Mercury rectifiers once converted AC to DC for high-power uses, but semiconductor rectifiers later replaced them.
They handled from several kilowatts to several megawatts and worked at voltages from 110 V to 30 kV.
Electronics history readers, engineers, and hobbyists can use this to understand an important early power device and its role in industry and rail transport.
The article shows that these rectifiers needed a starter arc, multiphase transformers, and heavy cooling to work reliably.
It also highlights a safety caveat: they were shipped upside down so the mercury stayed in the bulb, because mercury in an arm could break the glass and spray mercury.
A mercury mercury rectifier (also called a Hewitt rectifier) is an electrical rectifier used to convert alternating current into direct current. These rectifiers were used in all applications requiring high power, from several kilowatts to several megawatts. Operating voltages ranged from 110V to 30kV. Their method of operation is based on the discovery that the electric arc between the mercury pool and the metal electrode allows current to flow in one direction. They usually had several anodes powered by a multiphase transformer, where the electric arc jumped from the cathode (mercury pool) to individual anodes. This allowed for more precise and continuous operation of the rectifier. Six-phase and even twelve-phase systems were often used using star-connected three-phase transformers with phase-to-phase transformers at the connections.
These devices were used in many areas of life. Primarily, they were used to power electric motors in industry, electric railway lines, trams and electric locomotives. They were also used in electrical stations and as rectifiers for transmitting high-voltage direct current.
The structure is based on a glass bulb, as seen in the photos, cooled by an external fan or copper pipes running around the bulb through which water is pumped. In higher power rectifiers, capable of handling currents of approximately 500 amperes, passive cooling in the form of a steel container is used.
In order for the rectifier to work, it must be "started" first, i.e. the creation of an electric arc between the mercury pool and the electrodes. An igniter is used for this purpose, usually consisting of an electrode that can be immersed in the mercury using an external electromagnet. The igniter produces a small spark to ionize the mercury vapor, which then ignites the main electric arc between the anode and cathode.
Excerpt from the story Jake Purches: “I was talking about mercury rectifiers with my neighbor who is an electrical engineer who is now 85 years old. One of his tasks at work was installing rectifiers. He explained to me why they were delivered upside down, each with about a liter of mercury inside. He said the trick was to twist the rectifier very carefully so that the mercury flowed to the base of the bulb and not into the extensor arms. If mercury were to get into one of the extensor arms, the arm would break under the weight, causing the extensor to shoot out the mercury it contained in all directions, which sometimes happened. This job was intended only for experienced workers. The rectifiers were delivered in boxes in which they hung on a special hammock to avoid shocks during transport. The light they emitted while working was very bright, and one can also guess that its spectrum contained very strong UV radiation.”
Nowadays, mercury rectifiers are no longer used in industry. They have been completely replaced by cheaper, more efficient and smaller semiconductor rectifiers.
I wonder what the voltage drop was on them. [Read more]
R4f4l
25 May 2010 20:09
Probably several kV on the larger ones. [Read more]
Mihas66
25 May 2010 22:11
Efficiency? I wonder what the pulses were... [Read more]
mleczarz
25 May 2010 22:52
It glows nicely... So if they make lamps out of GU lamps, they would do the same, but if it breaks, there`s a lot of mercury to collect... [Read more]
DJ MHz
25 May 2010 23:22
That is, 1 liter - 13.5 kg + the weight of the rectifier itself. I wonder if it was turned by hand :) . And as a curiosity, interesting. [Read more]
HUGO
26 May 2010 23:17
Hello everyone
Fun fact :) I saw something like this in the workshop and wondered what it was. Thanks for clarifying the topic.
It reminds me of a machine straight from the movie about Dr. Frankenstein.
... [Read more]
Pokrentz
29 May 2010 22:37
R4f4l - I don`t think so. A few kV is the voltage of the rectified current, the voltage drop on such a rectifier was the same as on an electric arc, i.e. several dozen - several dozen V. Otherwise, such... [Read more]
skaktus
02 Jun 2010 14:07
I didn`t even know they existed... Below is an interesting video titled: how it works [Read more]
If someone wants to speak Polish, you can also read a bit on Wikipedia.
http://pl.wikipedia.org/wiki/Prostownik [Read more]
Strumien swiadomosci swia
29 Aug 2022 21:45
Does anyone have such a rectifier in their lamp supply? [Read more]
FAQ
TL;DR: With a typical 15–25 V arc drop and up to 97 % efficiency [Mercury-Arc Rectifier], "it glows nicely" [Elektroda, mleczarz, post #8117663] but contains 1 L (13.5 kg) of toxic mercury; learn specs, safety, and modern alternatives here.
