Housing Crank Pinion Dial Float Float Cover glass Gasket Pointer magnet Tank + A B F DC power Tank resistor N S E Float 14-38 Figure 14-69.  A DC electric fuel quantity indicator uses a variable resistor in the tank unit, which is moved by a float arm. Figure 14-68.  Simple mechanical fuel indicators used on light aircraft with fuel tanks in close proximity to the pilot. Digital indicators are available that work with the same  variable resistance signal from the tank unit. They convert  the variable resistance into a digital display in the cockpit  instrument head.  [Figure 14-70]  Fully digital instrumentation  systems, such as those found in a glass cockpit aircraft,  convert the variable resistance into a digital signal to be  processed in a computer and displayed on a flat screen panel.  Electric fuel quantity indicators are more common than  mechanical indicators in modern aircraft. Most of these units  operate with direct current (DC) and use variable resistance in  a circuit to drive a ratiometer-type indicator. The movement  of a float in the tank moves a connecting arm to the wiper on  a variable resistor in the tank unit. This resistor is wired in  series with one of the coils of the ratiometer-type fuel gauge in  the instrument panel. Changes to the current flowing through  the tank unit resistor change the current flowing through one  of the coils in the indicator. This alters the magnetic field  in which the indicating pointer pivots. The calibrated dial  indicates the corresponding fuel quantity.  [Figure 14-69]  Figure 14-67.  A float-type sight gauge fuel quantity indicator. 