The Turn Of The Century Electrotherapy Museum
(C) Jeff Behary 2011 

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The Electrician, July 17, 1887




The first thing to test is the vacuum. The usual way is to pass the spark from an induction coil through. If one terminal of a Ruhmkorff coil, giving about a half-inch spark, is connected to the terminal of the lamp, and the other is led to a wire tied round the bulb, and the lamp gradually exhausted, the discharge is purple or lilac at first, with pinkish lights round the carbon. As the vacuum gets better the discharge becomes bluer, and if the bulb is made of German glass it gradually changes to bright green. The green is distributed through the bulb at first, but as the vacuum gets better it is confined to the internal surface of the glass; it then appears only in patches on the glass; and finally it disappears altogether. It is difficult to get it to disappear altogether. If the bulb is made of English or lead glass the discharge is never of the brilliant apple-green seen in the case of German glass; it is of a bluish colour till it disappears.

The primary coil need not be supplied by a primary battery, as the cells are very troublesome. It is best, therefore, to connect it with a dynamo and resistance, or to work it from a secondary battery. If it is put in circuit with a dynamo with resistance in series to reduce the electromotive force, a lamp should be put in shunt to the primary coil and contact breaker, otherwise the sparking is troublesome and soon destroys the contacts. One terminal of the secondary is led to a plate and the other to a loop about an inch off, so that when the lamp is taken by the tip and held in the loop the

terminals touch the plate. In testing lamps with very thin or weak carbons it is best to take the lamp in the hand and touch one terminal of the coil only, as, if the lamp has a powerful discharge passed through it the carbon is often broken. Sometimes the carbon remains some minutes stuck against the glass after the discharge. Lamps will also frequently glow in the dark after the discharge.

A large number of lamps can be tested for vacuum very quickly. Those which do not show either a bright apple green on the glass if German, or no discharge at all, aro rejected. Any rejected lamps are examined; if there are any cracked at the sealing in they are sent back to the glass blowers' room to be repaired if possible. To repair such a lamp, the nose is held for a moment in the blowpipe flame, which cracks it. The lamp is left till the vacuum is destroyed by the slow leakage thus produced. The nose is then knocked off, and a new exhausting stem is sealed on. It requires some practice to do this neatly, but with care a lamp may be repaired so well that no one could tell it had been exhausted twice. The cracked end is then very slowly and carefully heated and worked till the crack is gone. The lamp is then sent to the pump room in the usual course. If the lamp is merely badly exhausted, a new exhausting tube is sealed on, and the lamp pumped again.

The current opinion is that the vacuum in a fairly well exhausted lamp is about one in a million, and that the Ruhmkorff coil is an infallible means of testing the vacuum.

The writer has come to the conclusion that this is not the case, and that the vacuum generally obtained is not more than about 1 in 20,000. The whole question demands further experiment, as the results obtained so far are conflicting, and there are not enough data to draw conclusions with certainty. In the first place, mercury vapour must have some tension. It seems to have been generally assumed by those who have worked at high vacua that it has no tension. Itegnault found the vapour tension of mercury at 20°C.—which may be taken as the average temperature in a pump room—to be -0372 mm.; this would give 1 in 20,000 as the maximum vacuum obtainable by any mercury pump. Apart from Regnault's determination, the mercury must have an appreciable vapour tension, for it evaporates perceptibly at ordinary temperatures. If a piece of gold leaf is held near mercury it is soon amalgamated. It seems, therefore, probable that the mercury pump ceases to take out air when the pressure is about equal to that of the mercury vapour itself. This would show why a Geissler pump goes on taking air out after a Sprengel stops, as the mercury in a Sprengel, being violently broken up, would give off vapour more easily. The action of the pump would of itself lead one to suppose that some such action takes place. If a Geissler pump, with a bulb the tame size as, say, one lamp and the phosphoric and other tube3 were set to exhaust the lamp, beginning with a pressure of 10 mm. of mercury, it would reach a vacuum of 1 in a million in 14 strokes, if the glass gave off no air. The glass, of course does give off air, so the pump takes considerably longer. But if the pump really brought the vacuum up to one in a million, it is not likely that the gas would be given off gradually, so that the vacuum soon rose to one in 200,000 or 300,000, and then rose to one in a million very slowly, never getting much better. If the Geissler is worked till it shows a vacuum that would generally be called one in a million, and then left for some hours, the vacuum will apparently have fallen to about what would be called one in 100,000. Even if the lamp be heated very strongly and be pumped for several days, it will show a so-called vacuum of only one in 20,000 or so after standing, though it showed one in a million at the last stroke of the pump. When a Geissler pump is worked till it shows a so-called vacuum of one in a million, it goes on for some time with only slight impiovement of the vacuum. If the vacuum weie really one in a million or so, and the air delivered by the pump were approximately equal to the air coming off the glass*, which is the accepted theory, the vacuum would immediately fall to one in 500,000, if the pump were allowed to miss a stroke. It does not do this, however; the apparent vacuum falls very slowly. This is easily explained on the mercury Yapour theory, as the bulb of the pump would

