An arc lamp or arc light is a lamp that produces light by an electric arc. The carbon arc light,
which consists of an arc between carbon electrodes in air, invented by Humphry Davy in the early
1800s, was the first practical electric light. It was widely used starting in the 1870s for
street and large building lighting until it was superseded by the incandescent light in
the early 20th century. It continued in use in more specialized applications where a high
intensity point light source was needed, such as searchlights and movie projectors until
after World War II. The term is now used to refer to gas discharge
lamps, which produce light by an arc between metal electrodes through an inert gas in a
glass bulb. The common fluorescent lamp is a low-pressure mercury arc lamp. The xenon
arc lamp, which produces a high intensity white light, is now used in many of the applications
which formerly used the carbon arc, such as movie projectors and searchlights.
Operation An arc is the discharge that occurs when a
gas is ionized. A high voltage is pulsed across the lamp to "ignite" or "strike" the arc,
after which the discharge can be maintained at a lower voltage. The "strike" requires
an electrical circuit with an igniter and a ballast. The ballast is wired in series
with the lamp and performs two functions. First, when the power is first switched on,
the igniter/starter sets up a small current through the ballast and starter. This creates
a small magnetic field within the ballast windings. A moment later the starter interrupts
the current flow from the ballast, which has a high inductance and therefore tries to maintain
the current flow; it cannot, as there is no longer a 'circuit'. As a result, a high voltage
appears across the ballast momentarily - to which the lamp is connected, therefore the
lamp receives this high voltage across it which 'strikes' the arc within the tube/lamp.
The circuit will repeat this action until the lamp is ionized enough to sustain the
arc. When the lamp sustains the arc, the ballast
performs its second function, to limit the current to that needed to operate the lamp.
The lamp, ballast and igniter are rated matched to each other; these parts must be replaced
with the same rating as the failed component or the lamp will not work.
The colour of the light emitted by the lamp changes as its electrical characteristics
change with temperature and time. Lightning is a similar principle where the atmosphere
is ionized by the high potential difference between earth and storm clouds.
The temperature of the arc in an arc lamp can reach several thousand degrees Celsius.
The outer glass envelope can reach 500 degrees Celsius, therefore before servicing one must
ensure the bulb has cooled sufficiently to handle. Often, if these types of lamps are
turned off or lose their power supply, one cannot restrike the lamp again for several
minutes. However, some lamps can be restruck as soon as they are turned off.
The Vortek water-wall plasma arc lamp, invented in 1975 by David Camm and Roy Nodwell at the
University of British Columbia Vancouver, Canada, made the Guinness Book of World Records
in 1986 and 1993 as the most powerful continuously burning light source at over 300 kW or 1.2
million candle power. Carbon arc lamp
In popular use, the term arc lamp means carbon arc lamp only. In a carbon arc lamp, the electrodes
are carbon rods in free air. To ignite the lamp, the rods are touched together, thus
allowing a relatively low voltage to strike the arc. The rods are then slowly drawn apart,
and electric current heats and maintains an arc across the gap. The tips of the carbon
rods are heated and the carbon vaporizes. The carbon vapor in the arc is highly luminous,
which is what produces the bright light. The rods are slowly burnt away in use, and the
distance between them needs to be regularly adjusted in order to maintain the arc. Many
ingenious mechanisms were invented to effect this automatically, mostly based on solenoids.
In one of the simplest mechanically-regulated forms the electrodes are mounted vertically.
The current supplying the arc is passed in series through a solenoid attached to the
top electrode. If the points of the electrodes are touching the resistance falls, the current
increases and the increased pull from the solenoid draws the points apart. If the arc
starts to fail the current drops and the points close up again. The Yablochkov candle is a
simple arc lamp without a regulator, but it has the drawbacks that the arc cannot be restarted
and a limited lifetime. History
The concept of carbon-arc lighting was first demonstrated by Sir Humphry Davy in the early
19th century, using charcoal sticks and a 2000-cell battery to create an arc across
a 4-inch gap. He mounted his electrodes horizontally and noted that, because of the strong convection
flow of air, the arc formed the shape of an arch. He coined the term "arch lamp", which
was contracted to "arc lamp" when the devices came into common usage.
The arc lamp provided one of the first commercial uses for electricity, a phenomenon previously
confined to experiment, the telegraph, and entertainment.
Carbon-arc lighting in the U.S.
