slab of iron or gun-metal
with emery and water, the emery passing a sieve of 80 threads to the
inch. The glass base on which the section is to be mounted for
grinding is placed on a bit of iron or copper plate over a Bunsen
burner, and three or four drops of natural Canada balsam are placed
upon it. The section is placed on the plate to heat at the same time.
The temperature must not rise so high as to cause any visible change
in the Canada balsam, except a slight formation of bubbles, which rise
to the surface, and can be blown off. The heating may require to be
continued, say, up to twenty minutes. The progress of the operation
is tested by examining the balsam as to its viscous properties.
An exceedingly simple and accurate way of testing is to dip a pair of
ordinary forceps in the balsam, which may be stirred a little to
secure uniformity. The forceps are introduced with the jaws in
contact, and, as soon as withdrawn, the jaws are allowed to spring
apart, thus drawing out a balsam thread. In a few moments the thread
is cold, and if the forceps be compressed, this thread will bend.
The Canada must be heated until it is just in such a state that on
bringing the jaws together the thread breaks. The forceps may open to
about three-quarters of an inch. If the Canada is more viscous, so
that the thread does not break, the section when cemented by it will
most probably slip on the slide. On the other hand, if the balsam is
more brittle, it will crumble away during the grinding.
Assuming that the proper point has been reached, the section is
mounted with the usual precautions to avoid air bubbles, i.e. by
dropping one edge on the balsam first. When all is cold, the surface
of the section may be ground on an iron plate with emery passing the
80 sieve, till it is about 1/40 inch thick. From this point it must
be reduced on ground glass by flours of emery and water; the rough
particles of the former may be washed out for fine work.
The process of grinding should not take more than half an hour if the
section is properly cut, etc. Beyond this point the allowable
thickness must depend on the nature of the rock; a good general rule
is to get the section just so thin that felspars show the yellow of
the first order in a polarising, microscope. The section is then
finished with, say, two minutes emery or water of Ayr-stone dust. It
is better not to have the surface too smooth.
To transfer the section, the hard Canada round the sides is scraped
away, and the section itself covered with some fresh Canada from the
bottle. It is then warmed till it will slip off when a pin, or the
invaluable dentist's chisel, is pressed against one side. If the
section be very delicate, the cover slip should be placed over it
before it is moved to the proper slide. The Canada used for mounting
is not quite so hard as that employed in grinding, but it should be
hard when cold, i.e. not sticky.
The art of preparing Canada balsam appears to consist in heating it
under such conditions as will ensure its being exposed in thin layers.
I have wasted a good deal of time in trying to bake Canada in
evaporating basins, with the invariable result that it was either over
or under-baked, and got dark in colour during the process.
On reviewing the process of rock section-cutting and mounting as just
described, I cannot help thinking that, if properly systematised, it
could be made much more rapid by the introduction of proper automatic
grinding machinery. It also seems not improbable that a proper
overhaul of available gums and cements would be found to lead to a
cementing material less troublesome than Canada balsam.
§ 79. Cutting Sections of Soft Substances.
Though this art is fully treated of in books on practical biology, it
is occasionally of use to the physicist, and the following note treats
of that part of the subject which is not distinctly biological.
Soft materials, of which thin sections may be required, generally
require to be strengthened before they are cut. For this purpose a
variety of materials are available. The one most generally used is
hard paraffin. The only point requiring attention is the embedding.
The material must be dry.
This is accomplished by soaking in absolute alcohol, i.e. really
absolute alcohol made by shaking up rectified spirit with potassium
carbonate, previously dried, and then digesting for a day with large
excess of quick-lime, making use of an inverted condenser and finally
distilling off the alcohol without allowing it to come in contact with
undried air. After soaking for some time in absolute alcohol, the
material may be transferred to oil of bergamot, or oil of cloves, or
almost any essential oil. After soaking in this long enough to allow
the alcohol to diffuse out, the material may be lifted into a bath of
melted paraffin (melting at, say, 51° C.). The process of soaking is
in some cases made to go more rapidly by exhausting, and, if the
material will stand it, by raising the temperature over 100° C. The
soaking process may require minutes, hours, or days, according to the
size and density of the material; but a few hours are usually
sufficient.
When cold, the sections may be cut in any of the ordinary forms of
microtome.
Another way of embedding is to soak in collodion, and then precipitate
the latter in the material and around it by plunging into nearly
absolute alcohol. The collodion yields a harder matrix than the
paraffin.
Whatever form of cutting machine is employed, the art of sharpening
the knife is the only one requiring any particular notice. The
easiest way of obtaining a knife hard enough to sharpen, is to use a
razor of good quality. If it has to be ground, it is best to do this
on a fine Turkey stone which is conveniently rested on two bits of
rubber tubing, to avoid jarring the blade. Many stones are slightly
cracked, but on no account must the razor be dragged across a crack,
or the edge will suffer.
The necessary and sufficient condition is that the razor must be
worked in little sweeps over the stone, and pressed against the latter
by little more than its own weight, and the grinding must be regular.
The edge may be inspected under a microscope, and it must be perfectly
smooth and even before it will cut sections. A finishing touch may be
given on a leather strap, but it must be done skilfully, otherwise it
is better omitted.
The necessity for providing exceptionally keen and sharp edges arose
in the manufacture of phonographs, where the knife used to turn up the
wax cylinders must leave a perfectly smooth surface. In 1889 this was
being accomplished on an ivory lap fed with a trace of very fine
diamond dust.
I have had this method in mind as a possible solution of the
difficulty of razor-grinding, but have not tried it. I imagine one
would use a soft steel or ivory slip rubbed over with fine diamond
dust and oil by means of an agate. The lap used in the phonograph
works was rotated at a high speed.
§ 80. On the Production of Quartz Threads.
[Footnote: Since this was written an article on the same subject by
Mr. Boys appeared in the Electrician for 1896. The instructions
therein given are in accordance with what I had written, and I have
made no alteration in the text.]
In 1887 the important properties of fused quartz were discovered by
Mr. Vernon Boys (Philosophical Magazine, June 1887, p. 489, "On the
Production, Properties, and Some Suggested Uses of the Finest
Threads"). A detailed study of the properties of quartz threads was
made by Mr. Boys and communicated to the Society of Arts in 1889
(Journal of the Society of Arts, 1889). An independent study of the
subject was made by the present writer in 1889 (Philosophical
Magazine, July 1890, "On the Elastic Constants of Quartz Threads ").
There is also a paper in the Philosophical Magazine for 1894 (vol.
xxxvii. p. 463), by Mr. Boys, on "The Attachment of Quartz Fibres."
This paper also appeared in the Journal of the Physical Society at
about the same date, together with an interesting discussion of the
matter. In the American Journal, Electric Power, for 1894, there is a
series of articles by Professor Nichols on "Galvanometers," in which a
particular method of producing quartz threads is recommended. The
method was originally discovered by Mr. Boys, but he seems to have
made no use of it. A hunt through French and German literature on the
subject has disclosed nothing of interest--nothing indeed which
cannot be found in the papers mentioned.
§ 81. Quartz fibres have two great advantages over other forms of
suspension when employed for any kind of torsion balance, from an
ordinary more or less "astatic" galvanometer to the Cavendish
apparatus. In the first place the actual strength of the fibres under
longitudinal stress is remarkably high, ranging from fifty to seventy
tons weight per square inch of section, and even more than this in the
case of very fine threads; the second and more important point in
favour of quartz depends on the wide limits within which cylindrical
threads of this material obey the simplest possible law of torsion,
i.e. the law that for a given thread carrying a given weight at a
given temperature and having one end clamped, the twist about the axis
of figure produced by a turning moment applied at the free end is
proportional simply to the moment of the twisting forces, and is
independent of the previous history of the thread.