be nearly full of mercury vapour, a little air coming in to every stroke, so that the apparent vacuum would increase very slowly. If the pump is left for some hours the air would diffuse into the pump bulb, and the mercury vapour woukl also find its way into the phosphoric tube and lamp. This would also explain why the apparent vacuum should fall off so much when the pump is allowed to rest for a day or so.

To return to the spark from the induction coil. It has been already mentioned that no spark will pass through the bulb of a good Geissler pump while the mercury is descending, until the end of the tube from the lamp is uncovered. It was known in the last century that pure mercury vapour does not carry any spark. A Torricellian vacuum was made, the mercury having beon very thoroughly boiled in the tube. It was found that the spark from an induction coil did not pass. When a little air was admitted, however, the bright green light appeared at once. Mercury vapour, therefore, contrary to the statements generally made, will not pass a spark unless air is mixed with it. As already mentioned, mercury seems to sweep the air off the glass, so that the Geissler bulb contains pure mercury vapour when the mercury is descending, but the spark can pass immediately the end of the lamp tube is uncovered.

Another way of testing vacua, which is only applicable in experimental work, when the lamp is not wanted, is to open the bulb under mercury. It requires care to perform this operation properly. If a lamp is merely held with its nose under mercury, and if the nose is then broken with a pair of pliers, the mercury will leave a bubble of air about the size of a bean. Most of this has come in from the outside, as there is a film of air upon the outside of the lamp, and especially on the jaws of the pliers. If the nose be broken so as to make a very small hole, and if the jaws of the pliers or any such thing be held against the tip, bubbles of air will be seen to come into the lamp. The best way to prevent this source of error is to wet the lamp and pliers with water or oil. If a number of lamps which have broken terminals, or are wasters from some other cause than bad vacua, are taken and tested with the induction coil, and are then opened under mercury, it will be found that those which showed what would generally be considered the best vacua on the coil do not always show the least air when opened under mercury. The air bubble generally left when a lamp is opened under mercury would correspond to a vacuum of about one in 10,000 to one in 20,000; but sometimes the mercury fills the whole of the inside of the lamp, showing a perfect vacuum. It is probable that the air condenses against the glass, and that this test is of little or no value. Davy found that when the mercury was allowed to rise to 'he top of a barometer tube slowly it left a bubble, but when it was allowed to come up with a bump, the bubble did not appear and the air seemed to be flattened out against the glass.

Sometimes a lamp passes no spark, or shows bright green spots after it has been lying in the store for some time, but alter running it will show a much lower vacuum on the coil. If left for some time the vacuum will appear to improve again. It seems as if the gas inside condensed on the glass, and was driven off by the heat when the lamp was run. The passage of sparks also seems to bring off gas from the glass. This effect may be best observed while exhausting a lamp. The vacuum of the lamp can also be apparently reduced by merely heating it.

Sometimes lamps, especially those for high electromotive force, or badly exhausted, show a blue light flickering about on one end of the carbon. There is a curious phenomenon in connection with this blue light. If a lamp which shows it is run still brighter the blue light generally gets worse and leaves the terminal, and spreads until the whole of the inside of the lamp becomes filled with bright blue light. If the lamp is overrun for some time, the blue light begins to flicker, and the flashes get fewer till there is only an occasional flicker visible. Finally it disappears altogether. It cannot be made to appear again.

If the lamps are run on the pumps, any that show blue lights during the testing should be rejected and sent, back for re exhausting; but if they are not run on the pumps, lamps

for high electromotive force will occasionally show hlue lights, which disappear at once and may be disregarded if the lamps show green phosphorescence on the induction coil.

The whole question of high vacua and the adhesion of air to the glass demands thorough investigation. Bunsen is of opinion that carbon dioxide is dissolved by the film of water on the surface of the glass, and that, though solution of carbon ilioxide in water at ordinary pressures does not act on glass, still the capillary attraction exerts such enormous pressure that a sufficiently strong solution is made to act on the glass, forming carbonate of soda.

The writer hopes to be able to carry out some further experiments on high vacua within the year, and if they prove worth it, to publish any results that may throw light on the difficult question.

Jeff Behary, c/o The Turn Of The Century Electrotherapy Museum

(C) Jeff Behary , 2012