Mercury-arc valve

As a result mercury-arc valves, when used as intended, are far more robust and durable and can carry much higher currents than most other types of gas discharge tube.

Mercury-arc rectifiers or "converters" were used for charging storage batteries, arc lighting systems,[6] the DC traction motors for trolleybuses, trams, and subways, and electroplating equipment.

Operation of the rectifier relies on an electrical arc discharge between electrodes in a sealed envelope containing mercury vapor at very low pressure.

The mercury ions are attracted towards the cathode, and the resulting ionic bombardment of the pool maintains the temperature of the emission spot, so long as a current of a few amperes continues.

While the current is carried by electrons, the positive ions returning to the cathode allow the conduction path to be largely unaffected by the space charge effects which limit the performance of vacuum tubes.

Hot-cathode, gas discharge tubes such as the thyratron may also achieve similar levels of efficiency but heated cathode filaments are delicate and have a short operating life when used at high current.

The temperature of the envelope must be carefully controlled, since the behaviour of the arc is determined largely by the vapor pressure of the mercury, which in turn is set by the coolest spot on the enclosure wall.

Development of high-current rectifiers required leadwire materials and glass with very similar coefficients of thermal expansion in order to prevent leakage of air into the envelope.

Arc-backs can be damaging or destructive to the valve, as well as creating high short-circuit currents in the external circuit, and are more prevalent at higher voltages.

One example of the problems caused by backfire occurred in 1960 subsequent to the electrification of the Glasgow North Suburban Railway where steam services had to be re-introduced after several mishaps.

The solution was found to be to include grading electrodes between the anode and control grid, connected to an external resistor-capacitor divider circuit.

However, the tall porcelain column required to house the grading electrodes was more difficult to cool than the steel tank at cathode potential, so the usable current rating was limited to about 200–300 A per anode.

The direct current produced by a single-phase rectifier thus contained a varying component (ripple) at twice the power supply frequency, which was undesirable in many applications for DC.

The solution was to use two-, three-, or even six-phase AC power supplies so that the rectified current would maintain a more constant voltage level.

Three phase operation can improve the efficiency of the transformer as well as providing smoother DC current by enabling two anodes to conduct simultaneously.

As solid-state metal rectifiers became available for low-voltage rectification in the 1920s, mercury arc tubes became limited to higher voltage and especially high-power applications.

In the 1960s, solid-state silicon devices, first diodes and then thyristors, replaced all lower-power and lower voltage rectifier applications of mercury arc tubes.

Several electric locomotives, including the New Haven EP5 and the Virginian EL-C, carried ignitrons on board to rectify incoming AC to traction motor DC.

The largest ever mercury-arc rectifiers, built by English Electric, were rated at 150 kV, 1800 A and were used until 2004 at the Nelson River DC Transmission System high-voltage DC-power-transmission project.

Mercury arc valves were used extensively in DC power systems on London Underground,[14] and two were still observed to be in operation in 2000 at the disused deep-level air-raid shelter at Belsize Park.

This led to the mercury-arc rectifier at the Goodge Street shelter featuring in an early episode of Doctor Who as an alien brain, cast for its "eerie glow".

[16] Auckland's Museum Of Transport And Technology (MOTAT) still employs a Mercury arc valve to provide power to the tram which carries visitors between its two sites.

Steel tank rectifiers frequently required vacuum pumps, which continually emitted small amounts of mercury vapor.