Mercury Vapor Lamp ( 1901 ) | Hewitt pioneers fluorescent lighting

 "Experience is a dim lamp, which only lights the one who bears it." Louis-Ferdinand Celine, writer and physician

During the latter half of the nineteenth century, there was considerable interest in the effects of electric currents being passed through gases at low pressures confined in glass tubes. Germans Julius Plucker and Heinrich Geissler noticed that if vaporized mercury was used as the gas, a bluish glow was emitted.

 

A low pressure mercury vapor lamp ( right ) alongside three improved high pressure lamps from the 1930s.

An electrical engineer from the United States, Peter Cooper Hewitt ( 1861-1921 ), began to experiment with these mercury tubes and produced ones that emitted a great deal of an unflattering bluish-green light. Some of this light was in the ultraviolet and thus could not be seen. The fact that there is no emission in the red meant that the light made people appear like "bloodless corpses." The dangerous effects of the ultraviolet emission could be alleviated by coating the tube with a fluorescent chemical that absorbed the ultraviolet and then emitted the energy at higher wavelengths.

Hewitt patented his mercury vapor lamp in 1901 and formed a company with the American entrepreneur George Westinghouse to produce the lamps. Although, their primary use at the time was industrial and in photographic studios, Hewitt's mercury lamp was actually the precursor to modern fluorescent lighting.

Mercury lamps are cheap to make and have a long life. It takes quite a while for the mercury to warm up and auxiliary electrodes are needed to start the excitation. Mercury lamps have a rather low luminous efficiency when compared to other discharge lamps. They also have the disadvantage in that they cannot be switched on and off quickly.