Hi John, The original unit of capacitance was the farad, however as this is a very large value it was divided by a million to the units of micro farads (µf). Since the greek letter mu (µ) isn't commonly available on typewriters it was usually printed as mfd. For very small values this is divided by a million again to µµf usually written mmf. Today it's usually called pico farads (pf). Your 0.1 µf is a common value and may be written as .1mf. In earlier times parts were usually made in rounded values, 0.1, 0.2, 0.5, etc. and submultiples. These values leave gaps in values between the tolerance (accuracy) and what seems like strange values replaced them. (0.1, 0.12, 0.15, 0.18, 0.22, 0.27, 0.33, 0.39, 0.47, 0.56, 0.68, 0.82). The upper limit with 5% error of one value meets the lower -5% value of the next highest. If you're replacing an old value use the closest value in the new range. For most paper and electrolytic capacitor applications they,re not critical and can be substituted with the next readilly available value, 0.15 or .18 might be harder to find, a 0.22 will usually work fine. Same with electolytics, old vslues were often 8µf and 16µf, use 10µf & 22µf respectively. Small mica tuning capacitors are normally reliable but if you replace them they need to be the same value, as they set the tuned frequency of circuits.
Don Black.
: would . 1 be read as .11mfd or the closest like .15mfd and .2 be read as .22mfd? --sorry