What are those red and green lights on the curling stones?

If the ball goes out of bounds in a football game, for example, it’s up to the referees to call the violation. But in a high-level curling match, the stones are somewhat self-controlled, thanks to the red and green lights above each.

As long as the curler releases the stone before the front of it crosses the first red line (the “hog line”), the lights will remain green and play will continue. If you see the red lights come on, it means the curler has kept their hand on the handle too long, committing a hog line violation. The stroke is disqualified and the stone is quickly removed so that it does not interfere with other stones on the curling sheet.

The detection system, called “Eye on the Hog”, dates back to the late 1990s, when Eric Salt, an engineering professor at the University of Saskatchewan, suggested to some of his advanced electrical engineering students to invent one for a class project. With his help, they did, and the Canadian engineering company Startco modified their design and brought it to market. The Canadian Curling Association quickly adopted it for official use, and its popularity grew from there.

Basically, there’s a magnetic strip set under the ice slightly behind the hog line – or, more accurately, the length of a stone’s radius before the edge of the hog line. [PDF]. Inside the stone is an electrical circuit whose current is affected when you touch the handle. If you still touch the handle when the stone passes through the magnet in the ice, it will trigger the battery operated red LEDs. Release the handle before the stone reaches the magnet and the green lights will flash until the stone gets there [PDF]. At this time, the flashing will change to solid green to indicate that the validation was valid.

This system also explains why the player throwing (“throwing”) the stone does not wear a glove on his throwing hand: he is not allowed to do so, because gloves inhibit the touch sensor. [PDF].

Due to the high caliber of Olympic curlers, hog line violations are not common in Olympic curling matches, so you don’t often see the red lights in action. But they came in handy when Italy’s Stefania Constantini kept her hand on the hilt a bit too long in a mixed doubles match against Norway on February 3. (Italy still won.)

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