It is to be noted, however, that the torsional resilience of quartz as
tested by the above law is not so perfect but that our instrumental
means allow us to detect its imperfections, and thus to satisfy
ourselves that threads made of quartz are not things standing apart
from all other materials, except in the sense that the limits within
which they may be twisted without deviating in their behaviour from
the law of strict proportionality by more than some unassigned small
quantity, are phenomenally wide.
A torsion balance--if we except the case of certain spiral
springs--is almost always called upon for information as to the magnitude
of very small forces, and for this purpose it is not essential merely
that some law of twisting should be exactly obeyed, but also that the
resistance to twisting of the suspension should be small.
Now, regarded merely as a substance possessing elastic rigidity,
quartz is markedly inferior to the majority of materials, for it is
very stiff indeed; its utility depends as much as anything upon its
great strength, for this allows us to, use threads of exceeding
fineness. In addition to this it must be possible, and moreover
readily possible, to obtain threads of uniform section over a
sufficient length, or the rate of twist per unit length of the thread
will vary in practice from point to point, so that the limits of
allowable twist averaged over the whole thread may not be exceeded,
and yet they may be greatly overpassed at particular points of the
thread.
It is interesting to note that in the case of quartz we not only have
a means for readily producing very uniform cylindrical threads, but
that the limits of allowable rate of twist are so wide that a small
departure from uniformity of section produces much less inconvenience
than in the case of any other known substance.
§ 82. There are three methods generally in use for drawing quartz
fibres, all depending on the fact that quartz when fused is so viscous
that it may be drawn into threads of great length, without these
threads breaking up into drops, or indeed without their showing any
sign of doing so. The surface tension of the melted quartz must,
however, be very considerable, as may be seen by examining the shape
of a drop of the molten material, and this suffices to impress a
rigidly cylindrical form upon the thread, the great viscosity
apparently damping down all oscillation.
The first method is the one originally employed by Mr. Boys. A needle
of quartz is melted somewhere in its length and is then drawn out
rapidly by a light arrow, to which one end of the needle is attached,
and which is projected from a kind of crossbow.
A modification of this method, which the writer has found of service
when very thick threads are required, is to replace the bow and arrow
by a kind of catapult.
The third method, which yields threads of almost unmanageable
fineness, depends on the experimental fact that when a fine point of
quartz is held in a high pressure oxygen gas blow-pipe flame, the
friction of the flame gases suffices to overcome the tendency of the
capillary forces to produce a spherical drop, and actually causes a
fine thread to be projected outwards in the direction of the flame.
§ 83. A preliminary operation to any method is the production of a
stick of fused quartz. This is managed as follows. A rock crystal or
quartz pebble is selected and examined. It must be perfectly white,
transparent, and free from dirt. Surface impurity can of course be
got rid of by means of a grindstone. The crystal is placed in a
perfectly clean Stourbridge clay crucible, furnished with a cover, and
heated to bright redness for about an hour in a clean fire or in a
Fletcher's gas furnace. The contents of the crucible are turned out
when sufficiently cool on to a clean brick or bit of slate. It will
be found that the crystal is completely broken up and the fragments
must be examined in case any of them have become contaminated by the
crucible, but this will not have happened if the temperature did not
rise beyond a bright red heat.
The heap of fragments being found satisfactory, the next thing is to
fuse some of the pieces together. Unless the preliminary heating has
been efficiently carried out this will prove an annoying task, because
a rock crystal generally contains so much water that it splinters
under the blow-pipe in a very persistent manner. There are two ways
of assembling the fragments. One is to place two tiles or bricks on
edge about the heap of quartz lying upon a third tile, so that the
heap occupies the angular corner or nook formed by the tiles (Fig.
64).
The oxygas blow-pipe previously described is adjusted to give its
hottest flame, the bags being weighted by at least two hundredweight,
if of the size described (see § 15).
The tip of the inner cone of the blow-pipe is brought to bear directly
upon one of the fragments, and if the operation is performed boldly it
will be found that the surface of the fragment can be fused, and the
fragment thus caused to hold together before the lower side gets hot
enough to suffer any contamination from the tile or brick. A second
fragment may be treated in the same way, and then a third, and so on.
Finally, the fragments may be fused together slightly at the corners,
and a stick may thus be formed. Of course a good deal of cracking and
splitting of the fragments takes place in the process; the best
pieces to operate upon are those which are well cracked to begin with,
and that in such a way that the little fragments are interlocked.
An alternative method which has some advantages is to arm a pair of
forceps with two stout platinum jaws, say an inch and a half long, and
flattened a little at the ends. The fragments are held in these
platinum forceps and the blow-pipe applied as before. This method
works very well in adding to a rod which has already been partly
formed, but the jaws require constant renewals. The first fragment
which is fused sufficiently to cohere may also be fused to a bit of
tobacco pipe, or hard glass tube or rod, and the quartz stick
gradually built up by fusing fresh pieces on to the one already in
position.
Fig. 64.
Since the glass or pipeclay will contaminate the quartz which has been
fused on to it, it is necessary to discard the end pieces at the close
of the operation. A string of fragments having been collected and
stuck together, the next step is to fuse them down into a uniform rod.
This is easily done by holding the string in the blow-pipe flame and
allowing it to fuse down. Twisting the fused part has a good effect
in assisting the operation. It is desirable to use a large jet and as
powerful a flame as can be obtained during this part of the operation.
The final result should be a rod, say two or three inches long and
one-eighth of an inch thick, which will in most cases contain a large
number of air bubbles. Since the presence of drawn-out bubbles cannot
be advantageous, it is often desirable to get rid of them, and this
can conveniently be done at the present stage. The process at best
is rather tedious; it consists in drawing the quartz down very fine
before an intense flame, in order to allow the bubbles to get close
enough to the surface to burst. A considerable loss of material
invariably occurs during the process; for whenever the thin rod
separates into two bits the process of flame-drawing of threads goes
on, and entails a certain waste; moreover, the quartz in fine
filaments is probably partially volatilised.
Sooner or later, however, a sufficient length of bubble-free quartz
can be obtained. It must not be supposed that it is always necessary
to eliminate bubbles as perfectly as is contemplated in the foregoing
description of the treatment, but for special purposes it may be
essential to do so, and in any case the reader's attention is directed
to a possible source of error.
It may be mentioned in connection with this matter that crystals of
quartz may look perfectly white and clear, and yet contain impurity.
For instance, traces of sodium are generally present, and lithium was
found in large spectroscopic quantity in five out of six samples of
the purest crystals in my laboratory. The presence of lithium in rock
crystal has also been detected by Tegetmeier (Vied. Ann, xli. p.
19, 1890).
After some practice in preparing rods and freeing them of bubbles the
operator will notice a distinct difference in the fusibility of the
samples of quartz he investigates, though all may appear equally pure
to the unaided eye. It should be mentioned, however, that high
infusibility cannot always be taken as a test of purity, for the most
infusible, or rather most viscous, sample examined by the writer
contained more lithium than some less viscous samples.
Fig. 65.
During the process of freeing the quartz from bubbles the lithium and
sodium will be found to burn away, or at all events to disappear.
