The nominal magnification reading in a electron microscope may be no more accurate than 5% even when the instrument is new. For any accurate work, the instrument needs calibrating with a known standard of length.
Whenever an instrument is calibrated, the lens currents must be cycled (by a "standardization button", if it is fitted), or by cycling the lens currents up to maximum and then down to the operating value. In this way, the effects of hysteresis are standardized. The value of the lens currents for each magnification should be recorded, so that the conditions can be reproduced.
While working at low magnifications, care should be taken to measure the grating spacings only in the central area of the plate. The image distortion is always at its worst in the low magnification range, and is most noticeable near the edges of the plate.
Cat. #80050 : 2160 lines/mm or 54864 lines/inch line grating. This is a finer ruled grating and may be used up to magnification of about 120,000x.
Cat. #80051 : 2000 lines/mm cross ruled grating. The grating is ruled in two directions at angles. It permits both a magnification check and also gives an easy visual impression of any image distortion, through distortion of the ruled pattern.
Cat. #80014 : Beef liver catalase crystals have lattice spacings of 8.75 nm and 6.85 nm. These are easily visible, and can be used to calibrate the upper end of the magnification range of a microscope.