Editing Germicidal lamp (section)

==Low-pressure mercury lamps==
Low-pressure mercury lamps are very similar to a [[fluorescent lamp]], with a [[wavelength]] of 253.7&nbsp;nm (1182.5&nbsp;THz).

The most common form of germicidal lamp looks similar to an ordinary fluorescent lamp but the tube contains no fluorescent [[phosphor]]. In addition, rather than being made of ordinary [[borosilicate glass]], the tube is made of [[fused quartz]] or [[Vycor|vycor  7913]]<ref name="vycor7913">{{cite web |title=Corning Vycor® 7913 UV-Transmitting Glass |url=http://www.matweb.com/search/datasheettext.aspx?matid=748 |website=www.matweb.com |publisher=Corning |access-date=3 December 2019}}</ref> glass. These two changes combine to allow the 253.7&nbsp;nm ultraviolet light produced by the [[Mercury (element)|mercury]] arc to pass out of the lamp unmodified (whereas, in common fluorescent lamps, it causes the phosphor to [[Fluorescence|fluoresce]], producing [[visible light]]). Germicidal lamps still produce a small amount of visible light due to other mercury radiation bands.

An older design looks like an [[incandescent lamp]] but with the envelope containing a few droplets of mercury. In this design, the incandescent filament heats the mercury, producing a vapor which eventually allows an arc to be struck, [[short circuit]]ing the incandescent filament.

As with all [[gas-discharge lamp]]s, low- and high-pressure mercury lamps exhibit [[negative resistance]] and require the use of an external [[ballast (electrical)|ballast]] to regulate the current flow. The older lamps that resembled an incandescent lamp were often operated in series with an ordinary 40&nbsp;W incandescent "appliance" lamp; the incandescent lamp acted as the ballast for the germicidal lamp.