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. [Footnote: "Barometer" tube is merely very thick-walled glass tubing, and makes particularly bad barometers, which are sold as weather glasses.]
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.
Fig.
37.
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part of the oxygas flame; in a moment the metals will fuse together at the point of contact, when they may be withdrawn.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.