In the United States, there were attempts to produce arc lamps commercially after 1850
but the lack of a constant electricity supply thwarted efforts. Thus electrical engineers
began focusing on the problem of improving Faraday's dynamo. The concept was improved
upon by a number of people including William Staite and Charles F. Brush. It was not until
the 1870s that lamps such as the Yablochkov candle were more commonly seen. In 1877, the
Franklin Institute conducted a comparative test of dynamo systems. The one developed
by Brush performed best, and Brush immediately applied his improved dynamo to arc-lighting
an early application being Public Square in Cleveland, Ohio, on April 29, 1879. In 1880,
Brush established the Brush Electric Company. The harsh and brilliant light was found most
suitable for public areas, such as Cleveland's Public Square, being around 200 times more
powerful than contemporary filament lamps. The usage of Brush electric arc lights spread
quickly. Scientific American reported in 1881 that the system was being used in:
800 lights in rolling mills, steel works, shops, etc.
1,240 lights in woolen, cotton, linen, silk, and other factories
425 lights in large stores, hotels, churches, etc.
250 lights in parks, docks, and summer resorts 275 lights in railroad depots and shops
130 lights in mines, smelting works, etc. 380 lights in factories and establishments
of various kinds 1,500 lights in lighting stations, for city
lighting, etc. 1,200 lights in England and other foreign
countries. A total of over 6,000 lights which are actually
sold There were three major advances in the 1880s:
The arcs were enclosed in a small tube to slow the carbon consumption.
Flame arc lamps were introduced where the carbon rods had metal salts added to increase
light output and produce different colours. František Křižík invented a mechanism
to allow the automatic adjustment of the electrodes. In the U.S., patent protection of arc-lighting
systems and improved dynamos proved difficult and as a result the arc-lighting industry
became highly competitive. Brush's principal competition was from the team of Elihu Thomson
and Edwin J. Houston. These two had formed the American Electric Corporation in 1880,
but it was soon bought up by Charles A. Coffin, moved to Lynn, Massachusetts, and renamed
the Thomson-Houston Electric Company. Thomson remained, though, the principal inventive
genius behind the company patenting improvements to the lighting system. Under the leadership
of Thomson-Houston's patent attorney, Frederick P. Fish, the company protected its new patent
rights. Coffin's management also led the company towards an aggressive policy of buy-outs and
mergers with competitors. Both strategies reduced competition in the electrical lighting
manufacturing industry. By 1890, the Thomson-Houston company was the dominant electrical manufacturing
company in the U.S. Nikola Tesla received U.S. Patent 447920, "Method of Operating Arc-Lamps",
that describes a 10,000 cycles per second alternator to suppress the disagreeable sound
of power-frequency harmonics produced by arc lamps operating on frequencies within the
range of human hearing. Around the turn of the century arc-lighting
systems were in decline, but Thomson-Houston controlled key patents to urban lighting systems.
This control slowed the expansion of incandescent lighting systems being developed by Thomas
Edison's Edison General Electric Company. Conversely, Edison's control of direct current
distribution and generating machinery patents blocked further expansion of Thomson-Houston.
The roadblock to expansion was removed when the two companies merged in 1892 to form the
General Electric Company. Arc lamps were used in some early motion-picture
studios to illuminate interior shots. One problem was that they produce such a high
level of ultra-violet light that many actors needed to wear sunglasses when off camera
to relieve sore eyes resulting from the ultra-violet light. The problem was solved by adding a
sheet of ordinary window glass in front of the lamp, blocking the ultra-violet. By the
dawn of the "talkies", arc lamps had been replaced in film studios with other types
of lights. In 1915, Elmer Ambrose Sperry began manufacturing his invention of a high-intensity
carbon arc searchlight. These were used aboard warships of all navies during the 20th century
for signaling and illuminating enemies. In the 1920s, carbon arc lamps were sold as family
health products, a substitute for natural sunlight.
Arc lamps were superseded by filament lamps in most roles, remaining in only certain niche
applications such as cinema projection, followspots, and searchlights. Even in these applications
conventional carbon arc lamps are being pushed into obsolescence by xenon arc lamps.
See also
References
Bibliography Braverman, Harry. Labor and Monopoly Capital.
New York: Monthly Review Press. MacLaren, Malcolm. The Rise of the Electrical
Industry during the Nineteenth Century. Princeton: Princeton University Press.
Noble, David F.. America by Design: Science, Technology, and the Rise of Corporate Capitalism.
New York: Oxford University Press. pp. 6–10. Prasser, Harold C.. The Electrical Manufacturers.
Cambridge: Harvard University Press. External links
Arc Lamp - Interactive Java Tutorial National High Magnetic Field Laboratory