The arc lamp was the earliest type of electric illuminant. Early electrical experimenters found that if two carbon rods were connected in an electric circuit and the circuit was closed by touching the tips of these rods together, upon separating the carbons, the current continued to flow across the gap, forming an arc that produced light. Sir Humphrey Davy, in 1801, first demonstrated an elementary form of an arc lamp using a battery of 2,000 cells, although the first commercial application was not until 1876. In that year, Charles Brush invented the open carbon arc and applied it to lighting streets; this was several years before Edison's first commercial incandescent lamp. Arc lamps depended on the tendency of an electric current to continue flowing across a gap after the circuit was broken. This principle is similar to the flash of flame at the opening of a knife switch or circuit breaker in a circuit carrying a heavy current. The early types of arc lamps employed two carbon electrodes, held end to end. To start the lamp, power was applied and the circuit broken by pulling the electrodes apart, and the arc formed in the gap between the electrodes. As the arc lamp burned, the electrodes were consumed and required periodic replacement. The electrodes' life depended on their chemical composition - at first, pure carbon was used which required frequent replacement (some early lamps had duplicate electrodes just to last one night), but later electrodes utilized additional minerals and elements that provided a brighter arc and lasted much longer. The lamp itself was simple - it was little more than a support and feeder mechanism for the two electrodes as well as a mechanism that separated the electrodes when power was applied. In the rare case the lamp was operated on a multiple circuit, additional parts were necessary to limit the current consumed by the lamp to avoid shorting out the circuit.
In general, arc lamps were rugged and adaptable to many uses and had the advantage of relatively high efficiency and concentration of the light source along with some control of the color of light. The necessity of trimming and frequent replacement of the electrodes, and the presence of undesirable combustion byproducts were two main objections to their use. The complicated mechanism within arc lighting luminaries that automatically separated the two carbon rods apart by a specified distance was also a handicap in comparison to the simple convenience and flexibility of incandescent lamps. These factors led to the gradual replacement of arc lights by incandescent lamps for practically all types of general and street lighting service. They were, however, used for years for high-powered searchlight projection and also for motion picture projectors where concentrated, high brightness light sources were essential. Certain types of arc lamps were well adapted to photochemical uses through the decades for blueprinting, photoengraving and commercial photography because of their efficiency in emitting blue and violet light to which photographic materials are most sensitive.
Types of Arc lights - Open and Enclosed
The earliest type of arc lamp was called the open arc because it was operated with the carbon electrodes exposed to the atmosphere. At the time of its introduction it was the most powerful artificial illuminant known and received much recognition. These earlier lamps had many disadvantages, the principal objections being the unsteadiness of the light and the rapid consumption of the carbons (it was not uncommon to replace the carbons every 8 to 10 burning hours). These lamps operated in series at about 50 volts, the number on a single lighting loop limited by the maximum voltage the central station generator could provide.
An improved type was introduced around the mid-1890s known as the enclosed arc where the electrodes operated within a glass globe. Since this inhibited the amount of air that could enter the arc, this had an immediate benefit of dramatically increasing the electrodes' life (typically 100-125 burning hours). The longer life of the electrodes outweighed the fact that these luminaries were less efficient than the open arc lamps. Both types of arc lamps are also known as "crater arcs". The tip of the positive electrode assumed the shape of a hollow crater and emitted about 90 percent of the light from the lamp, relatively little coming from the arc itself or the negative electrode.
It was found in later studies that certain chemicals could be combined with the carbon to form the electrodes that provided a highly luminous arc. This led to the development of the flame-carbon arc, so called because the arc stream provided most of the light. The efficiency of this type of lamp was three times that of the old open-arc lamp and about ten times that of the enclosed type.
Arc Lamp Operation
Arc lamps operated best on circuits operated by special generators or transformers that closely regulated the circuit current at a constant value. In practice, most early series arc currents ranged from 4 to 10 amperes, but the most common circuits operated at 5.5, 6.6 or 7.5 amperes (6.6 amperes would later be established as the standard current for series circuits). The arc voltage of the various types of lamps ranged from 50 to 80 volts and since the lamps were connected in series, the voltage of the system depended on the number of lamps - in practice, the circuit would typically be operated at several thousand volts. These lamps were designed for operation on either direct current or alternating current circuits; the exception was the magnetite arc, which required direct current. In order to operate these magnetite arc lamp circuits, mercury-arc rectifiers were utilized to do the necessary conversion from AC to DC.
In a few rare cases, arc lamps were operated in multiple at secondary distribution voltages. Since the conductivity of the arc is non-linear, it increases greatly with increased current. As the arc heated up, the the ionized gases in the arc decreased in resistance allowing still more current to flow. This was a fundamental characteristic of all arc and electric discharge types of light sources. This is not an issue when operated in series as the power source holds the current at a constant value, however on multiple circuits, additional equipment is required to limit the current available and prevent a short-circuit arc. This additional equipment took the form of a resistance when used on DC circuits or a reactance coil when used on AC circuits, and often consumed just as much energy as the arc itself!
Arc streetlighting had the advantage of higher luminous efficiency than the carbon-filament streetlighting lamps available through the early 1910s. The arcs provided a bright, white, concise light source that was ideal for illuminating large and open areas. Most arc lights were mounted on long truss arms, on span wires over intersections, or in some cases, on high towers where it was desired to illuminate broad areas of town at night. On many of these luminaries, a pulley system was utilized to lower the fixture to make it easier to service the fixture and replace the arc carbons. One company offered special poles with counterbalanced and pivoted arms for the same purpose. Other arc lights were placed upon the tops or fixed to the sides of poles and climbing was required for their maintenance.
Once improved tungsten-filament incandescent lamps were introduced in the early 1910s (particularly the "Mazda C" type), carbon-arc streetlighting went into a rapid decline. They simply could not compete with the incandescent light with its longer life and relatively maintenance-free fixtures (the only maintence needed was relamping), and the light was much steadier. With the right luminaries, incandescent lighting was just as white and almost as intense as the old arc lamps. In addition, they consumed less energy for the same amount of light output. Therefore carbon arc street lighting had became a thing of the past by the late 1920s in most locations in the United States.
Early Arc-lamp Streetlights
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