Electrometers are instruments that measure electric charge and/or electric potential difference. Many devices can measure voltage and charges with very-low leakage currents to 1 femtoampere (fA) or less. There are three main types of electrometers: vibrating reed, valve, and solid-state. Vibrating-reed electrometers contain a variable capacitor and a fixed-input electrode. As the capacitance varies, an electric charge is forced into and out of the capacitor. Valve electrometers contain an unbiased vacuum tube with a very high gain and input resistance. The input current flow into the high-impedance grid, amplifying the generated voltage. Solid-state electrometers contain a solid-state amplifier with field-effect transistors (FETs).