A rod of quartz, say three inches long, one-sixteenth of an inch in
diameter, and free from bubbles for half an inch of its length, even
when examined by a strong lens, is suitable for drawing into threads.
The rod is manipulated exactly in the manner described under
glass-blowing, and is finally drawn down at the bubble free part into
a needle, say 0.02 inch in diameter (No. 25 on the Birmingham wire
gauge), and 2 inches long.
Fig. 66.
There is one peculiarity about fused quartz which renders its
manipulation easier than that of glass--it is impossible to break
fused quartz, however suddenly it be thrust into the blow-pipe flame.
A rod having a diameter of three-sixteenths of an inch--and perhaps
much more--may be brought right up to the tip of the inner cone of
the oxy-gas flame and held there-till one side fuses, the other being
comparatively cool, without the slightest fear of precipitating a
smash. In seven years' experience I have never seen a bit of once
fused quartz broken by sudden heating; whether it might be done if
sufficient precautions were taken I do not know.
The reason of the fortunate peculiarity of quartz in this respect is,
I presume, to be found in the fact that quartz once it has been fused
is really a very strong material indeed, and is also probably the
least expansible substance known. From some experiments of the writer
upon the subject, it may be concluded that at the most quartz which
has been fused expands only about one-fifth as fast as flint-glass, at
all events between 20° and 70° C.
§ 84. Drawing Quartz Threads.
The thick end of the rod of quartz is held in the fingers or
occasionally in a clip. The end of the fine point is attached to a
straw arrow by means of a little sealing-wax. The arrow is laid on
the stock of a crossbow in the proper position for firing. See
Figs. 67 and 68, which practically explain themselves.
The needle is heated by the blow-pipe till a minute length is in a
state of uniform fusion; the arrow is then let fly, when it draws a
thread out with it. The arrow is preferably allowed to strike a
wooden target placed, say, 30 feet away from the bow, and a width of
black glazed calico is laid under the line of fire to catch the thread
or arrow if it falls short. The general arrangements will be obvious
from the figure.
The bow is of pine in the case where very long thin threads are
required, though for ordinary purposes I have found a bow of
lance-wood succeed quite as well. The trigger of the bow consists of
a simple pin passing through the stock and fastened at its lower end
to a string connected with a board which can be depressed by foot. In
the figure an ordinary trigger is shown, but the pin does just as
well.
Fig 67.
The arrow is made out of about 6 inches of straw, plugged up aft by a
small plug of pine or willow fastened in with sealing-wax, and
projecting backwards one-eighth of an inch. This projection serves a
double purpose: it gives a point of attachment for the quartz needle,
and on firing the bow it forms a resisting anvil on which the string
of the bow impinges. The head of the arrow is formed by a large
needle stuck in with sealing-wax, and heavy enough to bring the centre
of gravity of the arrow forward of one-third of its length, the
condition of stability in flight.
Fig. 68.
It is not necessary to employ any feathering for these arrows; though
I have occasionally used feathers or mica to "wing the shaft" no
advantage has resulted therefrom.
To get fine threads a high velocity is essential. This is obtained by
considering (and acting upon) the principles involved. The bow may be
regarded as a doubly-tapering rod clamped at the middle. After
deflection it returns towards its equilibrium position at a rate
depending in general terms on the elastic forces brought into play,
directly, and on the effective moment of inertia of the rod, inversely
(see Rayleigh, Sound, vol. ii. chap. viii.) If the mass of the
arrow is negligible compared with the bow, the rate at which the arrow
moves is practically determined by that attained by the end of the
bow, which is a maximum in crossing its equilibrium position.
The extent to which the arrow profits by this velocity depends on the
way the bow is strung. It will be greatest when the string is
perpendicular to the bow when passing its equilibrium position; or in
other words, when the string is infinitely long. Since the string has
mass, however, it is not permissible to make it too long, or its
weight begins to make itself felt, and a point is soon reached at
which the geometrical gain in string velocity is compensated for by
the total loss of velocity due to the inertia of the string. In
practice it is sufficient to use a string 10 per cent longer than the
bow.
It is well to use a light fiddle string, served with waxed silk at the
trigger catch; if this be omitted the gut gets worn through very
quickly. In order to decide how far it is permissible to bend the
bow, the quickest way is to make a rough experiment on a bit of the
same plank from which the bow is to be cut, and then to allow a small
factor of safety. In the figure the bow is of lance-wood and is more
bent than would be suitable for pine.
The bow itself is tapered from the middle outwards just like any other
bow. If thick threads are required, the above considerations are
modified by the fact that quartz opposes a considerable resistance to
drawing, and that consequently the arrow must not only have a high
velocity, but a fair supply of energy as well; in other words, it
must be heavy. A thin pine arrow instead of a straw generally does
very well, but in this case the advantage of using pine for the bow
vanishes; and in fact lance-wood does better, owing to the greater
displacement which it will stand without breaking. This of course
only means that a greater store of energy can be accumulated at one
bending.
I had occasion to investigate whether the unavoidable spin of an arrow
about its axis produces any effect on the thread, and for this purpose
made arrows with inertia bars thrust through the head, i.e. an arrow
with a bit of wire run through it, perpendicular to its
length--forming a cross in fact--the arms of the cross being weighted
at the extreme ends by shot. This form of arrow has a considerable
moment of inertia about its longer axis, and consequently rotates less
than a mere straw, provided that the couples tending to produce rotation
are not increased by the cross arm, or the velocity too much reduced.
Shooting one of these arrows slowly, I could see that it did not
rotate, and when fired at a high velocity, it generally arrived at the
target (placed at varying distances front bow) with the arms nearly
horizontal, thus showing that it probably did not rotate much.
I did not succeed in this at the first trial, by any means. The
threads got in this way were no better than those made with a single
straw, whence we may conclude very provisionally that the spin of the
arrow has only a small effect, if any, on the quality of the threads.
Feathering the arrow, in my experience, tends, if anything to make it
spin more; for one thing, because it is practically impossible to lay
the feathering on straight.
After the arrow is shot, it remains to gather in the thread, and if
the latter is at all thin, we have a rather troublesome job. In a
thread thirty or forty feet long, the most uniform part generally lies
in the middle if the thread is thin, i.e. of the order of a
ten-thousandth of an inch in diameter. If the thread is thick the
most uniform part may be anywhere. The part of the thread required is
generally best isolated by passing a slip of paper under it at each
end and cementing the thread to the paper by means of a little
paraffin or soft wax, and then cutting off the outer portions. One
bit of paper may then be lifted off the calico, and the thread will
carry the other bit. In this way the thread may be taken to a
blackened board, where it may be mounted for stock.
By passing the two ends of the thread under a microscope, or rather by
breaking bits off the two ends and examining them together, it is easy
to form an Opinion as to uniformity.
Mr. Boys has employed an optical method of examining threads, but the
writer has invariably found a high-power microscope more convenient
and capable of giving more exact information as to the diameter of the
threads.
The beginner--or indeed the practised hand--need not expect to get a
thread of the exact dimensions required at the first shot. A little
experience is necessary to enable one to judge of the right thickness
of the needle for a thread of given diameter. The threads are so
easily shot, however, that a few trials take up very little time and
generally afford quite sufficient experience to enable a thread of any
required diameter to be prepared.
It is no use attempting to heat an appreciable length of needle; if
this be done the thread almost invariably has a thick part about the
middle of its length.. It is sufficient to fuse at most about
one-twentieth of an inch along the needle before firing off the bow.
