THE ARC LAMP.
_General Discussion of the Electrical Arc._--The name of the electrical
arc lamp is derived from the arch-like appearance of the vapors which
give out the light when the carbons are placed horizontally. The
horizontal arc was the earliest form, hence the name which it carries to
this day.
The arc proper is due to the vapors of volatilized carbon or other
materials forming the electrodes, which may be consumed by the passage
of an electrical current from one electrode to another through the
intervening medium. In order that an arc may be formed, it is necessary
first to bring the electrodes together. This, if the circuit is properly
arranged, starts the current and when the circuit is partly interrupted,
as by slowly separating the points of the electrodes, the current passes
through the intervening space, with the result that a high degree of
heat (about 3,500 centigrade) is produced. This results in volatilizing
the carbon or any other material of which one or both electrodes may
consist.
As long as the distance between the electrode points is small, the
current will be quite strong and a hissing or frying sound will be given
out. In order to keep the current within bounds during the time that the
electrodes are together or while they are separated only a very
short distance, some resistance, or reactance in the case of
alternating-current arcs, is always connected in series in the circuit.
If this were not done, there would be a short circuit at the time of
starting or striking the arc.
The arc formed with very short separation of electrodes is generally
spoken of as a low tension arc and requires very hard carbons and about
25 volts. This type of arc is very little used for illuminating
purposes.
If the distance between the electrodes is increased gradually, the light
becomes very unsteady and flickers considerably until at a certain point
it begins to improve and give the long quiet arc. This condition will
occur when, with direct current, the electrodes are about one-eighth of
an inch apart. It will then be found that the voltage across the arc is
from 45 to 50 volts, which is the best voltage to use with open arcs. If
the separation be carried still further, the arc will grow longer and
become flaming until finally it breaks entirely.
The resistance of the arc is closely proportional to the cross section
of the electrodes and increases with the distance of the arc gap. It
acts, however, very much as though there were a small counter e. m. f.
set up within it.
The color of the light given off varies with the length of the arc
somewhat, but depends mainly upon the material of which the electrodes
consist. In the so-called flaming arcs, the peculiar color is obtained
by certain chemicals imbedded in the material composing the electrodes.
Whenever an arc is allowed to burn down until it reaches the electrode
holders, a greenish light is given off which is due to the
volatilization of the metal--usually brass--in these holders.
The light of a strong arc is extremely injurious to the eyes and should
only be viewed through colored glass. Many very painful experiences have
resulted from persons gazing upon arcs of 200 or 300 amperes, such as
are used sometimes in cutting away metals of old buildings, etc.
The most powerful arcs known at the present time are those used in some
steel mills for refining steel. These use upward of 10,000 amperes.
[Illustration: FIGURE 3.]
[Illustration: FIGURE 4.]
The length of the ordinary arc varies from one thirty-second of an inch
to one inch. The light is not of much use and is rather unsteady until
the electrodes have assumed a shape somewhat similar to that shown in
Figure 3 for direct current, and Figure 4 for alternating current. With
direct-current arcs, a crater is formed at the bottom of the positive
electrode and, from this crater, about 80 per cent of the light is
emitted. Where the light is wanted in a downward direction, the crater
is always formed at the top and for this purpose the top electrode must
be made positive; that is, the electricity must flow from the top
electrode into the lower one. In some cases, where special illumination
effects are desired, the bottom electrode is made the positive with the
result that most of the light is thrown upward. In such cases strong
shadows are thrown against the ceiling and the lamp is said to be
burning “upside down.”
The positive electrode can always be distinguished from the negative (a)
by the shadows cast; (b) by the form of the electrodes; and (c) by the
fact that since it is heated to a greater degree, it will, when the lamp
is turned off, remain hot for some time after the negative electrode has
cooled off.
In case the arc is drawn out very long and operated in this way for a
considerable time, the crater will almost wholly disappear and the
electrodes will appear rounded off.
In an alternating-current circuit, the positive and negative poles
reverse generally about 120 times per second and both electrodes in the
alternating-current arc are positive and negative to the same degree.
They are therefore very nearly alike, except that the heat rising from
the lower one increases slightly the volatilization of the upper. The
positive electrode in the direct-current arc is consumed approximately
twice as fast as the negative electrode. The consumption of the two
electrodes in an alternating-current arc is about equal and a crater
much smaller than the kind formed in a direct-current arc is, therefore,
formed on each electrode, instead of only on the positive electrode as
in the case of the direct-current arc.
