Mercury Arc Rectifiers

 by Damyy L

A Rectifier is a device that primarily serves  to convert high alternating current to direct current. Alternating current is  the flow of charge in a backward and forward direction while direct current is the flow of charge in one direction only . Mercury Arc Rectifiers were used extensively in the 18th century and it has a significant impact on the history of electrical converters in the electricity industry.

It was a very popular device until the 1970s, because of the invention of solid state thyristors and many other solid state devices, predominantly because solid state devices are more reliable, they cost less to maintain, and pose less environmental risk .

The mercury arc rectifier was discovered in 1902 by an electrical engineer from the United States; Peter Cooper Hewitt. The operation of the Mercury Arc Rectifier relies on an electrical arc discharge between electrodes  in a sealed envelope containing mercury vapour at very low pressure.The cathode used in the rectifier setup has a distinctive characteristic given that it is made up of a pool of liquid mercury, allowing it to function as a self-renewing cathode that does not deteriorate over time.

The mercury arc rectifier as the name implies makes use of the chemical element Mercury. 

Mercury is a metal  which means it can conduct electricity, it has two valence electrons that are free to move.

How it Works:

A mercury arc rectifier is simply made up of a cathode and anodes; the cathode is a pool of mercury liquid, and the anode, which is connected to the source of alternating current, is made of carbon or steel, which is essentially positively charged.The cathode is heated, causing the mercury in its liquid state to vapourize. The anode then attracts the negatively charged electrons, while the ionised mercury vapour returns to the cathode.This creates an electric arc an discharge between  the two electrodes .Due to this process, electrons can only flow through the tube in one direction—from the cathode to the anode. 

There are two main types of Mercury Arc Rectifiers; glass tube and steel valve

Glass tube rectifier:

Glass tube rectifier uses a glass bulb-like setup, with two or more  anodes that are typically connected to an AC current. As previously explained, it functions as follows: The liquid mercury is ignited with an external source (excitation cathode) ; it turns to vapour and rises within the bulb. The carbon-based anodes then attract the negatively charged electrons from the vapour and this  rectifies the alternating current. The ionised vapour condenses and returns to the surface of the cathode , this process also aids in maintaining the high temperature of the cathode.

Steel Valve Rectifier:

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According to the website detailed-pedia, steel valve rectifiers are used when dealing with  larger valves, it is made up of a steel tank with ceramic insulators for the electrodes , with a vacuum pump system to counteract slight leakage of air into the tank around imperfect seals.

 

The mercury arc rectifier was an invaluable instrument in the 18th and 19th centuries, particularly in the field of power electronics because it could generate  direct current, which was required at the time by numerous industries, particularly in the domains of radio transmission and electrolysis . Even the first batteries used to recharge electric cars were made using mercury arc rectifiers. Mercury arc rectifiers have since been replaced by semiconductors and solid state devices, but this was an inventive and useful technology nonetheless.