3. Electromagnetic Fields
We can't talk about weird human interface devices without talking about the theremin. If you think you don't know what a theremin is, you should be happy to note that you are mistaken - it lent the otherworldly operatic singer "woowooooo" sound to the original Star Trek theme and punctuated the soundscapes of countless old horror and sci-fi classics.
A theremin consists of two metal antennas that detect the position of the player's hands. The distance from one antenna determines the pitch of the note played, while the distance from the other determines the volume, and sound is output through a speaker.
When we talk about pitch and volume, what we actually mean is the frequency and amplitude - respectively - of the resulting sound waves. Picture your basic sinusoidal wave being broadcast on an oscilloscope. The wave's amplitude is its "height" - the vertical distance from crest to trough. The greater a sound wave's amplitude, the louder it is. Frequency isn't exactly a measure of a wave's width (the horizontal distance between two crests), but the width of a wave directly relates to its frequency - the wider a wave, the lower its frequency, and the deeper its pitch.
How do the antennas know where your hand is? First off, they are not actually antennas, since they are not used for either broadcasting or receiving a radio frequency. Instead, they serve the same role as plates in a capacitor.
Put simply, a capacitor is a device that maintains an electromagnetic field between two electrically conductive plates. In the case of the teremin, you may think that the two antennas correspond to the two plates that form an EM field, but that's not the case. Each antenna serves as a plate for a separate EM field - and your hands serve as the other plates. The distance of your hand from the antenna determines the capacitance of the EM field - how much energy is stored within it.