This can be done by means of the smaller oxygas blow-pipe jet
described in the article on blow-pipes for glass-blowing, § 14. The
flame must of course be turned down so as to be of a suitable size. A
sufficiently small flame may be got from almost any jet.
If the needle be not equally heated all round, the thread tends to be
curly; indeed by means of the catapult, threads may be pulled which,
when broken, tend to coil up like the balance-springs of watches, if
only care be taken to have one side of the needle much hotter than the
other.
§ 85. When examining bits of threads, say thicker than the
two-thousandth of an inch, under the microscope it is convenient to
use a film of glycerine stained with some kind of dye, in order to
render the thread more sharply visible. The thread is mounted beneath
a cover slip, and a drop of the stained glycerine allowed to run in.
Such a treatment gives the image of the thread a sharply defined edge
3 and the contrast between the whiteness of the thread and the colour
of the background allows measurements to be made with great ease.
On the whole the easiest way of measuring the diameter of a thick
thread is to use a measuring microscope, i.e. one in which the lens
system can be displaced along a plane bed by means of a finely cut
micrometer screw. The instruments made by the Cambridge Scientific
Instrument Company do fairly well. Direct measurements up to 0.0001
inch are easily made by means of a microscope provided with a Zeiss
"A" objective, and rather smaller differences of thickness can be made
out by it. For thin threads the method next to be described is more
fitting, because higher powers can be more conveniently used.
In this method an ordinary microscope is employed together with a
scale micrometer, and either an eyepiece micrometer, or a camera and
subsidiary scale. The eyepiece micrometer is the more convenient. If
a camera be employed, i.e. such an one as is supplied by Zeiss, it is
astonishing how the accuracy of observation may be increased by
attending carefully to the illumination of both the subsidiary scale
and of the thread. The two images should be as far as possible of
equal brightness, and for this purpose it will be found requisite to
employ small screens.
The detail of making a measurement by means of the micrometer eyepiece
is very simple. The thread is arranged on the stage so as to point
towards the observer, and the apparent diameter is read off on the
eyepiece scale. In order to calibrate the latter it is only necessary
to replace the thread by the stage micrometer, and to observe the
number of stage micrometer divisions occupying the space in the
eyepiece micrometer formerly occupied by the thread. It is essential
that both thread and stage micrometer should occupy the same position
in the field, for errors due to unequal distortion may otherwise
become of importance. For this reason it is best to utilise the
centre of the field only.
The same remark applies to measurements by means of the camera, where
the image of the thread is projected against the reflected image of
the subsidiary scale laid alongside the microscope. In this case the
value of the subsidiary scale divisions must be obtained from the
divisions of the stage micrometer, coinciding as nearly as possible
with the position occupied by the thread. Before commencing a
measurement the screens are moved about till both images appear
equally bright.
Threads up to about one twenty-thousandth of an inch in diameter may
be sufficiently well measured by means of a Zeiss "4 centimetre
apochromatic object-glass" and an eyepiece "No. 6" with sixteen
centimetre tube length. [Footnote: The objective certainly had "4 cm."
marked on it, but the focal length appeared to be about I.5 mm. only.]
§ 86. Drawing Threads by the Catapult.
The bow-and-arrow method fails when threads of a greater diameter than
about 0.0015 inch are required--at least if any reasonable uniformity
be demanded, and no radical change in the bow and arrow be carried
out.
Thus in the writer's laboratory a thread of about this diameter,
within 1/10000 of an inch-13 inches long and free from air
bubbles--was required. A fortnight's work by a most skilful operator
only resulted in the production of two lengths satisfying the
conditions.
The greatest loss of time occurs in the examination of the thread by
means of the microscope.
Threads for galvanometer suspensions are conveniently from 0.0001 to
0.0004 inch in diameter, and are much more easily made and got uniform
than thicker threads, to the production of which the catapult method
applies.
A reference to the diagram will make the construction of the
instrument quite clear. The moving end of the quartz is attached to a
small boxwood slider working on a tubular girder or between wires.
The quartz is secured in position by clamps shown at A and B, and
motion is imparted to the slider by a stretched piece of catapult
elastic (C). An easy means of regulating the pull of the elastic is
to hold it back by a loop of string whose length can be varied by
twisting it round a pin.
Fig. 69. [Footnote: For greater clearness of drawing, the tube
carrying the slider is shown somewhat higher above the base than is
convenient in practice; and the slide itself is shown too thin in the
direction of the hole through it.]
Since it is not permissible to allow the slider to rebound at the end
of its journey, some such arrangement of breaks as is shown must be
adopted. In the diagram the bottom of the slider runs on to a brass
spring between the girder and the base of the appliance, and so gets
jammed; the spiral spring acts merely as an additional guard. The
diagram does not show the lower spring very clearly; it is a mere
strip lying in the groove.
A rod of quartz, with a needle at one end, is prepared as before and
secured in the clamps. During the operation of fastening down the
clamps, there is some danger of breaking the needle, and consequently
it is advisable to soften the latter before and while adjusting the
second clamp.
The process of drawing a thread by this method is exactly similar to
the operation already described in connection with the arrow method.
Though short thick threads form the product generally obtained from
the catapult, it must not be supposed that thin threads cannot be
obtained in this way. If a short length of a very fine needle be
heated, it will be found to yield threads quite fine enough for
ordinary suspension purposes, but naturally not so uniform as those
obtained from the 40-foot lengths obtainable by the bow-and-arrow
method.
It is easy to make spiral quartz springs resembling watch
balance-springs by means of the catapult. All that is necessary is to
see that the quartz is rather unequally heated before the shot is
fired. In the future it is by no means impossible that such springs
may have a real value, for the rigidity of quartz is known to increase
as temperature rises. Hence it is probable that the springs would
become stiffer as temperature rises, even though they work chiefly by
bending, and little or not at all by twisting. As this is the kind of
temperature variation required to compensate an uncompensated watch
balance wheel, it may turn out to have some value.
§ 87. Drawing Threads by the Flame alone.
A stick of quartz is drawn down to a fine point, and the tip of this
point is held in the blow-pipe flame in the position shown in Fig.
70.
Fig. 70.
The friction of the flame gases is found to be sufficient to carry
forward the fused quartz and to draw it into threads in spite of the
influence of the capillary forces. If a sheet of paper be suspended
at a distance of two or three feet in front of the blow-pipe flame, it
will be found to be covered with fine threads tangled together into a
cobwebby mass. As this method is an exceedingly simple one of
obtaining threads, I have endeavoured to reduce it to a systematic
operation.
A sheet of cardboard, about two feet square, is painted dead black and
suspended horizontally, painted side downwards (Fig. 70, A), at a
height of about two feet above the blow-pipe flame. The latter is
adjusted so as to point almost vertically upwards and towards the
centre of the cardboard. A few half-inch pins are thrust through the
card from the upper surface and pushed home; about one dozen pins
scattered over the surface will be sufficient. Their object is to
prevent the threads being carried away round the edge of the screen.
The flame from the jet described so often is fed from gas bags
weighted to about eighty pounds per square foot of (one) surface,
i.e. "4-foot" bags require from three to four hundredweight to give
an advantageous pressure. [Footnote: The resulting threads were really
too fine for convenient manipulation, so that unless extremely fine
threads are required it will be better to reduce the pressure of the
gases considerably.]
