Ever noticed a painted yellow line in the parking lot around many supermarkets and retail stores? The magic yellow line emits a signal that causes carts to stop dead in their tracks, preventing carts from leaving the parking lot.
Now you can build your own portable yellow line — with up to a 20 foot range. Need I say more? Hint: it works inside the store.
Somebody going by the handle “Orthonormal Basis of Evil” has posted instructions for building this device to the Web site Instructables. There are eleven steps between the first act of assembly and the assault upon retail America. Most interesting from a math-and-physics standpoint are isolating the locking signal, which requires a Fast Fourier Transform (FFT), and recreating the locking signal, which is a nice exercise in electromagnetism. Essentially, the lesson is that a current in a wire creates a magnetic field, varying the current creates a fluctuating field, and these fluctuations can propagate through space to carry energy and influence electric charges elsewhere, possibly inducing new electric currents following the lead of the first. Here, the transmitting wire is either buried beneath the parking lot or carried on the person, and the “elsewhere” being influenced is inside the locking apparatus of the shopping cart’s front wheel.
(I may be mistaken here, but I think this is actually a case of near-field induction, rather than the typical far-field plane waves one often studies.)
Installing the device on its human user is an exercise in cybernetics, which “Orthonormal Basis of Evil” has chosen to illustrate in a way a little different from the electromagnetism textbooks I grew up with: