# Tactile illusions

This short video at iFixit provides a decent and accessible explanation of the Force Touch trackpad that’s now in the 13″ MacBook Pro and will be on the just-announced 12″ MacBook. It’s sort of a followup to Steps 11–15 of iFixit’s recent teardown of the MBP.

The trackpad uses four force gauges, one at each corner, to measure the force you exert when pressing down on it. It’s remarkably similar to the way my bathroom scale works. The force gauges are themselves very simple: short cantilever beams with strain gauges on one side. The strain gauges change resistance according to the curvature of the beams, the curvature of the beams changes according to the forces acting on them, and the four forces on the beams add up to the total force of your touch. By putting a gauge at each corner, the force is measured accurately regardless of where you touch.

I’m going to pick a nit with iFixit’s description of how strain gauges work. The video says

A strain gauge works off of a principle called Poisson’s Ratio, which says that as you stretch a wire, the diameter will decrease and the length will increase and that’ll increase the electrical resistance of that wire.

Every individual fact in that statement is true, and yet I cannot agree that Poisson’s ratio is why a strain gauge works. The electrical resistance of a wire does increase with length and does decrease with diameter, but these two effects are independent. And Poisson’s ratio (which is a number, not a principle) has nothing to do with electrical resistance; it is purely a measure of the relationship between the longitudinal strain in the direction of the applied force and the lateral strain at right angles to the applied force. Poisson’s ratio does play a role in the value of a strain gauge’s gauge factor, but even if a wire had a Poisson’s ratio of zero, you could still make a strain gauge out of it.

More interesting to me is the feedback the trackpad gives you. A set of electromagnets along one edge of the pad shake it laterally according to criteria that are a complete mystery to me. Somehow, though, buzzing the pad laterally gives the user the impression of downward motion. Everyone who’s tried it out says the feeling is uncannily like pushing down on a regular trackpad.

I assume there’s a scientific name for this trick, but so far none of the millions of tech pundits who’ve written about the Force Touch trackpad have dug in and found out what it is. Until I hear otherwise, I’m going to call it a tactile illusion, the obvious analog to an optical illusion. If I had had a real education and knew French, I’d come up with an analog for trompe l’œil. Does trompe l’doigt work?

Update 3/15/15 9:33 PM