On Laboratory Arts Part 8

(1890), P. 150:-

Bis.m.u.th 40 per cent

Lead 25 per cent

Tin 10 per cent

Cadmium 10 per cent

Mercury 15 per cent

This is practically one form of Rose's fusible metal with 15 per cent mercury added. It takes nearly an hour to set completely, and the apparatus must be clean and warm before it is applied.

As the result of several trials by myself and friends, I am afraid I must dissent from the claim of the author that such a cement will make a really air-tight joint between gla.s.s tubes. Indeed, the appearance of the surface as viewed through the gla.s.s is not such as to give any confidence, no matter what care may have been exercised in performing all the operations and cleaning the gla.s.s; besides which the cement is rigid when cold, and the expansion difficulty comes in.

On the other hand, if extreme air-tightness is not an object, the cement is strong and easily applied, and has many uses. I have an idea that if the joints were covered with a layer of soft wax, the result would be satisfactory in so far as air-tightness is concerned.

This antic.i.p.ation has since been verified.

In many cases one can resort to the device already mentioned of enclosing a rubber or tape-wrapped joint between two tubes in a bath of mercury, but in this case the gla.s.s must be clean and hot and the mercury also warm, dry, and pure when the joint is put together, otherwise an appreciable air film is left against the gla.s.s, and this may creep into the joint.

Perhaps the easiest way of making such a joint is to use an outer tube of thin clean gla.s.s, and bore a narrow hole into it from one side to admit the mercury; if the mercury is to be heated in vacuo, it is better to seal on a side joint. It is always better, if possible, to boil the mercury in situ, which involves making the wrapping of asbestos, but, after all, we come back to the position I began by taking up, viz. that the easiest and most reliable method is by fusion of the gla.s.s--all the rest are unsuitable for work of real precision.

I should be ungrateful, however, were I not to devote a few lines to the great convenience and merit of so-called "centering cement." This substance has two or three very valuable properties. It is very tough and strong in itself, and it remains plastic on cooling for some time before it really sets. If for any reason a small tube has to be cemented into a larger one, which is a good deal larger, so that an appreciable ma.s.s of cement is necessary, and particularly if the joint requires to have great mechanical strength, this cement is invaluable.

I have even used a plug of it instead of a cork for making the joint between a gas delivery tube and a calcium chloride tower. (Why are these affairs made with such abominable tubulures?)

The joint in question has never allowed the tube to sag though it projects horizontally to a distance of 6 inches, and has had to withstand nearly two years of Sydney temperature. The cement consists of a mixture of sh.e.l.lac and 10 per cent of oil of ca.s.sia.

The sh.e.l.lac is first melted in an iron ladle, and the oil of ca.s.sia quickly added and stirred in, to an extent of about 10 per cent, but the exact proportions are not of importance. Great care must be taken not to overheat the sh.e.l.lac.

APPENDIX TO CHAPTER I

ON THE PREPARATION OF VACUUM TUBES FOR THE PRODUCTION OF PROFESSOR ROENTGEN'S RADIATION

[Footnote: Written in May 1896.]

WHEN Professor Roentgen's discovery was first announced at the end of 1895 much difficulty was experienced in obtaining radiation of the requisite intensity for the repet.i.tion of his experiments. The following notes on the production of vacuum tubes of the required quality may therefore be of use to those who desire to prepare their own apparatus. It appears that flint gla.s.s is much more opaque to Roentgen's radiation than soda gla.s.s, and consequently the vacuum tubes require to be prepared from the latter material.

Fig. 39.

A form of vacuum tube which has proved very successful in the author's hands is sketched in Fig. 38. It is most easily constructed as follows. A bit of tubing about 2 centimetres diameter, 15 centimetres long, and 1.5 millimetre wall thickness, is drawn down to a point. The larger bulb, about 5 centimetres in diameter, is blown at one end of this tube. The thinner the bulb the better, provided that it does not collapse under atmospheric pressure. A very good idea of a proper thickness may be obtained from the statement that about 4 centimetres length of the tubing should be blown out to form the bulb. This would give a bulb of about the thickness of an ordinary fractionating bulb. Before going any further it is as well to test the bulb by tapping on the table and by exhausting it by means of an ordinary water-velocity pump.

The side tube is next prepared out of narrower tubing, and is provided with a smaller bulb, a blowing-out tube, and a terminal, to be made as will be described. This side tube is next fused on to the main tube, special care being taken about the annealing, and the cathode terminal is then sealed into the main tube. After using clean gla.s.s it is in general only necessary to rinse the tube out with clean alcohol, after which it may be dried and exhausted.

The success of the operation will depend primarily on the attention given to the preparation and sealing-in of the electrode facing the large bulb.

Preparation of Terminals. Some platinum wire of about No. 26 B.W.G--the exact size is unimportant--must be provided, also some sheet aluminium about 1 millimetre thick, some white enamel cement gla.s.s, and a "cane" of flint-gla.s.s tube of a few millimetres bore.

The electrodes are prepared by cutting discs of aluminium of from 1 to 1.5 centimetres diameter. The discs of aluminium are bored in the centre, so as to admit the "stems" which are made of aluminium wire of about 1 millimetre diameter. The stems are then riveted into the discs. The "stems" are about I centimetre long, and are drilled to a depth of about 3 millimetres, the drill used being about double the diameter of the platinum wire to be used for making the connections.

The faces of the electrodes--i.e. the free surfaces of the aluminium discs--are then hammered flat and brought to a burnished surface by being placed on a bit of highly polished steel and struck by a "set"

provided with a hole to allow of the "stem" escaping damage. The operation will be obvious after a reference to Figs. 39 and 40; it is referred to again on page 96.

