J. Russell Lemon asked on 02-04-02 at 22:31: >Dear Sirs: > >According to my Handbook of Chemistry and Physics, the yard is defined as: >3600/3937 of a meter. Therefore an inch would be 100/3937 of a meter >or about 2.54000508001016002032004064008128 cm. The web page >http://lamar.colostate.edu/~hillger/everyday.htm says that there are >exactly 2.54 cm in an inch. Which one is correct?
The ISO, and perhaps ISA before it, adopted an inch of 24.5 mm because that would allow a lathe or other machine tool to operate in either inches or millimetress if it had a gear wheel with 127 teeth. The US inch of 1/39.37 m was sligtly larger than this, and the British inch of 25.399 972 mm was slightly smaller. In 1959 a conference of the English-speaking nations decided to adopt the ISO inch of 25.4 mm. The US Geodetic Survey decided not to change their maps to the new value because they were going metric anyway. The old US foot is now known as the survey foot. Joseph B.Reid 17 Glebe Road West Toronto M5P 1C8 TEL. 416-486-6071
