Early Arc Lighting
The first industrial arc lights used carbon electrodes and the electrical discharge was
in open air.
By the early 1800s low pressure gas discharge lamps had become
available. Still only suitable for industrial use, they did produce large
amounts of light. Shown here is a 400W medium pressure mercury (MA/V) lamp. As
can be seen, the light output is a blue-green colour. The lamp also took
about 7 minutes to run up to full operating temperature. These lamps operated
hanging vertically but could be operated vertically if a magnetic deflector was
used to force the arc away from the inner glass wall.
The MA/V image on the left is shown at start-up. The 'mouse-over' shows the lamp at full running temperature. Note how the
arc shape changes with the internal vapour pressure. Unfortunately the images don't show how much brighter the arc has become at full running temperature.
The second lamp shown here developed similarly to the mercury lamp but it took the invention of a glass which was resistant to sodium to allow the lamp to go into production.
Using metallic sodium these low pressure lamps produced a yellow light making them
monochromatic. Due to this and their high efficiency they were commonly used in street
lighting. The arc tube is housed in a "Dewar" or vacuum outer jacket. This reduces heat loss and helps the lamp
maintain the high temperature required to evaporate sodium. The early SO/H lamp's outer jacket was reusable and this kept re-lamping costs down. However moisture could
become trapped in the jacket during use leading to starting issues. As a result this type of sodium lamp became obsolete in the 1960s with the introduction of the SOI/H lamps.
The video shows the construction of the SO/H lamp.
