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 a.s.sociated with some kind of permanent set taken by the fibres during the stage of pa.s.sage 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 sh.e.l.lac, but where very great accuracy is aimed at, the sh.e.l.lac (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 sh.e.l.lac, this is practically all that needs to be done. The supports should resemble large pins. The upper support will be a bra.s.s wire in most cases, and will require to be filed away as shown in the sketch (Fig. 71). It is then coated with sh.e.l.lac by heating and rubbing upon the sh.e.l.lac. As previously noted, the sh.e.l.lac 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 sh.e.l.lac, which is now cold. When the thread is in place, a soldering iron is put against the bra.s.s wire, and the sh.e.l.lac 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 sh.e.l.lac, and should this be the case, a hot iron must be brought up to the sh.e.l.lac 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 sh.e.l.lac 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 sh.e.l.lac is to prevent them being acted upon by the acid. The threads are not very easily washed acid free, but the process may be a.s.sisted 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 Leclanche 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 sh.e.l.lac 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 sh.e.l.lac 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 gla.s.s hair pencil.

This is best done by holding the thread horizontally with the a.s.sistance 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 gla.s.s 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 gla.s.s-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 bra.s.s, and the third is an alloy of silver and bra.s.s 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 bra.s.s, 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 const.i.tuents 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 a.s.sured. 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.