Electrolytic rectifiers are an artwork among old technologies. They were created in the beginning of 20th century and could transform an alternating current in a direct one, only with two pieces of metal in a solution.
The electrolytic consists of two electrodes submerged in a solution, one electrode is made of aluminium, the material of the other is not very important, it can be steel or lead, normally in a solution of ammonium phosphate or borax (the same used nowadays in electrolytic capacitors [WIKI1]) although the solution is not very important too.
Before getting into operation an electrical current should be passed through the cell constructed this way to create an oxide layer around the aluminium, it could be AC (alternating current) or DC (direct current), but if it was a DC the positive should be connected to the aluminium.
The only important thing about the electrolytic rectifier design is the aluminium, so before proceeding about the electrolytic rectifier lets talk about aluminium.
Aluminium is a fascinating metal, and the fact that it is very common makes us forget its amazing characteristics.
Aluminium is very chemically active, an it has more affinity with oxygen than magnesium. Magnesium was used in the old days in disposable flashbulbs, like the ones used in the Kodak Pocket Instamatic 100 camera  (this flashes were called Magicubes). Magnesium burns and is capable of extracting oxygen to burn from water or even from a carbon dioxide atmosphere, and it burns with a white flame.
In the case of aluminium this oxygen affinity is so big that when exposed to air that an oxide layer is formed in its surface so quickly that it isolates it from oxygen, making it more difficult to burn than magnesium.
But in pulverized form and at a high temperature, it burns, that’s why it is used in special effects in movies. In pulverized form together with iron oxide, to supply the oxygen it forms a mixture called thermite, used in incendiary bombs . As the mixture only ignites at high temperatures, the fuse and the bomb body were made of magnesium, that is easier to ignite and generates a high temperature burning.
This oxide formation property is useful in many ways, it is the oxide layer that protects anodized aluminium , in this case the oxide layer is created by an electrolytic process until it gets thick enough to protect aluminium.
This same oxide layer allowed the creation of electrolytic capacitors. Capacitors consist in two metal plates separated by a insulating layer. In a next post I will talk about capacitors; the important thing at the moment is that they are electronic devices that store electrical charge and the bigger the plates area and thinner the insulating layer (called dielectric), the greater their charge storage capacity. In the case of electrolytic capacitors a high capacity is reached by making the dielectric be the thin oxide layer, and winding the plates, to make the biggest area in the smallest possible volume.
That’s why you can’t solder aluminium with tin/lead solder, the oxide layer do not allow the solder not to stick, although you it can be soldered by welding (spot welding or thermite welding for instance) where the metal is fused to make the bond.
Aluminium is not magnetic, this means it is not attracted by a magnet. I remember that in my childhood I was intrigued by this. The house that we lived then had aluminium windows and corners and even the front door was of aluminium. Once I was playing with a magnet and passing it over the aluminium corner I felt a force pushing the magnet, but only when I moved the magnet. After a while I realized what happened, the magnet movement induced a current in the aluminium and this current generated a magnetic field that opposed to the magnet field, repelling it, according Lenz law.
It is the same principle of eddy current brakes used in trains, these are the brakes used for velocity regulation, before using the shoe-type or disk brakes (that ones that you hear squeaking), using an electromagnet instead of a magnet. It is this principle that is used to separate aluminium cans from garbage, using and alternating current instead of a magnet moving, what makes cans jump away.
Aluminium has a high thermal expansion coefficient. In a house with doors and windows with aluminium frames (and remember, I lived in one) you get used to crackling and popping noises, sometimes very loud ones. Aluminium expands with day heat slowly, at night as it cools, tensions accumulate in it, and it comes back to the smaller size at once, creating the popping sounds. In those silly TV shows that you can see at Discovery and Syfy about ghost hunters and hauntings it s common see people complaining about unexplainable cracking and popping noises, and you can clearly see the aluminium frames in the windows.
Aluminium is extracted from bauxite ore, not from chemical reactions, but by an electrolytic process, that consumes a vast amount of electric energy. Hence aluminium being expensive despite being the third most abundant element in Earth’s crust, only behind oxygen and silicon. The electrolytic process to extract aluminium is impressive; carbon electrodes submerged in an mix of cryolite and alumina (extracted from bauxite) kept liquid by an electrical current passage, electrolyzing alumina, making a very loud noise. It is a process that you will not see frequently in video, as the magnetic fields generated interfere with any camera.
Even so, high voltage distribution cables in the highest part of power poles, are made of aluminium with steel inside. They are called ACSR (aluminium conductor steel reinforced), or aluminium with steel core. Aluminium is not as good conductor as cooper, but it is lighter, cheaper (cooper is much more rare in Earth’s crust and despite the energy requirements to produce aluminium, it is still cheaper then cooper) and as it does not oxidizes (or better it oxidizes so quickly that the oxide layer protects it), the aluminium layer provides protection to the steel.
Another last detail about aluminium; at the house we lived, garage gates were made of aluminium, and once someone tried to break in. My parents bought a huge thick iron chain to lock it. After a while my father realized that the chain was rusted only in the contact points between iron and aluminium, the electric potential difference produced by the contact of the two different metals enhanced corrosion .
I saw the same phenomenon several years later, when I moved out from my parents house. We had and electric shower  that broke twice and as the man in the house I got to change the heating element. Both times the heating element opened exactly in the same contact with an aluminium rivet with it. The manufacturer didn’t realized (or didn’t care) that depending on the water characteristics that would be very common, if all the heating element was made of the same metal that would not happen.
