Various Capacitive Sensors

    Over the years, brilliant minds have used the science and nature of capacitors to create some truly amazing and accurate sensors so that electronics can measure and correspond with the physical world we see everyday.

    Flexible conductive plates are the main component of condenser microphones. Invented at Bell Labs in 1916, the condenser microphone is one of the earliest methods of capturing sound. A diaphragm, the flexible plate of a capacitor, is moved/deformed by sound waves in the air. As voltage is held constant across the plates, the capacitance can be measured as the distance between the plates changes.

http://www.mediacollege.com/audio/microphones/condenser.html
    Capacitive accelerometers are not the only kind of accelerometer, but it is effective. As a mass is accelerated, a plate is moved, thus the capacitance is effected/measured. A system known as MEMS (Micro Electo-Mechanical Systems) is often what makes up a capacitive accelerometer. These micro-mechanical systems can be made incredibly small and can be made to represent the figure to the right. These components can be strapped to rockets, used to deploy airbags, make your Mario Kart turn on your Wii video game console, and much much more.     
A little more in-depth look at capacitive accelerometers:            https://wiki.metropolia.fi/display/sensor/Acceleration+sensors
https://www.youtube.com/watch?v=i2U49usFo10

    One of the newest features in cell phones have people very excited. Fingerprint Scanners!! We're in the future 0_o. Looking over the security risks of this technology, it is a very elegant use of capacitors. By measuring the various voltages across several different capacitors, or measuring as a user slides their finger over the sensor, the cell phone or computer can map the ridges and valleys that make up a fingerprint. An important aspect of this technology is that the finger of the user is actually the other conducting plate in these capacitors! This method of sensing will be explored in further detail on the next page (Screens).
http://computer.howstuffworks.com/fingerprint-scanner3.htm

    Liquid level sensors use a changing dielectric to effect the capacitance. The parallel plates are charged to a voltage. As the liquid level rises, perpendicular to the vertical plates, more dielectric is introduced between the two plates, thus boosting the capacitance. A measurement at empty and one at full is all a processor needs to accurately map the amount of liquid in the container at any level. This method is often used in airplanes to measure the fuel level. The gauge to the right may look a little odd for a capacitor but do not fret, the parallel plates are merely concentric tubes.

http://cdn.racerpartswholesale.com/images/uploads/11875_2884_large.jpg