§ 49 Fusing Electrodes Into Glass.
This art has greatly improved since the introduction of the incandescent lamp; however, up to the present, platinum seems to remain the only substance capable of giving a certainly air-tight result. I have not tried the aluminium-alumina method.
Many years ago it was the fashion to surround the platinum wire with a drop of white enamel glass in order to cause better adhesion between it and the ordinary glass. 1 However, in the case of flint glass, if one may judge from incandescent lamps, this is not essential — a fact which entirely coincides with my own experience.
On the other hand, when sealing electrodes into German glass I have often used a drop of enamel with perfect results, though this is not always done in Germany. In all cases, however, in which electrodes have to be sealed in — especially when they are liable to heat — I recommend flint glass, and in this have the support of Mr. Rain (The Incandescent Lamp and its Manufacture, p. 131). The exact details for the preparation of eudiometer tubes are given by Faraday (Chemical Manipulation, § 1200).
In view of what has preceded, however, I will content myself with the following notes. Make the hole through which the wire is to protrude only slightly larger than the wire itself, and be sure that the latter is clean. Allow the glass to cool sufficiently not to stick to the wire when the latter is pushed in. Be sure that, on heating, the glass does not get reduced, and that it flows up to the wire all round; pull and push the wire a little with a pair of pincers, to ensure this.
It is not a bad plan to get the glass exceedingly fluid round the wire — even if the lump has to be blown out a little afterwards—as it cools. The seal should finally be well annealed in asbestos, but first by gradually moving it into the hot air in front of the flame.
It was observed by Professor J. J. Thomson and the author some years ago (Proc. Roy. Soc. 40. 331. 1886) that when very violent discharges are taken through lightly sealed-in electrodes in lead-glass tubes — say from a large battery of Leyden jars — gas appears to be carried into the tube over and above that naturally given off by the platinum, and this without there being any apparent want of perfection in the seal. This observation has since been confirmed by others. Consequently in experiments on violent discharges in vacuo where certainty is required as to the exclusion of air, the seals should be protected by a guard tube or cap containing mercury; this must, of course, be put in hot and clean, on hot and clean glass, and in special cases should be boiled in situ.
A well-known German physicist (Warburg, I think) recommends putting the seals under water, but I cannot think that this is a good plan, for if air can get in, why not water? which has its surface tension in its favour. The same reasoning prevents my recommending a layer of sulphuric acid above the mercury-a method used for securing air-tightness in “mercury joints” by Mr. Gimingham, Proc. R. S. 1874.
Further protection may be attained for many purposes by coating the platinum wire with a sheath of glass, say half an inch long, fused to the platinum wire to a depth of one-twentieth of an inch all round.
In some cases the electrodes must be expected to get very hot, for instance, when it is desired to platinise mirrors by the device of Professor Wright of Yale. In this and similar cases I have met with great success by using “barometer” tubes of about one-twelfth of an inch bore, and with walls, say, one-tenth of an inch thick. 2
This tube is drawn down to a long point — say an inch long by one-eighth of an inch external diameter, and the wire is fused in for a length, say, of three-quarters of an inch, but only in the narrow drawn-down part of the tube. At different times I have tried four such seals, and though the electrodes were red hot for hours, I have never had an accident — of course they were well annealed.
For directions as to the making of high vacuum tubes, see the section dealing with that matter.
§ 50. As economy of platinum is often of importance, the following little art will save money and trouble. Platinum is easily caused to join most firmly to copper — with which, I presume, it alloys — by the following method. Hold the platinum wire against the copper wire, end to end, at the tip of the reducing flame of a typical blowpipe — or anywhere — preferably in the “reducing” part of the oxygas flame; in a moment the metals will fuse together at the point of contact, when they may be withdrawn.
Such a joint is very strong and wholly satisfactory, much better than a soldered joint. If the work is not carried out successfully so that a considerable drop of copper-platinum alloy accumulates, cut it off and start again. The essence of success is speed, so that the copper does not get “burned.” If any considerable quantity of alloy is formed it dissolves the copper, and weakens it, so that we have first the platinum wire, then a bead of alloy, and then a copper wire fused into the bead, but so thin just outside the latter that the joint has no mechanical strength.
Notes
1 Hittorf and Geissler (Pogg. Ann. 1864, § 35; English translation, Phys. Soc. London, p. 138) found that it was impossible to make air-tight joints between platinum and hard potash glass, but that soft lead glass could be used with success as a cement.
2 “Barometer” tube is merely very thick-walled glass tubing, and makes particularly bad barometers, which are sold as weather glasses.