Two sticks of quartz are introduced and caused to meet just in front
of the inner cone--the hottest part of the flame. They are then
drawn apart so as to form a fine neck, which softens and is bent in
the direction of motion of the flame gases. When fusion is complete
the neck separates into two parts, and a thread is drawn from each of
them. By alternately lightly touching the rods together, and drawing
them apart, quite a mass of threads may be obtained in two or three
minutes, when the process should be stopped. If too many threads get
entangled in the pins, one gives one's self the unnecessary trouble of
separating them. On taking down the card it will be found that the
threads have been caught by the pins; but the card now being laid
black side upwards, the former easily slip off the points.
Threads at least a foot long, and perhaps vastly longer, may be
obtained by this method, and are extraordinarily fine. When I first
read Professor Nichols' statement (Electric Power, 1894) as to the
value of these fibres for galvanometer purposes, I was rather
sceptical on the ground that the threads would tend to get annealed by
being drawn gradually, instead of suddenly, from a place of intense
heat to regions of lower temperature.
Now annealing threads by a Bunsen makes them rotten. The threads
being immersed in the hot flame gases could only cool at the same rate
as the gas, and it was not--and is not--clear to me that annealing
of the threads can be avoided. On the other hand, it may be possible
that a thread cooled slowly from the first does not suffer in the same
way as a cold thread would do when annealed in a Bunsen flame.
Again the velocity of the gases is beyond doubt exceedingly high, so
that the annealing, even supposing it to be deleterious, might not be
carried very far. Threads drawn by this method and measured "dry,"
i.e. by mounting them on a slide without the addition of any liquid,
turned out to have a diameter of about 1/20000 of an inch.
I do not think I could manage to mount such fine threads without very
special trouble. All the threads lying on the board, however, were
found in reality to consist of three or four separate threads, and
there is no reason why several threads should not be mounted in
parallel, provided, of course, that they are equally stretched and
touching each other. Equality of tension in the mounting could be
secured by making one attachment good, then cementing the other
attachment to the other end of the threads, and "drawing" the two
attachments slightly apart at the moment the cement commences to set.
This method may turn out to be very valuable, for, so far as I can
see, the carrying power would be increased without an increase of
torsional stiffness of anything like so high an order as would be the
case were one thread only employed. On the other hand, the law of
torsion could hardly be quite so simple, at all events, to the second
order of approximations.
§ 88. Properties of Threads.
A large number of experiments on the numerical values of the elastic
constants of quartz threads have been made by Mr. Boys and his
students, and by the writer. As the methods employed were quite
distinct and the results wholly independent, and yet in good agreement
with each other, a rounded average may be accepted with considerable
confidence.
TENACITY OF QUARTZ FIBRES (BOYS).
Diameter of Thread.
Tenacity in Tons' Weight per Square Inch of Section.
Tenacity in Dynes per Square Centimetre.
Inches
Centimetres
0.00069
0.00175
51.7
8 X 109
0.00019
0.00048
74.5
11.5 X 109
Rounded mean of Boys' and Threlfall's results:
Young's Modulus at 20° C,
5.6 X 1011 C.G.S.
Modulus of Simple Rigidity at 20° C,
2.65 X 1011 C.G.S.
Modulus of Incompressibility,
1.4 X 1011 C.G.S.
Modulus of Torsion,
3.7 X 1011 C.G.S.
Approximate coefficient of linear expansion of quartz per degree
between 80° C. and 30° C. is 0.0000017 (Threlfall = loc. cit.).
This must be regarded with some suspicion, as the data were not
concordant. There is no doubt, however, about the extreme
inexpansibility of quartz.
Temperature coefficient of modulus of torsional rigidity per degree
centigrade, 22° to 98° C, 0.000133
Ditto, absolute simple rigidity, 0.000128 (Threlfall).
Limit of allowable rate of twist in round numbers is, one-third turn
per centimetre, in a fibre 0.01 cm. diameter.
The limiting rate is probably roughly inversely as the diameter.
Attention must be called to the rapid increase in the torsional
rigidity of these threads as the temperature rises. A quartz spiral
spring-balance will be appreciably stronger in hot weather.
§ 89. In the majority of instances in which quartz threads are
applied in the laboratory, it is desirable to keep the coefficient of
torsion as small as possible, and hence threads are used as fine as
possible.
It is convenient to remember that a thread 0.0014 cm. or 0.0007 inch
in diameter breaks with a weight of about ten grammes, and may
conveniently be employed to carry, say, five grammes. With threads
three times finer the breaking strength per unit area increases, say,
50 per cent. In ordinary practice--galvanometric work for
instance--where it is desirable to use a thread as fine and short as
possible to sustain a weight up to, say, half a gramme, it will be
found that fibres five centimetres long or over give no trouble
through defect of elastic properties. A factor of safety of two is
a fair allowance when loading threads.
No difficulty will be experienced in mounting threads having a
diameter of 0.0002 inch or over. With finer threads it is necessary
to employ very dark backgrounds (Mr. Boys uses the darkness of a
slightly opened drawer), or the threads cannot be sufficiently well
seen.
In the case of instruments in which threads remain highly twisted for
long periods of time, the above rule as to the safe limit of twist
does not allow of a sufficient margin; it is only applicable to
galvanometric and similar purposes.
The cause of the increase in tenacity as the diameter diminishes is at
present unknown. It is due neither to an effect of annealing
(annealed threads are rotten), nor is it a skin effect, nor is it due
to the cooling of the thread under higher capillary pressure. It is,
however, possible that it may be associated with some kind of
permanent set taken by the fibres during the stage of passage from the
liquid to the solid state.
§ 90. On the Attachment of Quartz Fibres.
For many purposes it is sufficient to cement the fibres in position by
means of ordinary yellow shellac, but where very great accuracy is
aimed at, the shellac (being itself imperfectly elastic and exposed to
shearing stress) imposes its imperfections on the whole system. This
source of error can be got over by soldering the threads in position.
Attempts were made by the writer in this direction, with fair success,
in 1889, but as Mr. Boys has carried the art to a high degree of
perfection, I will suppress the description of my own method and
describe his in preference. It has, of course, been frequently
repeated in my laboratory.
In many cases, however, if not in all, it may be replaced by Margot
soldering, as already described, a note on the application of which to
this purpose will follow.
A thread of the proper diameter having been selected, it is cut to the
right length. With fine threads this is not always a perfectly easy
matter. The best way is for the operator to station himself facing a
good light, not sunlight, which is too tiring to the eye, but bright
diffused light. The thread will be furnished with bits of paper stuck
on with paraffin at both ends, as already described.
A rough sketch of the apparatus--or, at all events, two lines showing
the exact length which the free part of the thread must have--are
marked on a smooth board, and this is supported with its plane
vertical. The thread is held against the board, and the upper piece
of paper is stuck lightly to the board with a trace of soft wax, so
that the lower edge of the paper is at any desired height above the
upper mark. This distance is measured, and forms the length of thread
allowed to overlap the support. A second bit of paper is attached
below the lower mark, a margin for the attachment of the lower end
being measured and left as before. The thread will be most easily
seen if the board is painted a dead black.
If it is desired to attach the thread to its supports merely by
shellac, this is practically all that needs to be done. The supports
should resemble large pins. The upper support will be a brass wire in
most cases, and will require to be filed away as shown in the sketch
(Fig. 71). It is then coated with shellac by heating and rubbing
upon the shellac. As previously noted, the shellac must not be
overheated.
