The Enigma machine was invented by Arthur Scherbius in 1918 and was built to be a device for companies to encrypt their information. The name Enigma is not one specific machine, but a range of rotor based, cipher encryption devices which evolved in complexity over it's lifetime.
For more information and a lot more detail, I recommend a visit to www.cryptomuseum.com
Die Handelsmaschine (the commercial machine, or trade machine) was the first cipher machine to be sold commercially under the name Enigma. It was quite a large and bulky device which had four non-removable cipher wheels each with 28 contacts on each side.
The cipher machine could be switching between ciphering, deciphering and plain text (so it could be used as a standard typewriter). It was replaced by a newer printing Enigma in 1926. There are currently no known surviving examples of this model.
Die schreibende Enigma (the printing Enigma) replaced the Handelsmachine in 1924. This machine has a mechanism similar to a typewriter with type-bars rather than the Handelsmaschine's rotating print wheel. It had 60 keys and had a clever method to switching between letters and numbers/symbols while still enciphering using just the letters A-Z on the four non-removable cipher wheels.
Due to various production and reliability issues, it was not released before 1926 and, due to a high price, probably did not sell that many machines. In 1929, the final printing model of Enigma, the Enigma Model H29 replaced it, but in the meantime, in 1924, a newer model using lamps rather than a printing method quickly replaced the print machines.
This model, the Enigma H29, was the last of the printing Enigma machines. Introduced in 1929, it had eight cipher wheels with four used to encipher the message and the other four controlling how the rotors turned over.
A large machine, weighing nearly 60kg, it was not particularly portable and had many reliability issues. It was soon to be replaced by the more popular lamp based machines which were much smaller, lighter and cheaper.
Enigma A was the first of the Glow lamp type machines and was introduced in 1924. Currently, there does not appear to be a photo of this machine as it was succeeded by Enigma B in the same year, but it is described in the patent DE407804 which gives an idea of how it looked.
It initially had two main rotors as well as the reflector. Later additions made it so that the UKW reflector could also be rotated manually to one of it's 26 settings. The 26 keys were set in two rows with the lamps set above each row. The keys and light panel were initially blank so the customer could write their own array of letters/numbers on them. After each key was pressed, a larger key marked "Antriebstaste" or driving button had to be pressed to actually rotate the rightmost rotor.
Enigma B improved on the A model in a number of ways and was first introduced in late 1924. There are two versions, the early one had two rows of keys at the front with the rotors placed centrally and the lamp panel now at the back of the machine. This version was also supplied with 28 keys and 28 positions on each rotor to the Swedish General Staff, the W character having been removed and the extra characters Å, Ä & Ö added. This initial version had two rotors plus a movable UKW reflector which also moved when the second rotor's notch moved it. The separate button to move the rotors has also been replaced with each key now moving the rotor before making the electrical connection.
The second version of this model was much more like the Enigma layout we see mostly with the lampboard between the keyboard and the rotors at the back. The keys and lampboard are laid out in the more familiar three levels. This machine had two rotors (with either 26 or 28 positions) marked with numbers and a third rotor marked with letters (A-Z or A-Ö). This changed the movable reflector for a fixed internal one. Also, this machine was the first to be able to remove the three main rotors so that they could have their places switched, meaning there was a further 6 different orders in which they could be preset. Another first was on this machine was the introduction of the Ringstellung (ring-setting) which meant the ring engraved with the numbers/letters could be rotated relative to the wiring.
Enigma C replaced model B from late 1924. It is very similar to the Enigma B version 2 with a few improvements. These include the three moving rotors, each with numbers 1-26 with a fixed UKW. Each row of the keyboard is now stepped up to make typing easier. A power switch and external terminals are added meaning it can be run either from the internal battery or from an external 4V power supply plus the battery option can be set to change the lampboard to bright or dark mode. This is the last model Enigma which has the keyboard set out in alphabetic order rather than the more common one found on typewriters (QWERTZ)
In 1926, the Enigma D was developed to replace the C model. A few improvements were made from Enigma C like the top lid to access the internal rotors now having a hinge with two screws rather than the removable one from before which had four to remove it plus the keyboard is now in the German typewriter QWERTZ layout. This machine was the main commercial machine made available and came with a standard set of three rotors.
