How do anti-theft devices on shopping carts know they’re tripping at the edge of the parking lot?

I know that anti-theft devices on shopping carts are designed to trip at the edge of the parking lot, but how? And how come the wheels are locked half the time when the cart is very close (or even inside) to the store, not near the edge of the property? – The squeaky wheel

We’ve come a long way since Elmer Isaacks patented the first wheel lock for shopping trolleys in 1968. Until then, anyone could take a shopping cart with them at will and leave it in the nearest park, pond or private garden when they got bored. Thank goodness such things are unheard of in today’s wheellock-enabled utopia.

Isaack’s wheellock reinforced the cart’s perimeter with a series of magnets under the pavement. When a carriage rolled across the line, a magnetic mechanism pushed a rod through a hole in the wheel, like a stick through the spokes of a bicycle.

Modern systems are more sophisticated, if less elegant: today’s perimeter is just a buried wire carrying electrical impulses. An induction coil in the wheel picks up these impulses and forwards them to an on-board computer, which recognizes the moment and reacts by activating an internal, motor-driven braking system.

These different approaches to the same problem have one thing in common: both can be overcome by simply holding the ratchet wheel a few feet above the gauge line, usually by doing a quick shopping cart wheelie as you cross. (Some stores now use two locking wheels on opposite corners, making this exploit impossible without elevating the entire cart overhead, like Hulk.)

So, when locking wheels are so easy to beat, why bother? Well, they’re still better than nothing; Each cart saved costs $150 that you don’t spend. But it’s also because not only do these systems combat cart theft, many also combat “pushout theft,” also known as “filling the cart and leaving without paying.”

When a cart pulls into the store, the lock activates and locks as they walk out the door without going through a checkout line first. It’s a cute system, but you can see how it can occasionally get confused and imprison an innocent cart. Since the average pushout costs $560, it’s worth the retailer’s bother — especially when that hassle falls mostly on folks like you and me, Squeaky, with our square-wheeled carts.

Questions? send them to dr.know@wweek.com.