The thread is cut off below the lower slip of paper, and the upper
support being conveniently laid in a horizontal position on another
dead-black surface, the thread is carried to it and laid as designed
against the shellac, which is now cold. When the thread is in place,
a soldering iron is put against the brass wire, and the shellac
gradually melted till it closes over the thread.
Fig. 71.
The iron is then withdrawn and the thread pulled away from the point
for one-twentieth of an inch or less. This ensures that the thread
makes proper contact with the cement, and also that it is free from
kinks; of course, it must leave the cement in the proper direction. A
similar process is next carried out with respect to the lower
attachment, and the ends of the thread are neatly trimmed off.
Both ends of the thread being secured, the next step is to transfer
the upper support to a clip stand, the suspended parts being held by
hand, so that the weight comes on the thread very gradually. In this
way it will be easily seen whether the thread is bent where it enters
the shellac, and should this be the case, a hot iron must be brought
up to the shellac and the error rectified.
When both the support and the suspended parts are brought nearly to
the required bearing, the hot iron is held for a moment close up to
each attachment, the hand being held close below but not touching the
suspended parts, and both attachments are allowed to straighten
themselves out naturally.
These details may appear tiresome, and so they are when written out at
length, but the time occupied in carrying them out is very short, and
quartz threads break easily, unless the pull upon them is accurately
in the direction of their length at all points.
In the event of its being decided to attach the thread by soldering,
the process is rather more expensive in time, but not otherwise more
troublesome.
Fig. 72. Fig. 73.
The thread being cut as before to the proper length, little bits of
aluminium foil are smeared all over with melted shellac and suspended
from the thread replacing the paper slips before described. It is
important that no paraffin should be allowed to touch the thread
anywhere near a point intended to be soldered. The thread is hung up
from a clip stand by one of the bits of foil, and the lower end is
washed by dipping it into strong nitric acid for a moment and thence
into water. The object of smearing the foils all over with shellac is
to prevent them being acted upon by the acid. The threads are not
very easily washed acid free, but the process may be assisted by means
of a fine camel's-hair pencil.
Some silvering solution made as described (§ 65) is put into a test
tube; the thread, after rinsing with distilled water, is lowered into
the solution so far as is required, and is allowed to receive a
coating of silver. It has been observed that the coating of silver
must not be too thick--not sufficiently thick to be opaque. A watch
may be kept on the process by immersing a minute strip of mica
alongside the thread.
The silvered thread is rinsed with distilled water and allowed to dry.
Meanwhile the other end of the thread may be silvered. When both ends
are silvered the process of coppering by electro deposit is commenced.
A test tube is partially filled with a ten per cent solution of
sulphate of copper, and several copper wires are dipped into it to
form an anode. The thread is lowered carefully into the solution so
as not to introduce air bubbles, and the silvered part is allowed to
project far enough above the surface of the solution to come in
contact with a fine copper wire. The circuit is closed through a
Leclanché cell and a resistance box.
It is as well to begin with a fair resistance, say 100 ohms out in the
box, and the progress of the deposit is watched by means of a
low-power microscope set up in front of the thread. If the copper
appears to come down in a granular form, the resistance is too small
and must be increased; if no headway appears to be made, the
resistance must be diminished.
As soon as a fair coat of copper has come down, i.e. when the
diameter of the thread is about doubled, the process is interrupted.
The thread is withdrawn, washed, dipped in a solution of chloride of
zinc, and carefully tinned by dragging it over a small clean drop of
solder on a soldering bit.
During this part of the process the shellac is apt to get melted if
the iron is held too close, so that it is advisable to begin by making
the thread somewhat over long. The end of the thread must only be
trimmed off at the conclusion of the operation, i.e. after the thread
is soldered up. The thread is attached to the previously tinned
supports much in the same way as has been described under the head of
shellac attachments. It does not very much matter whether both ends
are coppered before one is soldered up or not. At the conclusion of
the whole process the superfluous copper and silver are dissolved off
by a little hot strong nitric acid applied on a glass hair pencil.
This is best done by holding the thread horizontally with the
assistance of clip stands.
If the thread is too delicate to bear brushing, the nitric acid may be
applied by pouring out a big drop into a bit of platinum foil and
holding this below the thread so as to touch it lightly. The
dissolving of the copper and silver is, of course, followed by copious
washing with hot water. This process is more laborious than might be
imagined, but it may be shortened by heating the platinum foil
supporting the water (Fig. 74).
Fig. 74
The washing part of the process is, in the opinion of the writer, the
most difficult part of the whole business, and it requires to be very
thorough, or the thread will end by drawing out of the solder. In
many cases it is better to try to do without any application of nitric
acid at all, but, of course, this involves silvering and coppering to
exact distances from the ends of the thread--at all events, in
apparatus where the effective length of the thread is narrowly
prescribed.
It is important not to leave the active parts of the thread
appreciably silvered, for the sake of avoiding zero changes due to the
imperfect elasticity of the silver. In this soldering process
ordinary tinman's solder may be employed; it must be applied very
free from dust or oxide.
§ 91. Other Modes of soldering Quartz.
Thick rods of quartz may be treated for attachment by solder in the
same way as glass was treated by Professor Kundt to get a foundation
for his electrolytically deposited prisms. [Footnote: See Appendix at
end of book.]
The application of a drop of a strong solution of platinum
tetrachloride to the rod will, on drying, give rise to a film of the
dry salt, and this may be reduced in the luminous gas flame. During
the process, however, the quartz is apt to get rotten, especially if
the temperature has been anything approaching a full red heat. The
resulting platinum deposit adheres very strongly to the quartz, and
may be soldered to as before. This method has been employed by the
writer with success since 1887, and may even be extended to thick
threads.
It was also found that fusible metal either stuck to or contracted
upon clean quartz so as to make a firm joint. In the light of M.
Margot's researches (already described), it occurred to me that
perhaps my experience was only a special case of the phenomena of
adhesion investigated with so much success by M. Margot. I therefore
tried whether the alloy of tin and zinc used for soldering aluminium
would stick to quartz, and instantly found that this was indeed the
case.
Adhesion between the alloy and perfectly clean quartz takes place
almost without rubbing. A rod of quartz thus "tinned" can be soldered
up to anything to which solder will stick, at once. On applying the
method to thick quartz threads, success was instantaneous (the threads
were some preserved for ordinary galvanometer suspensions); but when
the method was applied to very fine threads, great difficulty in
tinning the threads was experienced. The operation is best performed
by having the alloy on the end of an aluminium soldering bit, and
taking care that it is perfectly free from oxide before the thread is
drawn across it. There was no difficulty in soldering a thread
"tinned" in this manner to a copper wire with tinman's solder, and
the joint appeared perfect, the thread breaking finally at about an
inch away from the joint.
I allow Mr. Boys' method to stand as I have written it, simply because
I have not had time as yet to make thorough tests of the durability of
"Margot" joints on the finest threads; but I have practically no
doubt as to its perfect applicability, provided always that the solder
can be got clean enough when melted on the bit. Very fine threads
will require to be stretched before tinning, in order to enable them
to break through the capillary barrier of the surface of the melted
solder.
§ 92. Soldering.
It is almost unfair to the arts of the glass-blower or optician to
describe them side by side with the humble trade of soldering.
Nevertheless, no accomplishment of a mechanical kind is so serviceable
to the physicist as handiness with the soldering bit; and, as a rule,
there is no other exercise in which the average student shows such
lamentable incapacity. The following remarks on the subject are
therefore addressed to persons presumably quite ignorant of the way in
which soldering is carried out, and do not profess to be more than of
the most elementary character.
