My car is due for a routine service, its 50 Mm service, and this has started me thinking about units. When I was a boy I learned about rods, poles and perches and tons, cwt, quarters, pounds and ounces. When I moved to secondary school I put away these childish things and, after a brief flirtation in A-level physics with the electrostatic cgi units, found the MKS system. And thought that that was that.
Note that my car’s service is not its 50 mm service but, one thousand million times (or, as natives of the USA would say, 1 billion times) bigger, its 50 Mm service. For some unfathomable reason the people at the garage call this a 50’000 km service.
I certainly have referred more than once in my life to 10 dB. Indeed, I’m not sure that I’ve ever seen anyone write 1 B. Given that deci is not a standard subdivision of anything, why was it chosen as the base unit?
In aviation we know how high we are because we have barometers in our aircraft masquerading as altimeters. Of course, these are only accurate when the current sea-level atmospheric pressure of the region is set into them and so, at irregular intervals, a flight service specialist or air traffic controller will radio the local atmospheric pressure to you. And, at least in North America, that reading will be in inches of mercury! But the people giving the weather forecast on the public radio talk of hecto-Pascals. Hecto? I am pleased to see that there is a web site dedicated to getting rid of this strange unit. I don’t know the author of that site, Gene Nygaard, but his frustration certainly shews through and, as he points out, the other number used in aviation for atmospheric pressure is in tenths of hecto-pascals.
I had better leave this confusing area after mentioning a peculiar search result from Google:
Hecto. The SI prefix for 102, abbreviated h (e.g., 1 hm = 102 meters).
I hope that you can all notice what happened there. It took me a while.