The roulette wheel is the center of the game - the spirit and the soul of the game.
Roulette wheels come in different sizes, but generally they are around 3 feet (about one meter) in diameter. You then have online roulette wheels which are only as big as your monitor. Beside the deceivingly simple look threads a lot of clever and complicated mathematical design has gone into it.
The wheel is sitting on a spindle, allowing it to rotate freely when the dealer (traditionally called the "croupier" in roulette games) spins the handle that's attached to the wooden top that traditionally covers the outer part of the spindle.
The wheel itself is surrounded by a wooden or plastic cover that creates a slanted wall into which the ball (also called marble in some places) is thrown. Modern roulette wheels also have security glasses surrounding the roulette to prevent cheaters from blowing air into the wheel when the ball slows down and thus affecting the game (see our article on classic methods of roulette table cheating for examples).
The heart of the roulette wheel consists of numbered slots or pockets that catch the ball. The roulette wheel is designed so that the ball will always settle down in one presumably random slot - the slanted walls and the curvy inner surface of the wheel will always make the ball roll off towards the opening in which the slots are residing. The ball will inevitably fall into one of them.
Of course, the wheel pockets are marked. Every pocket has a number against it and a color. Numbers are evenly divided (from 1 to 36) between red and black colour. Zeros (0 and 00 in American Roulette table) are green. The principal difference between American and European roulette wheels is that American wheel has a double zero ("00"), while European roulette doesn't. Presence of additional slot affects the odds and the house edge: it is 2.7% for European roulette and 5.26% for American styled games. American roulette wheel therefore has 38 slots (1 to 36, zero and double zero), while European one has 37 (1-36 and the zero).
There is a magic and symbolism to the roulette wheel is the distribution of numbers and colours on the roulette wheel. It is designed in such a way that the distribution of numbers and colours is statistically even and provides a fair (equal) chance of any numbers playing down. This is done to normalize the game odds, which is necessary for more or less complicated roulette betting. The colours alternate; so do pairs of numbers across the wheel. If you carefully study the roulette wheel you will soon see a clear pattern in disposition of the numbers. While it's self evident that no two throws of a ball are dependent on each other, the normalization is needed because of the possibility of placing multiple bets under different conditions and odds. This disposition doesn't bear a lot of relevance for online casinos due to the way the software works (software program can produce mathematically even results regardless of actual disposition of the roulette wheel); technically speaking, the software instantly generates a random game outcome and it is then presented to the player as a visual projection of the ball falling down into particular slot of the wheel).
The roulette wheel has its own industry around it; there are vendors that dedicate themselves to the craft of making roulette wheels. Despite being simple, roulette wheels for luxury casinos have to answer to very high standards, and very precise measurements are made to make sure that the wheels actually bow down to the industry requirements. For instance, certain tolerances in a wheel dimensions lay within margin of 1/10 of a millimeter. Even with this, it is factual that no two roulette wheels are alike, and as the roulette relies on physics of the wheel, every wheel is essentially biased, no matter how small the bias is. Some of the vendors compete solely on the basis of measures taken to prevent the bias of the wheel.
The problem with a biased wheel is that an experienced cheater can eventually locate the flaw and exploit it to his advantage. While this seems to be tremendously complicated and severely improbable, the fact of life is that there are cheaters who do this specific kind of thing. They are armed with analytical tools and (mostly illegal) electronic gadgets that help them track a particular wheel and, based on statistical deviations and analysis of the physics of the wheel predict where the ball will (or will not) land.
Bias is a partiality that consistently influences game in an unfair way. If, for example, one of the pockets of the wheel is wider and its walls are slanted differently, the likelihood that in the long run that particular slot will attract the ball more times that any other slot would rises. A mechanical flaw in a roulette wheel that's known to the player and an advanced betting management scheme to compensate for the imprecise statistical predictions is an awesome material for a cheater. Even things as little as dust or invisible defects of the ball can affect the odds of the particular roulette wheel, exposing the fairly balanced scheme to abuse. The casinos of course don't sleep at the switchboard either: all roulette wheels are routinely examined and maintained.
The bias of course does not exist in computer software and so the online casinos are not subjects to physical defects; the random number generators are agressively tested by the vendors, players and outside organizations to make sure that they are not producing biased results. While there are known instances of fraudulent online casinos deliberately rigging the random number generators, it's a very unlikely occurence that will never slip by unnoticed.