For laboratory purposes three kinds of solder are in general
sufficient. One is the ordinary tinman's solder composed of lead and
tin. The second is "spelter," or soft fusible brass, and the third is
an alloy of silver and brass called silver solder.
Tinman's solder is used for most purposes where high temperatures are
not required, or where the apparatus is intended to be temporary. The
"spelter," which is really only finely granulated fusible brass, is
used for brazing iron joints. The silver solder is convenient for
most purposes where permanency is required, and is especially suited
to the joining of small objects.
§ 93. Soft tinman's solder is made by melting together two parts of
grain tin and one of soft lead--the exact proportions are not of
consequence--but, on the other hand, the purer the constituents the
better the solder. Within certain limits, the greater the proportion
of tin the cleaner and more fusible is the solder. It is usually
worth while to prepare the solder in the laboratory, for in this way a
uniform and dependable product is assured. Good soft lead is melted
in an iron ladle and skimmed; the temperature is allowed to rise very
little above the melting-point. The tin is then added little by
little, the alloy stirred vigorously and skimmed, and sticks of solder
conveniently cast by sweeping the ladle over a clean iron plate, so as
to pour out a thin stream of solder. If the solder be properly made
it will have a mat and bright mottled surface, and will "crackle" when
held up to the ear and bent.
Perhaps the chief precaution necessary in making solder is to exclude
zinc. The presence of a very small percentage of this metal entirely
spoils the solder for tinman's work by preventing its "running" or
flowing smoothly under the soldering bit.
Fig. 75.
Fig. 76.
Fig. 77.
§ 94. Preparing a Soldering Bit.
The wedge-shaped edge of one of the forms of bit shown in the sketch
is filed to shape and the bit heated in a fire or on a gas heater. A
bit of rough sandstone, or even a clean soft brick, or a bit of tin
plate having some sand sprinkled over it, is placed in a convenient
position and sprinkled with resin.
As soon as the bit is hot enough to melt solder it is withdrawn and a
few drops of solder melted on to the brick or its equivalent. The
iron or bit is then rubbed to and fro over the solder and resin till
the former adheres to and tins the copper head. It will be found
advisable to tin every side of the point of the bit and to carry the
tinning back at least half an inch from the edge.
If the solder obstinately refuses to adhere, the cause is to be sought
in the oxidation of the copper, or of the solder, or both--in either
case the result of too high a temperature or too prolonged heating.
The simple remedy is to get the iron hot, and then to dress it with an
old file, so as to expose a bright surface, which is instantly passed
over the resin as a means of preserving it from oxidation. If the
process above described be now carried out, it will be found that the
difficulty disappears.
Before using the iron, wipe off any soot or coke or burned resin by
means of an old rag. An iron tinned in this way is much to be
preferred to one tinned by means of chloride of zinc.
A shorter and more usual method is carried out as follows: The
solution of chloride of zinc is prepared by adding bits of zinc to
some commercial hydrochloric acid diluted with a little (say 25 per
cent) of water. The acid may conveniently be placed in a small glazed
white jar (a jam pot does excellently), and this should only be filled
to about one-quarter of its capacity. An excess of zinc may be added.
It may be fancy, but I prefer a soldering solution made in this way to
a solution of chloride of zinc bought as a chemical product. The jar
is generally mounted on a heavy leaden base, so as to avoid any danger
of its getting knocked over, for nothing is so nasty or bad for tools
as a bench on which this noxious liquid has been upset (Fig. 78).
Fig. 78.
To tin a soldering bit, a little of the fluid is dipped out of the jar
on to a bit of tin plate bent up at the edges--a few drops is
sufficient--and the iron is heated and rubbed about in the liquid
with a drop of solder. If the iron is anything like clean it will tin
at once and exhibit a very bright surface, but quite dirty copper may
be tinned by dipping it for a moment in the liquid in the pot and then
working it about over the solder. An iron so tinned remains covered
with chloride of zinc, and this must be carefully wiped off if it is
intended to use the iron with a resin or tallow flux in lead
soldering.
One disadvantage of this process is that the copper bit soon gets
eaten into holes and requires to be dressed up afresh. On the other
hand, an iron so tinned always presents a nice clean solder surface
until the next time it is heated, when it generally becomes very dirty
and requires to be carefully wiped before using.
In my experience also an iron so tinned is more easily spoiled as to
the state of its surface, "detinned," in fact, by overheating than
when the tinning is carried out by resin and friction. When this
happens, the shortest way out of the difficulty is the application of
the old file so as to obtain a perfectly fresh surface. No one who
knows his business ever uses an iron that is not perfectly clean and
well tinned.
The iron may be cleaned from time to time by heating it red hot and
quenching it in water to get rid of the oxide, which scales off in the
process.
§ 95. Soft Soldering.
In the laboratory the chief application of the process is to copper
soldering during the construction of electrical apparatus and to zinc
soldering for general purposes.
In ninety-nine cases out of every hundred where difficulties occur
their origin is to be traced to dirt. There seems to be some
inexplicable kink in the human mind which renders it callous to
repeated proofs of the necessity for cleaning surfaces which it is
intended to solder. The slightest trace of albuminous or gelatinous
matter or shellac will prevent solder adhering to most metals and the
same remark applies in a measure to the presence of oxides, although
these may be removed by chloride of zinc or prevented from forming by
resin or tallow. A touch with an ordinarily dirty hand--I refer to a
solderer's hand--will often soil work sufficiently to make the
adherence of solder difficult.
The fluxes most generally employed are tallow for lead, resin or
Venice turpentine for copper, chloride of zinc for anything except
lead, which never requires it. The latter flux has the property (also
possessed by borax at a red heat) of dissolving any traces of oxide
which may be formed, as well as acting as a protecting layer to the
metal.
We may now turn to the consideration of a simple case of soldering,
say the joining of two copper wires. The wires are first cleaned
either by dipping in a bath of sulphuric and nitric acids--a thing no
laboratory should be without--or by any suitable mechanical means.
The cleaned wires are then twisted together--there is a regulation
way of doing this, but it presents no advantage in laboratory
practice--and the joint is sprinkled over with resin, or painted
with a solution of resin in alcohol.
The iron, being heated and floated with solder, is held against the
joint, the latter being supported on a brick, and the solder is
allowed to "sweat" into the joint. Enough solder must be present to
penetrate right through the joint. Nothing is gained by rubbing
violently with the iron. If the copper is clean it will tin, and if
it is dirty it won't, and there the matter ends.
Beginners generally use too small or too cold a bit, and produce a
ragged, dirty joint in consequence. If the saving of time be an
object, the joint may be twisted together on ordinarily dirty oxidised
wires and heated to, say, 200° C. It is then painted with chloride of
zinc and soldered with the bit.
There is a difference of opinion as to the relative merits of chloride
of zinc and of resin as a flux in soldering copper. Thus the standing
German practice is, or was, to employ the former flux in every case
for soldering electric light wires, while in England the custom used
to be to specify that soldering should be done by resin, and this
custom may still prevail; it lingers in Australia at all events.
However, it is agreed on all hands that when chloride of zinc is used
it must be carefully washed off. I have known of an electrical
engineer insisting on his workmen "licking" joints with their tongues
to ensure the total removal of chloride of zinc; it has a horrible
taste; and I have occasionally pursued the same plan myself when the
soldering of fine wires was in question.
In any case, it is very certain that chloride of zinc left in a joint
will ruin it sooner or later by loosening the contact between copper
and solder.