The general form of alternating-current arc carbons is given in Figure
4. The small elevations shown in the cuts are due to impurities and do
not appear with first-class carbons.
When arc lamps are operated on alternating-current circuits, the best
voltage for the arc is about 28; and consequently, for the same quantity
of light, the current must be increased so that the amperage of
alternating-current lamps is always much greater than that of
direct-current lamps.
The alternating-current arc is much noisier than the direct-current arc,
but with very high frequencies this noise ceases.
In general, arc lamps do not work very well on low frequencies. The time
at which the current is practically zero is long enough to allow the
vapor between the electrode points to cool off sufficiently to interfere
with successful operation.
Any arc light is affected by draughts of air and can even be blown out.
If this occurs often, there will be rapid feeding, a short arc, and
great waste of electrode.
A magnet held close to an arc can be made to blow it out or force it to
one side. This fact is made use of in some lightning arresters.
Generally speaking, arc lamps are of two kinds, _open_ and _enclosed_.
The enclosed arc operates at a much higher voltage and is but little
used about theaters. The open arc is almost universally used for stage
work and this is about the only place where it is still considered
useful. This kind of arc lamp is, however, very hazardous in localities
where inflammable material abounds and for this reason it is always
enclosed with wire mesh when possible.
Lens lamps can be tightly enclosed since none of the light is wanted
except that which passes through the lens in front.
[Illustration: FIGURE 5.]
[Illustration: FIGURE 6.]
The so-called flood lamps are usually provided with wire gauze in front
of the arc, which prevents the escape of pieces of the electrodes and
also prevents parts of scenery, etc., from coming in contact with the
arc.
The lamp houses should be of such dimensions that, with the highest
amperage the lamp is capable of using, the outer walls will not become
excessively hot.
Illustrations of standard lens and flood lamps, as made by the Chicago
Stage Lighting Company, are shown in Figures 5 and 6.
_Operation of Arc Lamps._--From the standpoint of operation, arc lamps
may be divided into two classes, viz.: _hand-feed_ and _automatic-feed_.
The hand-feed lamp is generally used in theaters and is practically the
only kind admitted on the stage, or for stage illuminating purposes.
Only a very few houses now use arc lamps for general illumination.
The operation of hand-feed lamps[1] is ordinarily quite simple and will
be fully treated under the head of “Projection”, so that we may now
consider only the automatic lamps. At the present time these are used
mostly, if at all, for the illumination of the exterior of the theater.
[1] Full diagrams and descriptions are in another work of the
authors’, entitled “Electricians’ Operating and Testing Manual,” so
that no space need be given to these here, save in a very general way.
The operator should first familiarize himself with the construction and
principles upon which the mechanism of his lamp is based. For this
purpose he should remove the outer jacket, thus exposing the working
mechanism; turn on the current; and endeavor to learn the significance
of each part. It is of course necessary that the operator understand the
hazards due to manipulating live wires and that he should be very
careful not to make short circuits or grounds which might destroy parts
of the lamp.
Automatic-feed lamps are usually trimmed in the following manner: Bring
the lamp within reach; remove the globe; take out the lower electrode;
let down the upper electrode rod and thoroughly clean it with crocus
cloth. This upper electrode rod is the principal thing that concerns the
lamp trimmer; it must be perfectly straight and care must be exercised
not to bend it accidentally; it must be clean so that the clutches may
properly grip it; it must not be greasy. If it grows dirty or greasy, it
will soon become pitted from the current that passes from the contacts
to it.
The next operation is to remove the upper electrode and place it in the
lower electrode holder. (The length of electrode necessary should be
known. The lower one generally burns out first--it being shorter--and if
the arc reaches the lower electrode holder, will begin to consume it; if
the lower carbon is too long, the arc is liable to reach the upper
electrode holder and destroy it.) The upper electrode may then be placed
in position and aligned with the lower. To do this it is best to turn it
about and try it until it aligns in all positions. The two electrodes
should form a straight line, up and down, no matter which way the upper
is turned.
In some forms of enclosed lamps, the clutch grips the electrode direct.
