figure 28-1 is a diagram that illustrates the principle of an electronic analytical balance. the pan rides above a hollow metal cylinder that is surrounded by a coil and fits over the inner pole of a cylindrical permanent magnet. an electric current in the coil creates a magnetic field that supports or levitates the cylinder, the pan and indicator arm, and whatever load is on the pan. the current is adjusted so that the level of the indicator arm is in the null position when the pan is empty. placing an object on the pan causes the pan and indicator arm to move downward, which increases the amount of light striking the photocell of the null detector. the increased current form the photocell is amplified and fed into the coil, creating a larger magnetic field, which returns the pan to its original null position. a device such as this, in which a small electric current causes a mechanical system to maintain a null position, is called a servo system. the current required to keep the pan and object in the null position is directly proportional to the mass of the object and is readily measured, digitized, and displayed. the calibration of and electronic balance involves the use of standard mass and adjustment of the current so that the mass of the standard is exhibited on the display. most electronic balances are equipped with a built-in standard weight, thus making periodic recalibration simple and convenient. with some, periodic recalibration is performed automatically.