Very often it is requisite to solder together two extensive flat
surfaces--for instance, in "chucking" certain kinds of brass work.
The surfaces to be soldered must be carefully tinned, most
conveniently by the help of the blow-pipe and chloride of zinc. After
tinning, the surfaces are laid together and heated so as to "sweat"
them together; the phrase, though inelegant, is expressive.
96. Soldering Tin Plate.
If the plate be new and clean, a little resin or its solution in
alcohol is all that is necessary as a flux. If the tin plate is rusty
the rust must be removed and the clean iron, or rather mild steel,
surface exposed. The use of chloride of zinc is practically essential
in this case. Tin plate is often spotted with rust long before it
becomes rusty as a whole, when, of course, it may be regarded as worn
out, and such rust spots are most conveniently removed by means of the
plumber's shave-hook. The shave-hook is merely a peculiarly shaped
hard steel scraping knife on a handle (Fig. 79).
Fig. 79
With tin plate the soldering of long joints is often necessary. The
plate must be temporarily held in position either by binding with iron
wire, fastening by clamps, or holding by an assistant. The flux is
applied and the iron run slowly along the joint. Enough solder is
used to completely float the tip of the iron. By arranging the joint
so that it slopes downward slightly, and commencing at the upper end,
the solder may be caused to flow after the iron, and will leave a
joint with the minimum permissible amount of solder in it. By
regulating the slope, heat of iron, etc, any desired quantity of
solder may be run into the joint.
§ 97. Soldering Zinc.
Zinc alloys with soft solder very easily, and by so doing entirely
spoils it, making, it "crumbly," dirty, and preventing it running.
Consequently, in soldering up zinc great care must be taken to prevent
the solder becoming appreciably contaminated by the zinc. To this end
the zinc surfaces are cleaned by means of a little hydrochloric acid,
which is painted on instead of chloride of zinc. Plenty of solder is
melted on to the work, and is drawn along over the joint by a single
slow motion of the soldering bit. The iron must be just hot enough to
make the solder flow freely, and it must never be rubbed violently on
the zinc or allowed to linger in one spot; the result of the latter
action will be to melt a hole through the zinc, owing to the tendency
of this metal to form an easily fusible alloy with the solder.
The art of soldering zinc is a very useful one in the laboratory. The
majority of physicists appear to overlook the advantages of zinc
considered as a material for apparatus construction. It is light,
fairly strong, cheap, easily fusible, and yet hard and elastic when
cold. It may be worked as easily as lead at a temperature of, say,
150° to 200° C, and slightly below the melting-point (423° C.) it is
brittle and may & powdered. The property of softening at a moderate
temperature is invaluable as a means of flattening zinc plate or
shaping it in any way. During the work it may be held by means of an
old cloth. Zinc sheet which has been heated between iron plates and
flattened by pressure retains its flatness very fairly well after
cooling.
§ 98. Soldering other Metals.
Iron.
The iron must be filed clean and then brushed with chloride of zinc
solution. Some people add a little sat ammoniac to the chloride of
zinc, but the improvement thus made is practically inappreciable. If
the iron is clean it tins quite easily, and the process of soldering
it is perfectly easy and requires no special comment.
Brass.
The same method as described for iron succeeds perfectly. The brass,
if not exceedingly dirty, may be cleaned by heating to the temperature
at which solder melts (below 200° C.), and painting it over with
chloride of zinc, or dipping it in the liquor. If now the brass be
heated again in the blow-pipe flame, it will be found to tin perfectly
well when rubbed over with solder.
German Silver, Platinoid, Silver, and Platinum are treated like iron.
With regard to silver and platinum the same precautions as recommended
in the case of zinc must be observed, for both these metals form
fusible alloys with solder.
Gold when pure requires no flux. Standard gold, which contains copper,
solders better with a little chloride of zinc.
Lead must be pared absolutely clean and then soldered quickly with a
hot iron, using tallow as a flux. Since solder if over hot will
adhere to lead almost anywhere, plumbers are in the habit of specially
soiling those parts to which it is not intended that solder shall
adhere. The "soiling" paint consists of very thin glue, called size,
mixed with lampblack; on an emergency a raw potato may be cut in half,
and the work to be soiled may be rubbed over with the cut surface of
the potato.
Hard Carbon or gas coke may be soldered after coating with copper by
an electrolytic process, as will be described.
§ 99. Brazing.
Soldering at a red heat by means of spelter is called brazing.
Spelter is soft brass, and is generally made from zinc one part,
copper one part; an alloy easily granulated at a red heat; it is
purchased in the granular form.
The art of brazing is applied to metals which will withstand a red
heat, and the joints so soldered have the strength of brass.
The pieces to be jointed by this method must be carefully cleaned and
held in their proper relative positions by means of iron wire. It is
generally necessary to soften iron wire as purchased by heating it red
hot and allowing it to cool in the air; if this is not done the wire
is usually too hard to be employed satisfactorily for binding.
Very thin wire--i.e. above No. 20 on the Birmingham wire gauge--does
not do, for it gets burned through, and perhaps allows the work
to fall apart at a critical moment.
The work being securely fastened, the next step is to cover the
cleaned parts with flux in order to prevent oxidation. For this
purpose "glass borax" is employed. "Glass" borax is simply ordinary
borax which has been fused for the purpose of getting rid of water of
crystallisation. The glass borax is reduced to powder in an iron
mortar, for it is very hard, and is then made up into a cream with a
little water. This cream is painted on to the parts of the work which
are destined to receive the solder.
The next step is to prepare the spelter, and this is easily done by
mixing it with the cream, taking care to stir thoroughly with a
flattened iron wire till each particle of spelter is perfectly covered
with the borax. The mixture should not be too wet to behave as a
granular mass, and may then be lifted on to the work by means of the
iron spatula.
Care must be taken to place the spelter on those parts only which are
intended to receive it, and when this is done, the joint may be
lightly powdered over with the dry borax, and will then be ready for
heating.
If the object is of considerable size it is most conveniently heated
on the forge; if small the blowpipe is more convenient. In the
latter case, place the work on a firebrick, and arrange two other
bricks on edge about it, so that it lies more or less in a corner. A
few bits of coke may also be placed on and about the work to increase
the temperature by their combustion, and to concentrate the flame and
prevent radiation. The temperature is gradually raised to a bright
red heat, when the spelter will be observed to fuse or "run," as it is
technically said to do.
If the cleaning and distribution of flux has been successful, the
spelter will "run" along the joint very freely, and the work should be
tapped gently to make sure that the spelter has really run into the
joint. The heating may be interrupted when the spelter is observed to
have melted into a continuous mass. As soon as the work has fallen
below a red heat it may be plunged into water, a process which has the
effect of cracking off the glass-like layer of borax.
There is, however, some risk of causing the work to buckle by this
violent treatment, which must of course be modified so as to suit the
circumstances of the case. If the joint is in such a position that
the borax cannot be filed off, a very convenient instrument for its
removal by scraping is the watchmaker's graver, a square rod of hard
steel ground to a bevelled point (Fig. 80).
Fig. 80.
Several precautions require to be mentioned. In the first place,
spelter is merely rather soft brass, and consequently it often cannot
be fused without endangering the rest of the work. A good protection
is a layer of fireclay laid upon the more delicate parts, such for
instance as any screwed part.
Gun-metal and tap-metal do not lend themselves to brazing so readily
as iron or yellow brass, and are usually more conveniently treated by
means of silver solder.