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Like resistors, capacitors are generally manufactured with values to two significant digits. Also, small capacitors for general purposes have practical values greater than 1 pf and less than 1 µf. As a result, a useful convention has developed in reading capacitance values. If a capacitor is marked "47," its value is 47 pf. If it is marked .047, its value is .047 µf. Thus, whole numbers express capacitance values in picofarads while decimal fractions express values in microfarads. Any capacitor manufactured with a value of 1 µf or greater is physically large enough to be clearly marked with its actual value.
A newer nomenclature has developed, where three numbers are printed on the body of the capacitor. The third digit in this case works like the multiplier band on a resistor; it tells the number of zeros to tack onto the end of the two significant digits. Thus, if you see a capacitor marked "151," it is not a precision component. Rather, it is an ordinary capacitor with a capacitance of 150 pf. In this nomenclature, all values are given in picofarads. Therefore you might well see a capacitor marked 684, which would mean 680000 pf, or 0.68 µf.