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The
Electrician, July 17, 1887 INCANDESCENT
LAMP MANUFACTURE. BY J.
SWINBURNE. Testing.
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.
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