Draw machine, in full
Gravity-pick machines hold the balls in a clear chamber with paddles turning in opposite directions. The mixing runs for a set time, then a door in the base opens and the balls drop through one at a time into a display rail. Powerball, Mega Millions and EuroMillions draws use this type, which is also the one most familiar from televised draws because the balls stay visible throughout.
Air-mix machines work by blowing rather than dropping. A fan in the base pushes the balls around the chamber, and when a valve opens the airflow carries balls up a tube one after another. Daily digit games commonly use this design, and Eurojackpot draws with it. Ball weight and diameter are held to tight tolerances in both designs.
The controls around the machine matter more than the machine. Operators keep several machines and several ball sets, weigh and measure the balls, store them securely between draws, and select which machine and which set will be used shortly before the draw, often at random. Draws happen in front of independent observers, usually an accounting firm, and the equipment is checked before and after. The 1980 Pennsylvania Lottery fraud, in which balls were weighted, is why much of this procedure exists.
- Lottery mechanics
- Lottery machine, Ball machine
A worked example
A major draw typically runs like this. The operator selects one of its machines and one of its ball sets shortly before air time, rather than using the same pair every week. For Powerball the chosen set of 69 white balls and 26 red balls is weighed and inspected, then loaded. Independent auditors watch, the draw is recorded, and the equipment is checked again before it goes back into storage.
Sources
In the Pennsylvania Daily Number draw of 24 April 1980, every ball except those numbered 4 and 6 was weighted, the draw produced 666, and unusual betting patterns exposed the plot.
Wikipedia — 1980 Pennsylvania Lottery scandal
See also
Where it comes up on this site
Draw machine: frequently asked
They are made to tight tolerances and weighed before use, and operators keep several ball sets so no single set gets enough use for wear to show. The procedure exists because it once failed. In April 1980 the Pennsylvania Daily Number was rigged by weighting every ball except the 4s and 6s, the draw came up 666, and the plot was caught by the betting patterns rather than by any inspection of the equipment. The inspections came afterwards.
A real mechanical bias would show up over thousands of draws as numbers appearing more often than chance allows, which is what audits and published frequency data exist to catch. Rotating machines and ball sets is meant to stop drift building up in the first place. Either way, each draw stays independent of the last.