How does your phone's screen know exactly where you touched it?

Your phone's screen knows where you touched it because your body conducts electricity. The glass is coated with a transparent conductor, and when your finger comes close, it disturbs the screen's electric field. That disturbance shows up as a change in capacitance, the ability of two nearby conductors to hold electric charge between them.

The trick for finding the exact spot is a grid. Under the glass run invisible lines, some horizontal and some vertical, that cross without touching. Every crossing acts as a tiny capacitor. The phone fires quick voltage pulses into the lines one at a time and listens on the crossing lines. Where your finger is, the capacitance at that crossing drops. By checking every crossing, the phone can pinpoint one touch or several at once, which is why pinch-to-zoom works.

This also explains a winter annoyance. Ordinary gloves are electrical insulators, so some capacitive screens simply cannot feel your finger through them. Special gloves fix this with a patch of conductive thread on the fingertip. Older resistive screens worked differently: pressing pushed two layers together until they touched, so any rigid object, gloved finger included, would do.

The idea is older than you might think. The first finger-driven touchscreen was built by Eric Johnson at Britain's Royal Radar Establishment, who described it in 1965. In the early 1970s two CERN engineers, Frank Beck and Bent Stumpe, made a transparent touchscreen that went into use in 1973. It could already handle several fingers, but that feature was switched off in software.