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Research & Development A short history of the mercury-arc rectifier

Editor: Florian Richert

Invented in 1902, the mercury-arc rectifier made it possible to supply high-voltage di-rect current (HVDC) for major operations, such as powering radio transmitters and the New York subway network. Although mercury-arc rectifiers were gradually phased out in the 1970s, their legacy is proving extremely valuable for the renewable energy sec-tor.

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How did mercury-arc rectifiers change the world?
How did mercury-arc rectifiers change the world?
(Source: Public Domain / Unsplash)

What is a mercury-arc rectifier?

The mercury-arc rectifier is used to convert (or rectify) alternating current into direct current. The basic form is a glass bulb valve featuring one or more arms with steel or carbon anodes, and a cathode consisting of a self-restoring pool of liquid mercury. This liquid cathode sets it apart from many other types of rectifiers, making it more durable and capable of transmitting much higher electrical currents. The AC heats up and va-porizes the mercury, causing the current to travel through the vapor to the other side of the bulb. The current is prevented from traveling backwards, so the effect is like that of a diode.

A brief history

The mercury-arc rectifier was invented by US electrical engineer Peter Cooper Hewitt at the turn of the 20th century. This revolutionary device is also known as the Hewittic rectifier, mercury-arc valve or mercury-vapor rectifier. Hewitt further refined his origi-nal glass bulb design and developed the steel tank variant in 1908. These large-scale rectifiers were primarily used for a range of high-voltage direct current tasks, such as powering industrial engines, radio transmitters and traction motors for subways and streetcars. Mercury-arc rectifiers were also used to recharge the batteries of early electric cars in the late 18th and early 19th centuries, before the combustion engine took over.

Mercury-arc rectifiers were the leading technology for high-voltage rectification until they were superseded by semiconductor rectifiers in the 1970s. The phase-out didn’t happen overnight, though: mercury-arc rectifiers were still being used to power the New York subway system well into the 1990s.

How did mercury-arc rectifiers change the world?

Mercury-arc rectifiers made high-voltage direct current (HVDC) a viable technology. This was a significant achievement, as HVDC has a higher transient efficiency and is less expensive than high-voltage alternating current (HVAC) when it comes to transfer-ring bulk electricity over long distances. Mercury-arc rectifiers are placed at either end of the long-distance system. They convert AC into HVDC in preparation for the journey through an underground or underwater cable, then convert it back into AC at the other end. The result is a kind of ‘electricity superhighway’ that offers more stability, reliabil-ity and transmission capacity than AC transmission systems.

In the late 1970s, HVDC system manufacturers switched to solid-state devices such as thyristor valves, because they had no inverse arc faults and could predict performance far more accurately than their mercury-arc predecessors. Thyristors are also easier to produce, operate, test, and repair.

HVDC in the renewable energy sector

The benefits of HVDC make this technology particularly suitable for the growing renew-able energy sector. Renewable energy is often farmed in remote locations and has to travel vast distances to reach the cities and towns where it is consumed. Examples include offshore wind turbines, solar panels in arid, inhabited locations, and hydroelec-tricity plants in mountainous regions. HVDC enables clean energy to reach its destina-tion with fewer lines and minimal loss of energy.
Mercury-arc rectifiers and HVDC are also playing a key role in the global shift towards continental-scale power grids. According to Circuit Digest, the evolution of the global energy economy will likely make ultra-high-voltage direct current transmission systems increasingly important. In 2018, the HVDC market was valued at USD 7.4 billion, while a Future Market Insights report predicts it will reach USD 14.36 billion by the end of 2026—no doubt spurred on by the positive trends in the renewables sector.