On the Starting of Hg Vapor Discharge Tubes

Abstract

In a commercial method of starting the Cooper Hewitt Hg vapor lamp, a current of about 0.7 amp. is first made to flow in a circuit containing an inductance and a Hg vacuum switch or ``shifter.'' This shifter is then opened magnetically and the resulting arc therein is unstable and begins to go out. But the inductance acts to keep the current flowing and develops a high voltage which is impressed as a negative kick of about 1000 volts, in the average case of starting, on the Hg pool cathode of the main tube. This kick, as a result of the action of a strip of tinfoil on the outside of the cathode bulb surrounding the Hg pool edge and connected to the main anode, starts the arc. The whole process results in the transfer of nearly the entire current from the shifter arc to the main arc in a time of the order of a few microseconds. The initiation of the cathode spot is attributed to the action of a high electric field at the Hg pool edge existing between this edge and the tinfoil on the outside of the glass. In accord with this theory is the fact that anything which can be done to cause the Hg to wet the glass at the pool edge at a number of points (thereby increasing the field) such as by baking a little carborundum powder on the glass at the edge, helps starting enormously. If the anode of the tube is disconnected, simple electroscopes show that the walls of the tube everywhere become charged to a negative potential of the order of 10,000 volts as a result of the kicks. High speed electrons are thus evidently shot up the tube. Ionization produced by these probably forms the beginning of the positive column. After initiation, the current in the tube rises to a maximum at a rate limited by the inductance in the circuit and then decreases to a low value sometimes going out altogether. This decrease is shown to be due to an extreme deficiency of Hg vapor in the tube caused by the electrical clean-up of Hg on the walls. Violent voltage surges set in across the tube at this time and a hissing noise is heard. The region of aggravated low pressure can be seen as glowing weak and reddish compared to the blue white of the rest of the tube. This appearance vanishes when, as a result of the rising vapor pressure due to heating, the hiss and surges stop. The surge period may endure many seconds in cold weather before the arc is able to pick up.