Mercury Arc Rectifiers

Mercury arc rectifiers (also known as Cooper-Hewitt or Hewittic rectifiers) were extensively used to provide DC in high power applications, powers ranging from kilowatts up to a few megawatts, at voltages ranging from 110V to 30KV. Their operation is based on the discovery that an arc between a pool of mercury and a metal anode only allows current to pass in one direction. Multiple anodes are typically used, fed from a multiple-phase transformer, the arc jumping from the cathode pool to each anode in sequence. There may be three, six or even twelve transformer phases, each feeding one anode. Six and twelve-phase systems used star-connected three-phase transformers with interphase transformers between the star common connections.

Construction is either a glass bulb, as shown in the pictures below, cooled by an external fan or water jacket, or a steel tank for very large units with capacities above about 500 amps. 

To initiate the arc, an ignitor is used, typically consisting of an electrode which can be dipped in the mercury pool using an external electromagnet. The ignitor draws a small spark to ionise the mercury vapour, initiating the main arc.

Mercury arc rectifiers have now been made totally obsolete by semiconductors, although there are a few still in service in old installations. I recently (Jan 2002) saw a working installation on display at the Museum of Transport & Technology in Auckland, New Zealand, where they use two banks of 4 big glass MARs to provide DC to run their tram service - you could see the arc glow intensity changing in intensity as the tram changed speed!

New! Want to know more ? Check out these Detailed articles on Mercury Arc Rectifiers from old engineering publications. Also see this advertisment for a small 'Nevitron' single-pase MAR. More technical info in this 1964 book

Installation and Maintainance instructions for Hackbridge & Hewittic Group Ltd. MARs :
Small single-bulb types 40/100,100/200 and 100/200ET     Large single-bulb types ST     Group bulb units type GT and 1/2GT

German MAR page (Incidentally, the German for MAR is "quecksilberdampfgleichrichter")

Thanks to Jake Purches for this interesting snippet : I was talking about Mercury Arc rectifiers with my neighbour who is a former electrical engineer, now 85 years old. One of his jobs was to install Mercury arc rectifiers and he explained to me how they came delivered upside down, with the mercury, a quart in all, would be sitting in the large glass bulb. He said the trick was to invert the bulb very carefully to make sure the mercury would flow into the base cap and NOT into the rectifier arms! If the mercury did enter an arm, the weight of it would snap it off and a quart of mercury would be shooting out all over the place, which did happen occasionally. This job was left to the experienced chaps of course. The MARs themselves would be delivered in crates that had a hammock arrangement so that the MAR bulb would be hanging, or suspended to avoid shock. The light from them was extremely bright of course, and probably very strong in the UV spectrum too.

The picture below is the tiny mercury arc rectifier in my glassware collection, which is from a welder (bulb approx. 4" diameter). This had an outer electrode consisting of a conductive coating, most of which has fallen off. I believe this was used as an ignitor, presumably with a high voltage impulse or RF drive to ionise the mercury vapour and initiate the arc. If anyone knows the rating  of this type of bulb, please let me know!

Big glass-bulb mercury arc rectifiers : 3 phase unit approx. 18" high, Siemens Museum, Munich (Left), and  6-phase rectifier, approx. 36" high,  Deutsches Museum, Munich.

Circuit diagram of mercury-arc rectifier unit.

(left) Large 12-phase steel-tank rectifers, each supplying  4000 Amps at 420V, English Electric Co., (right) working rectifier on display at the Cambridge Museum of  Technology, UK

Thanks to Nick Catford for the picture below, showing one of two MARs still in service (Jan 2001), powering a lift in Belsize Park, London. The blade of the cooling fan can be seen immediately below the bulb.

Thanks to  'nasadowsk' for the following : The NYC subway system still uses these in a few locations, but is converting to solid state. They also use mechanical AC-DC conversion on some of the oldest (90+ years) lines. A number of railroads in the US  experimented with mercury Arcs (commonly called ignitrons* here), notebly the Virginian, Pensylvainia, and New Haven. The idea was to use
commonly available 60hz ac power, instead of the 25 hz that A.C. electric trains required at the time. Early applications were freight, but later passenger trains had them. New haven's stuff was plagued by complexity (the trains had to support not only 11kv 25hz AC overhead, but 600V dc third rail), and fires (this is bad publicity, and rather annoying too). Ultimately, they fixed these problems, but by then, solid state equipment was available.
But the advantages of rectifer locomotives and trains weren't had - Virginian depowered in the 60's, New Haven depowered all but 2 lines, and the PRR never converted to 60 hz, and ultimately abandoned rectifers for all but suburban lines. Today, only Metro-North, which now runs the former New Haven line, takes full advantage of the technology - they converted to 60hz AC in 1983. (Note on power: the US uses 60hz AC, instead of 50 like the rest of the world)
* Ignitrons are controllable mercury-arc rectifiers, which operate rather like modern thyristors, using a trigger signal to control a large current. They are still used in some very high power applications.

From Stephan Meyn : I recently ran into this Mercury Arc Rectifier in a museum in Kandos, NSW Australia. It was used to charge the batteries for the coal mine's electric engines. As you can see by the person next to it it is about 4 foot in size.

Another type of small mercury-arc rectifier - the 'Rectigor'

Thanks to Anthony Richards for this picture of one of 28 Steel-tank MARs still in service (Jun 2002) driving 1200hp motors in a bar rolling mill in Newcastle, Australia.

Thanks to Robert Reay for this pic from an old book of a portable MAR based DC supply unit. I suspect moving it whilst operating would have been a bad idea due to the mercury sloshing about....

The excellent Amberley Museum has recently acquired a MAR with all its control gear, made by Hackbridge & Hewittic Electric, and are planning to get it into a working condition for demonstrations. Remarkably, it was in service up until January 2005, a working life of well over 50 years! This was part of a battery charging system at a power substation at Croydon (South London).

Rectifier in cubicle.

Part of the battery control gear - a semi-automatic tap-changer (this unit is about six feet high). Two huge solenoids operate a rotary switch arrangement, which includes resistance coils to keep the current flowing as the switch moves between contacts.

Operating instructions - click image to see full-res version.

Tungar rectifiers

These were small gasfilled rectifiers, used for low-voltage applications such as battery chargers. The one pictured below is marked "Edison-Swan Electric 68506". The small black device shown underneath is the modern semiconductor equivalent!