I elements are systems with time delay of the 1st order and thus contain only one energy storage element. A typical example found in everday use is a tank or a bunker, whose input variable is the mass flow (mass per unit of time) and its output variable is the fill level. When there is a inlet flow which is constant in time, the fill level increases linearly until the maximum fill level is reached. An example for an electrical I element is a capacitor, in which the input variable selected is the charging current and the output variable is the capacitor's voltage: when the charge current is constant the capacitor voltage increases linearly (theoretically ad infinitum).
Even a DC motor used to position the slide carriage of a machine tool, constitutes an I element, if the armature voltage is selected as the system's input variable and the slide carriage's position as the output variable (see the following Figure). If the armature voltage is constant the motor operates at constant speed and resulting in the slide carriage moving forward at a constant speed. The slide carriage's position thus changes at a linear rate.