Ans. Because, on account of the arc produced, they would gradually wear away and could not be replaced conveniently.
In the case of mercury, the excess vapor is condensed to liquid form in the large glass bulb or condensing chamber of the tube and gravitates back to the cathode, where it is used over and over again.
Fig. 2,119.—General Electric series mercury arc rectifier outfit; view showing method of replacing a tube. The illustration also shows tube carrier and drip tray.
Ques. In the operation of rectifiers, how is the heat generated in the bulb dissipated?
Ans. In small rectifier sets the heat generated is dissipated through the tube to the air, and in large tubes such as used in supplying 40 to 60 kw. for constant current flaming arc lights operating at 4 or 6.6 amperes, the tubes are immersed in a tank of oil, and cooled similar to the arrangement used for oil insulated water cooled transformers.
Ques. What results are obtained with oil cooled tubes?
Ans. In practice it is found that the life of oil cooled tubes is greatly increased and temperature changes do not affect the ability to start up as in the air cooled tubes.