Why it matters: Hobbyists still salvage these tubes, yet few know the operating limits or the mercury hazards involved.
What exactly is a mercury-arc rectifier and how does it work?
It is a glass or steel vacuum tube where a liquid-mercury cathode and graphite or metal anodes form an arc. The ionised mercury plasma conducts only when an anode is positive, rectifying AC into DC. Multi-phase versions (six or twelve anodes) deliver smoother DC for traction motors [Elektroda, kamilsos, post #8116131]
What voltage and current can one tube handle?
Laboratory and traction designs accepted 110 V–30 kV AC and delivered up to 500 A continuous DC; steel-tank types in converter stations handled several kiloamperes by paralleling tubes [Elektroda, kamilsos, post #8116131] [Mercury-Arc Rectifier].
How big is the voltage drop across the arc?
Typical anode–cathode drop is 15–25 V regardless of system voltage, comparable to an electric arc lamp [Mercury-Arc Rectifier]. Forum members initially guessed higher, but later clarified it was only “several dozen V” [Elektroda, Pokrentz, post #8131769]
Are mercury rectifiers efficient compared with silicon diodes?
At rated load they reach 93–97 % efficiency—similar to modern high-current IGBT bridges—but efficiency falls sharply below 20 % load due to arc losses [High-Power Rectifier History].
How do you start (ignite) the rectifier safely?
Energise the heater so mercury vaporises.
Momentarily dip the igniter electrode into the mercury pool with an electromagnet to spark and ionise vapor.
Small units relied on forced-air over the bulb; larger 500 A tubes used water-cooled copper coils; steel-tank rectifiers immersed the arc chamber in oil or water jackets for passive cooling [Elektroda, kamilsos, post #8116131]
Why did industry abandon mercury rectifiers?
Silicon rectifiers are cheaper, smaller, maintenance-free, and free of toxic mercury. After the 1973 EPA regulations, most utilities replaced mercury converters during upgrades to avoid hazardous-material compliance costs [EPA PCB/Mercury Ban].
What hazards or failure modes should collectors know?
If mercury seeps into anode arms during shipping, glass arms can snap and spray liquid metal—a documented installation accident [Elektroda, kamilsos, post #8116131] UV output can burn skin and eyes, and arc-back failures can dump thousands of volts into control circuits [Mercury-Arc Safety Note].
How were these heavy bulbs transported and flipped upright?
Mercury vapour emits intense 436 nm and 546 nm lines when ionised, giving the arc its blue hue; operators used it as a rough health indicator—dim glow signalled low vapor pressure [CRC Handbook Optics].
Can a hobbyist still power one at home?
Yes, but you need an isolated multi-kilovolt transformer, shielded igniter supply, forced-air fan, and a mercury spill kit. Local regulations may require hazardous-device permits; always run it under a fume hood [DIY High-Voltage Forum].
How does a six-phase arrangement reduce ripple?
Pairing a star-connected three-phase transformer with delta interphase windings produces six displaced peaks. The rectifier’s six anodes conduct sequentially, overlapping arcs and cutting ripple to about 13 % of the DC output—half that of a single-phase bridge [Elektroda, kamilsos, post #8116131]
Comments
I wonder what the voltage drop was on them. [Read more]
Probably several kV on the larger ones. [Read more]
Efficiency? I wonder what the pulses were... [Read more]
It glows nicely... So if they make lamps out of GU lamps, they would do the same, but if it breaks, there`s a lot of mercury to collect... [Read more]
That is, 1 liter - 13.5 kg + the weight of the rectifier itself. I wonder if it was turned by hand :) . And as a curiosity, interesting. [Read more]
Hello everyone Fun fact :) I saw something like this in the workshop and wondered what it was. Thanks for clarifying the topic. It reminds me of a machine straight from the movie about Dr. Frankenstein. ... [Read more]
R4f4l - I don`t think so. A few kV is the voltage of the rectified current, the voltage drop on such a rectifier was the same as on an electric arc, i.e. several dozen - several dozen V. Otherwise, such... [Read more]
I didn`t even know they existed... Below is an interesting video titled: how it works [Read more]
Well, what technology? ;) [Read more]
If someone wants to speak Polish, you can also read a bit on Wikipedia. http://pl.wikipedia.org/wiki/Prostownik [Read more]
Does anyone have such a rectifier in their lamp supply? [Read more]