The platinum wires may be most conveniently attached by melting one end of the piece of platinum wire in the oxygas blow-pipe till it forms a bead just large enough to pa.s.s into the hole drilled up the stem of the electrode. The junction between the stein and the platinum wire is then made permanent by squeezing the aluminium down upon the platinum wire with the help of a pair of pliers. It is also possible to fuse the aluminium round the platinum, but as I have had several breakages of such joints, I prefer the mechanical connection described.

Fig. 39. Sets for striking aluminium electrodes

Fig. 40.

i. Aluminium electrode.

ii. Aluminium electrode connected to platinum wire.

iii. Aluminium electrode connected to platinum wire and protected by gla.s.s.

iv. Detail of fastening platinum wire.

The stem and platinum wire may now be protected by covering them with a little flint gla.s.s. For this purpose the flint-gla.s.s tube is pulled down till it will just slip over the stem and wire, and is cut off so as to leave about half a centimetre of platinum wire projecting. The flint-gla.s.s tube is then fused down upon the platinum wire, care being taken to avoid the presence of air bubbles. At the close of the operation a single drop of white enamel gla.s.s is fused round the platinum wire at a high temperature, so as to make a good joint with the protecting flint-gla.s.s tube.

The negative electrode being nearly as large as the main tube, it must be introduced before the latter is drawn down for sealing. After drawing down the main tube in the usual manner, taking care not to make it less than a millimetre in wall-thickness, it is cut off so as to leave a hole not quite big enough for the enamel drop to pa.s.s through. By heating and opening, the aperture is got just large enough to allow the enamel drop to pa.s.s into it, and when this is the case the joint is sealed, pulled, and blown out until the electrode occupies the right position--viz. in the centre of the tube and with its face normal to the axis of the tube.

The gla.s.s walls near the negative electrode must not be less than a millimetre thick, and may be rather more with advantage, the gla.s.s must be even, and the joint between the flint gla.s.s and the soda gla.s.s, or between the wire and the soda gla.s.s, must be wholly through the enamel. The "seal" must be well annealed. It will be found that the sealing-in process is much easier when the stem of the electrode is short and when the gla.s.s coating is not too heavy. Half a millimetre of gla.s.s thickness round the stein is quite sufficient.

The diagram, of the tube shows that the main tube has been expanded round the edges of the cathode. This is to reduce the heating consequent on the projection of cathode rays from the edges of the disc against the gla.s.s tube.

The anode is inserted into its bulb in a quite similar manner. If desired it may be made considerably smaller, and does not need the careful adjustment requisite in sealing-in the cathode, nor does the gla.s.s near the entry wire require to be so thick.

More intense effects are often got by making the cathode slightly concave, but in this case the risk of melting the thin gla.s.s is considerably increased. No doubt, Bohemian gla.s.s might be used throughout instead of soft soda gla.s.s, and this would not melt so easily; the difficulties of manipulating the gla.s.s are, however, more p.r.o.nounced.

It will be shown directly that the best Roentgen effects are got with a high vacuum, and it is for this reason that the gla.s.s near the cathode seal requires to be strong. The potential right up to the cathode is strongly positive inside the tube, and this causes the gla.s.s to be exposed to a strong electric stress in the neighbourhood of the seal.

Although the gla.s.s-blowing involved in the making of a so-called focus tube is rather more difficult than in the case just described, there is no reason why such a difficulty should not be overcome; I will therefore explain how a focus tube may be made.

Fig. 41.

A bulb about 3 inches in diameter is blown from a bit of tube of a little more than 1 inch diameter. Unless the walls of the tube are about one-eighth of an inch in thickness, this will involve a preliminary thickening up of the gla.s.s. This is not difficult if care be taken to avoid making the gla.s.s too hot. The larger gas jet described in connection with the soda-gla.s.s-blowing table must be employed. In blowing a bulb of this size it must not be forgotten that draughts exercise a very injurious influence by causing the gla.s.s to cool unequally; this leads to bulbs of irregular shape.

In the method of construction shown in Fig. 41, the anode is put in first. This anode simply consists of a square bit of platinum or platinum-iridium foil, measuring about 0.75 inch by 1 inch, and riveted on to a bent aluminium wire stem.

As soon as the anode is fused in, and while the gla.s.s is still hot, the side tube is put on. The whole of the anode end is then carefully annealed. When the annealing is finished the side tube is bent as shown to serve as a handle when the time comes to mount the cathode.

Before placing the cathode in position, and while the main tube is still wide open, the anode is adjusted by means of a tool thrust in through this open end. This is necessary in view of the fact that the platinum foil is occasionally bent during the operation of forcing the anode into the bulb.

The cathode is a portion of a spherical surface of polished aluminium, a mode of preparing which will be given directly. The cathode having been placed inside the bulb, the wide gla.s.s tube is carefully drawn down and cut off at such a point that when the cathode is in position its centre of curvature will lie slightly in front of the anode plate.

For instance, if the radius of curvature of the cathode be 1.5 inches, the centre of curvature may lie something like an eighth of an inch or less in front of the anode.

The cathode as shown in Fig. 41 is rather smaller than is advantageous. To make it much larger than is shown, however, the opening into the bulb would require to be considerably widened, and though this is not really a difficult operation, still it requires more practice than my readers are likely to have had. The difficulty is not so much in widening out the entry as in closing it down again neatly.

Now as to making the anode. A disc of aluminium is cut from a sheet which must not be too thick--one twenty-fifth of an inch is quite thick enough. This disc is bored at the centre to allow of the stem being riveted in position. The disc is then annealed in the Bunsen flame and the stem riveted on.

The curvature is best got by striking between steel dies (see Figs.