Mercury-arc Valve - Operating Principles

Operating Principles

Operation of the rectifier relies on an electrical arc discharge between electrodes in a sealed envelope containing mercury vapor. A pool of liquid mercury acts as a self-renewing cathode that does not deteriorate with time. The mercury emits electrons freely, whereas the carbon anodes emit very few electrons even when heated, thus rectifying action occurs.

Once an arc is formed, electrons are emitted from the surface of the pool, causing ionization of mercury vapor along the path towards the anodes. 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.

Since current is carried both by electrons and ions, the conduction path is largely unaffected by the space charge effects which limit the performance of vacuum tubes, which means that the valve is capable of carrying very high currents at quite low arc voltages of typically 20-30 V and hence is a very efficient rectifier. Other hot-cathode gas discharge tubes such as the thyratron may also achieve similar levels of efficiency but their solid, heated cathode filaments are delicate and have a finite operating life, which can be very short when used continuously 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. A typical design maintains temperature at 40 degrees Celsius and a mercury vapor pressure of 7 millipascals.

The mercury ions emit light at characteristic wavelengths, the relative intensities of which are determined by the pressure of the vapor. At the low pressure within a rectifier, the light appears pale blue-violet and contains much ultraviolet light.