Looking at the picture, it is easy to assume that this is a 4-wheel machine, but actually, it is in fact a 3-wheel machine with a settable UKW reflector.
The Enigma K variant was a commercial machine introduced in 1927 with a few minor manufacturing differences, but it was generally identical to the Enigma D. A number of variants of the Enigma K were produced in later years, adding rotors with more turnover notches and rewiring depending on the end customer's requirements.
This machine is one of the main machines simulated in Virtual Enigma.
In 1926, the German Army (the Reichswehr, later Wehrmacht) began showing an interest in the Enigma machine and requested a special variant of the Enigma D. The prototype, ready in 1927, had three removable rotors and a fixed reflector (UKW), but had the addition of a plugboard (the Steckerbrett) on the front which added to the cryptographic security by an order of magnitude.
The Enigma I (Roman numeral one, so pronounced as Enigma Eins or Enigma One rather than the letter I), was only available to the Army with the wiring of the rotors, the reflector and the plugboard being exclusively for the military machines. Initially supplied with three rotors, a further two more were made available from Decmeber 1938 onwards giving a choice of five in total, the remaining two being held in a wooden storage box.
The steckerbrett plugboard went through a number of versions, initially using 26 single cables to swap letters, but the final version used double ended sockets with two contacts. When plugged between two sockets, the corresponding letters were switched over. Enigma Steckerbrett at Cryptomuseum.com
The M1, M2 and M3 were used by the German Navy from 1934 onwards and were all 3 rotor models. It was compatible with the Army's Enigma I as the initial five rotors had the same internal wiring and notch position. Each version had a few manufacturing differences but were generally the same. It had a smaller battery fitted to allow a 4V connector which could then accept a plug direct from the ship systems to power it. The rotors all have letters A-Z written around the edge rather than the numbers previously found on machines before.
Exclusively for the Kriegsmarine, an additional three rotors were supplied (VI, VII and VIII) each with two notches opposite each other rather than the standard single notch on all of the other rotors. This meant that secure messages which could not be read by the Army or standard Navy units were possible. Another difference on the rotors was the addition of two spring-loaded levers to replace the sprung-pin model on the Enigma I versions. Both levers needed to be pressed together to allow the ring position to be set.
This machine is one of the main machines simulated in Virtual Enigma.
The Enigma M4 is the next model up from the M3 and was for use by some divisions of the German Navy (Kriegsmarine) from 1942 and was used on both U-Boats and battleships. It added an additional rotor making four in total in use in the scramber making it a formidable challenge to crack. The four in the name isn't specifically due to it having four rotors but is just the next in the series.
There were eight rotors (named I to VIII) supplied which were identical to the ones from the M1 to M3 series. The first five were compatible with the original Enigma I model rotors (they would even work in that machine if fitted) with the other three wired the same as the M1-3.
To enable the additional rotor (Zusatzwalze) to be fitted, the UKW reflector was made thinner plus the extra was a thinner version and fitted to the left of the normal three rotors. Two rotors were supplied for this fourth thin wheel and were named Beta and Gamma but they could not be used in place of the other eight standard rotors. They looked similar to the normal rotors but had the spring-pin method of changing the ring position rather than the M1-4 lever method. The fourth wheel could be set at start, but did not turn automatically during encipherment.
The thin UKW was also supplied with a changeable second version and were named b and c. Note the lower case letters to distinguish them from the normal B reflectors. These new thin reflectors were wired differently from their normal Enigma I versions but if the b reflector was used with the Beta fourth rotor set to the letter A, it was compatible with the normally wired Enigma I B UKW. Likewise, the c reflector with the Gamma rotor was compatible with the standard C UKW meaning messages could be transferred across the different machines.
This is just a brief overview of the main Enigma models, for much more detailed information, photos and the other models which were created, please visit https://www.cryptomuseum.com/crypto/enigma/index.htm (can you tell it's one of my favourite sites on the web?)