It is the opposite process of the so called cathodic protection [WIKI2]. This term disturbed me for a long time, because on the bus stop I took to go to University, at Cardoso de Almeida avenue, was right in front of the cemetery, in front of a metal box in the iron fence where it was written “cathodic protection”. Years later with the Internet (or more precisely Google) I found what was that. In cathodic protection, metal rods or plaques connected the ground are put in contact with fences or gates to avoid corrosion, the small electrical potential generated by the different metal contact acts to avoid corrosion. The rods or plaques, if made of the right metal, are eroded instead of the gates or fences.
This takes us back to the electrolytic rectifier, its working is not so simple as it may seem at the first sight. It is more common the see texts about it in old books [STA] [DEE] (as it is not used anymore) and the best text I ever read about it is: [BRU].
You can find references to this type of rectifier in books from the 40’s, which means that they were still used at that time.
The reasons that lead electrolytic rectifiers to not being used anymore are several, in first place components with liquid parts change very much their characteristics when in movement, making it difficult packaging it; the liquid can freeze in low temperatures (that is why electrolytic capacitors normally have ethylene glycol in its solution, thus making this solution poisonous); if too much current passes by it the solution can heat and boil, causing a steam explosion; electrolysis of the solution can produce flammable or noxious gases. But maybe the most important reasons are: they can not be used at very high voltages, as the oxide layer is very thin; and that when it is not conducting it behaves just like an electrolytic capacitor, allowing the passage of alternating currents of higher frequencies, besides when not used for longs times they began to conduct in both directions.
But no disadvantage would depose them if there were no alternatives, and when they appeared, they ended being substituted first by mercury vapor rectifiers and solid state rectifiers like selenium a=or cooper oxide rectifiers, and later by germanium and silicon rectifiers. But this technology did not disappeared completely, the same basic structure is used today in electrolytic capacitors.
. Thermite is not only used in incendiary bombs, in fact it is not even used in incendiary bombs anymore. I used the example because was the first image in my mind, because of an image in an encyclopedia that I saw in my childhood. It is used for soldering rail tracks, it fuses the two track segments together. It does not explode, it does not produce toxic gases and smoke and it is not easy to ignite, making it relatively safe, specially in subway tunnels.
. Later studying the galvanic series [WIKI3] I realized that this would not be possible with a pure aluminium gate and a steel or iron chain. The aluminium would be eroded and not the iron. But the chain was in contact with a padlock with a brass body, making it possible. So the culprit probably was brass not aluminium that was protecting the chain.
. Electric showers are not common in USA, but are the rule here in Brazil (and in some other countries like Ecuador, old houses in UK, etc.). Here in Brazil gas lines are not common everywhere (usually in these places we use cylinders with a butane-propane mix), and electric energy is cheap (or at least it was), so electric showers are a good choice.
People from countries where gas, or central heaters are used (Chile and USA for instance) get horrified when they see an electric shower. On the other hand people used to electric showers are equally horrified when they see a gas heater (I know I was).
Even with water touching directly the heating element in the shower will not kill you because electricity prefers the easiest path, and even if the heating element breaks a wire is connected to ground where water passes. Some new models that have shielded heating elements in that water do not touch the heating element directly, this is done more to prolong heating element life than for safety.
Even in old type electrical installations where no ground wire exists (only neutral wire that is only grounded outside the house) they are pretty safe because pipes are still made of metal.
in fact carbon monoxide poisoning from gas heaters are far more common then electrocution (I never heard of one in the shower). I use electric showers since childhood and never got a shock (but in old houses with no ground wire if you have a cut on your fingers you can fell the small leakage current, because cuts are more sensitive to electricity).
A common complaint of electric showers is that changing the flow of water changes its temperature and its true, unless you use new models with electronic temperature control and sometimes a turbine to keep flow steady.
[STA]: STANLEY, Norman. The Electroluminescent Rectifier. Aurora, 16 NOV 2001. Available at: <http://www.sas.org/E-Bulletin/2001-11-16<http://www.sas.org/E-Bulletin/2001-11-16/chem/column.html>. Accessed at: 21 SEP 2008 (dead link).
[WIKI1]: Electrolytic capacitor. In: Wikipedia. Available at: <http://en.wikipedia.org/wiki/Electrolytic_capacitor>. Accessed at: 16 FEB 2016.
[WIKI2]: Cathodic protection. In: Wikipedia. Available at: <https://en.wikipedia.org/wiki/Cathodic_protection>. Accessed at: 16 FEB 2016.
[WIKI3]: Galvanic series. In: Wikipedia. Available at: <https://en.wikipedia.org/wiki/Galvanic_series>. Accessed at: 16 FEB 2016.
[BRU]: GÜNTHERSHULZE, Ing. A.; DE BRUINE, Walter A. Electric rectifier and valves. Chapter 7: Electrolytic rectifiers. Cambridge: John Wiley & Sons, 1928. The book was translated by de Bruine and he wrote the part on electrolytic rectifiers. Available at: <http://www.tubebooks.org/books/erav.pdf>. Accessed at: 16 FEB 2016.
[DEE]: DEELEY, Paul McKnight. Electrolytic Capacitors: Theory, Construction, Characteristics and Applications. [S.l.], 19 FEB 2016. Available at: <http://www.faradnet.com/deeley/book_toc.htm#toc>. Accessed at: 19 FEB 2016.
[SUFRAMA]: SUPERINTENDÊNCIA DA ZONA FRANCA DE MANAUS. Mercadoria Estrageira: Listagem Padrão de Insumos. Manaus, [S.d.]. Available at: <http://www.suframa.gov.br/servicos/estrangeiro/consultas/listageminsumos/EST_PoloProdutoTipo.asp?produto=0700>. Accessed at: 4 SEP 2009 (broken link).