How does a coin-op pool table know to return the cue ball when you scratch?

A STAFF REPORT FROM THE STRAIGHT DOPE SCIENCE ADVISORY BOARD

Dear Straight Dope:

How do pay-per-play pool tables know which ball is the white ball and return that one to you when you scratch? It can't be a difference in size, mass or density because that would alter how the balls react after they make contact with the cue ball. The only thing I can think of is some sort of metallic load built into the ball with the same properties as the material it is displacing (sort of like casino dice which are the same weight on all sides despite the different number of divots on each side) I'm assuming that this would enable the pool table to use magnetism to identify the cue ball. If so, is there any difference between how cue balls with and without the load react? How are they made? Do you have to buy a certain kind of cue ball in the event that the one you have is stolen? Are they more expensive? Then again, I'm probably way off track . . . sorry, I'm a philosophy major.

Una replies:

I recently pondered this myself after my lady and I used a coin-operated table for the first time, and she was surprised that after I scratched (as I frequently do) the cue ball would return to play. So I did a little investigating, and it turns out that you’ve guessed the answer–both magnets and different-sized cue balls are used, depending on the table.

For those not familiar with coin-operated pool tables, all the balls are held inside the table until the coins are inserted, after which they all fall out. As each ball is hit into a pocket, it rolls down a simple system of chutes that return it to a storage area, where it’s retained until the game is over and more money is inserted. However, since it’s common to scratch by sinking the cue ball, there has to be a system for returning just that ball. Otherwise, the game would be over the first time anybody scratched, which, if you’re playing against me, means within about 30 seconds.

Some coin-operated pool tables solve this problem by using cue balls that are a different size from the colored balls. The standard diameter for pocket billiard balls, as defined by the Billiard Congress of America, is 2-1/4 inches plus or minus .005 inch (ref 1). In the case of tables using oversized cue balls, the cue ball is typically 2-3/8 inches in diameter, or about 1/8 inch larger than the other balls. This system works fairly well, although some players allege that having a larger diameter cue ball throws off their shots and changes the dynamics of the game. My unprofessional opinion is that few hard-core or professional players use coin-operated tables in the first place, preferring a better-maintained and more professional table.

Magnetic cue ball return systems rely on a magnet embedded in the cue ball that triggers a sensor as the ball rolls down the return chute. This sensor causes a switch to flip that moves a gate, sending the cue ball into a separate path that returns it to the players. This system apparently works very well (based on calls to a couple local coin-operated pool table supply stores), although tables using magnetic balls seem to be far less common than ones using oversized balls. Magnetic cue balls are a bit more expensive than oversized ones, running around \$10 to \$15 each as opposed to \$4 to \$7 for oversized. I couldn’t find a significant difference in the cost of the tables themselves, but due to differences in features and design it was difficult to make a broad comparison. Magnetic cue balls are regulation size and weight and thus unlikely to affect play, although anecdotally speaking some say magnetic balls roll and respond differently. Given that some people will blame anything from the phase of the moon to the aurora borealis for a missed shot, don’t be surprised if your opponent claims that shifts in the Earth’s magnetic field were what spoiled their aim.

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