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Old 13th Sep 2017, 11:55 am   #41
Herald1360
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Isn't it convenient that the speed of light is approximately 300,000 kilometres per second?

Plenty good enough for frequency to wavelength conversions.

Even more convenient that it's about 1ft/nS when thinking about signal delays in old money.
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Old 13th Sep 2017, 12:12 pm   #42
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"...As an exception, the name of the kilogram, which is the base unit of mass, includes the prefix kilo, for historical reasons...."

In other words, "because they got it wrong".

Interesting definition, though. The gram is no longer an SI unit. I didn't know that.
The gram is as much an SI unit as the millimetre, or the microfarad, as a submultiple of the base unit. (a CGS gram may be different by a few parts per billion, as it's based on the mass of 1cc of water. But the CGS centimetre was defined differently from the SI metre, as well). I do recall a homework question in A level physics (1979), where a mass was given in grams, an object accelerating to so-many cm/sec in so-many seconds, what is the force? I put my answer in dynes, on the basis that the question was clearly in CGS units, I was marked down

It's a bit of an inconsistency, really, in SI, calling the base unit a kilogram. Would have been better calling it something else. Suggestions?

However, on the basis that a rose by any other name would smell as sweet, the actual mass that the kilogram is defined to be, is fairly well chosen. (I'd have made the metre a bit smaller, personally, but that's just me.)
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Old 13th Sep 2017, 2:10 pm   #43
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The other connection between the metre and the second is that a pendulum 1m. long takes -- as near as dn it is to swearing -- 1 second to swing from side to side. Possibly useful if you are ever stuck on a desert island and need to make yourself a set of measurement standards from scratch
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Old 13th Sep 2017, 3:16 pm   #44
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I like the variety of units used past and present, understating them explains many things, like why is the US gallon smaller than ours (to do with wine and beer gallons). My local hardware store still sells paraffin in gallons (or a half for the smaller, common 4 litre containers) with a conversion to litres pasted on the machine.

America went metric in 1868 by defining their yard as a certain fraction of a metre, saved making their own standard apparently.
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Old 13th Sep 2017, 3:33 pm   #45
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I don't think Americans use metric units for everyday shopping, measuring etc, only for science and engineering purposes.
I recently met my son's father in law (FIL in common parlance) who's an architect in LA. In conversation, we mentioned metric measurements, and he had no idea of what were. They use feet and inches, pounds, etc, as we older folks used to.
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Old 13th Sep 2017, 3:40 pm   #46
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Yes, the Americans still use imperial measurements to buy their sausages.
It's a cultural thing. A bit like speaking your own language.
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Old 13th Sep 2017, 4:37 pm   #47
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Americans are gradually coming into line, though. Five years ago, American data books on magnetic materials gave figures in gauss and oersteds. Now, they're at least in teslas and amp-turns per metre.

While the conversion between gauss and tesla is easy (10,000 G = 1 T), oersteds to amp-turns per metre is not (1000/4π Oe = 1 A/m). Trouble is, I now have a feel for magnitudes of magnetising field strength in oersteds. It'll take a generation or two to let things settle...

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Old 13th Sep 2017, 5:25 pm   #48
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It's a bit of an inconsistency, really, in SI, calling the base unit a kilogram. Would have been better calling it something else. Suggestions?
Perhaps the most logical suggestion would be to name the kilogram a “giorgi” with the symbol “G” in honour of Giovanni Giorgi who in 1901 proposed that the MKS system (which used the metre, kilogram and second as its fundamental units), extended with a fourth unit to be taken from the units of electromagnetism, be used as an international system.

Unfortunately, this would cause endless confusion with the “gram” and its SI unit symbol “g”. For example, if it was intended to specify a tracking force of 6 mG (milligiorgi) and MS word autocorrect changed this to 6 Mg (megagram) you would have specified a tracking force of 6 kg instead of 6 g.

It’s probably best to stick to the kilogram after all.
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Old 13th Sep 2017, 5:33 pm   #49
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I like that.

Don't have to have G for the giorgi, either - how about zeta (ζ)? No confusion then.
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Old 13th Sep 2017, 7:00 pm   #50
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We had enough trouble when the Siemens (mho) was brought in.

mS for milli Siemens is too easily confused with ms for milliseconds.
It would also be much better id base units were capitalised and not derived units

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Old 13th Sep 2017, 9:57 pm   #51
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It would indeed - probably a bit too late now. Instead, the capitalised units are the ones named after somebody. So the capitalised base units are amp (A), kelvin (K). Second is s (though I personally write sec so as never to confuse with siemens S); metre is m; then we move to two-lettered base units, kilogram is kg; candela is cd; and then to three letters, mole being mol. All the other SI units are derived units, usually a single capital letter when named after someone but occasionally two (e.g. Hz, Wb).

Siemens incidentally I have never used, nor mho (which there was a period when the abbreviation was upside-down capital omega). I prefer to write mA/V instead, as every conductance I've used is mutual conductance, of valve (occasionally transistor). And then, if load is in kilohms, multiply the two together to get gain (wow! That's brought just about right back to the OP's first post!).
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Old 14th Sep 2017, 7:30 am   #52
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Yes, the Americans still use imperial measurements to buy their sausages.
It's a cultural thing. A bit like speaking your own language.
That ISN'T what I wrote.
(Or perhaps it is, and my faculties are deteriorating quicker that I thought.)
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Old 14th Sep 2017, 7:43 am   #53
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As Joe Public, the BBC and uncle Tom Cobbley all call masses and weights in "Keeeloes"
I don't suppose it matters what the bit after the multiplier would have been :-(

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Old 14th Sep 2017, 7:52 am   #54
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"Keelo" would solve the problem. It no longer sounds like a multiple.
What would the symbols for Kilo-Keelo and Milli-Keelo be?
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Old 14th Sep 2017, 5:40 pm   #55
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Kilo-Keelo is a Meelo and a Milli-Keelo is just a lo.
Or did I get that wrong somewhere?

I went crazy with some guys who liked centiBels....argh.
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Old 14th Sep 2017, 7:55 pm   #56
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Just to go back in time a bit, there is also the unit beloved of old fashioned power engineers, the Daraf, a reciprocal Farad, named like the Mho. There was probably one for the Henry as well but I can't think of it at present.

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Old 14th Sep 2017, 8:10 pm   #57
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Wow, Ed! I've just googled it... inverse capacitance or elastance, measured in darafs! Thanks!! Never used that, don't believe I knew about the term! (I have some across susceptance though, like conductance, measured in mhos ℧ until SI replaced them by siemens). No abbreviation given for the daraf, rather a shame... And nothing about reciprocal inductance (of course, turning H upside-down for reciprocal henry would hardly work anyway!)
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Old 15th Sep 2017, 8:15 am   #58
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Reciprocal Henry sounds like someone from Viz comic.
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Old 15th Sep 2017, 1:52 pm   #59
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Reciprocal Henry sounds like someone from Viz comic.
Presumably with unfeasibley large dimensions!
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Old 15th Sep 2017, 4:45 pm   #60
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Surely the International Unit for sausages is the "link" [two sausages] ??
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