In such a case all of the upper electrode must be carefully examined to
see that it is straight and free from burs, and that it can pass freely
into the opening at the top of the inner globe. The successful operation
of enclosed arcs depends upon the confinement of the gas in the inner
globe. This globe must, therefore, be kept as tight as possible without
interfering with the operation of the electrodes which pass through it.
With enclosed arcs, the care of the inner globe is of great importance,
because impurities are cast off which soon coat the inner globe and
absorb much of the light.
The care of the outer globes in general is also an important matter. A
dirty globe looks very unsightly and absorbs much light.
The following points should be carefully considered in handling and
trimming lamps:
(1) Be sure that you understand your system and know whether it is a
constant-current or a constant-potential system of distribution. With
constant-current systems, the current is constant and the voltage over
the arc is regulated; while with constant-potential systems, the voltage
is constant and the current through the arc is regulated.
(2) With constant-current or series lamps, the line must never be
opened, but must be shunted around the lamp if a lamp is to be cut out.
(3) With constant-potential lamps, the lamp must never be shunted but
the circuit must be opened.
(4) In all cases each lamp should be controlled by a double pole switch.
(5) Constant-potential lamps cannot be operated without resistance in
the circuit; this resistance may be in the lamp itself or outside.
(6) Never handle high tension lamps without insulating yourself from the
ground; and handle live wires only with one hand at a time.
(7) Provide spark arresters for all open-arc lamps in the vicinity of
inflammable material.
(8) Never leave a lamp without globes where the wind can strike it. It
will be blown out or feed often, thus consuming the electrodes very fast
and at the same time yielding a very poor light.
Green light emitted by the lamp will indicate that the electrode
holders are burning. Strong shadows cast upwards indicate a lamp burning
“upside down”. The positive electrode retains heat longer than the
negative. The quality and size of electrodes has much to do with
successful operation. Always use the kind of electrodes recommended by
the maker of the lamp.
Direct-current arc lamps do not require much in the shape of reflectors
as they naturally throw most of the light downward, when the upper
electrode is positive. They should as a rule be suspended high.
Alternating-current arc lamps throw most of the light from the upper
electrode slightly below the horizontal and that from the lower
electrode somewhat above. If the light is wanted in a downward
direction, suitable reflectors must be provided.
_Testing of Arc Lamps._--The constant-potential arc lamp is usually
designed for a certain current and voltage. The enclosed arcs as a rule
operate singly on 110 volts, while open arcs are run two in series on
the same voltage. In order to test and see that the voltage and current
are right, an ammeter and a voltmeter are needed. The current and
voltage can both be adjusted by altering the resistance, which is always
in series with such lamps. To get the correct voltage over the arc, be
sure to connect the voltmeter to the two electrode holders so as to
eliminate any other potential drops that may affect the reading.
_Testing Carbons._--The color of the light and the steadiness of it can
of course only be determined by actual operation tests. The arc obtained
by using large electrodes with low current density is liable to rotate
around the electrodes, burn unsteadily, and flicker. This is due to the
fact that the arc tends to establish itself at the point of least
resistance. In order that the arc may burn uniformly, the current
density must be great enough to force all of the electrode points into
use.
As a rule the best electrode is the one that has the longest range from
the low voltage point of hissing to the high voltage point of flaming.
With such an electrode the greatest range in light can be obtained
without either the hissing or the flaming.
The same qualities that give an electrode long range, as above, also
indicate its purity and if we make a test for range, we shall therefore
at the same time make a test for purity.
The test for range can be carried out by any ordinary hand-feed lamp. To
make it, the electrodes are inserted and allowed to burn until their
points have assumed the proper shape. The arc can then be shortened
until the familiar hissing sound is heard. Note the voltage at which
this occurs, being careful to have the voltmeter connected so as to get
the voltage across the arc only. Now separate the electrodes slowly
until they begin to flame and note this voltage. Ordinarily the hissing
voltage will be about 42 and the flaming voltage about 62. The greater
the difference between the two, the better the carbons are assumed to
be. In making comparative tests on electrodes in this manner, care
should be taken that all of the conditions of current and size of
electrodes be the same.
The test for comparative life of electrodes is best made by arranging
the different electrodes so that the same current will pass through each
for the same length of time. If this is done, all that is necessary is
to weigh the electrodes before and after burning. The approximate useful
life of an electrode can be easily determined by burning it for a stated
length of time, noting the length consumed and comparing it with the
length available for burning.