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Old 11th Sep 2017, 4:39 pm   #21
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Indeed, the kilo is the only SI unit with a multiplier built in. I am happy with many measurement systems, they all have their place in the real world.
 
Old 11th Sep 2017, 5:04 pm   #22
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MKS measurements all the way for me. Imagine trying to do complicated calculations if we still used units like Scruples, Minims, Spans and Hands!

I recall some decades back reading an article on possible reasons for the collapse of the Roman empire. One theory posited was that their number-system greatly impeded all but the simplest of business calculations.

I can just imagine Roman O-level maths: "Caesar has MCMLXVII Denarii. He loans them to Brutus for VIII months at an interest-rate of XI percent. How much does Brutus have to repay in all, and what is the APR?"
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Old 11th Sep 2017, 5:14 pm   #23
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Agreed, MKS for proper measurement/calculation but some of the older length units are very handy, feet and inches for almost anything American and the barlycorn for shoe sizes (1/3rd inch plus offset), mind you I have yet to get grips with the euro shoe sizes.
 
Old 11th Sep 2017, 5:28 pm   #24
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I never understood all the fuss about MKS and CGS. The factors in both cases are of ten, so no fundamental difference can apply between the two....

The whole palaver comes about because of a fundamental flaw in the French Revolutionary system of measurement. The unit of Mass, the Gram (or Gramme) is not in the same order of magnitude as the unit of Length, the Metre...
Err... I'm not sure about that last bit. As Mass and Length are quite different concepts (as is Time), how can we say whether the units are the same order of magnitude or not?

Like, is a gram in the same order of magnitude as a second?
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Old 11th Sep 2017, 6:02 pm   #25
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Get the dimensions "right" and it all fits together without many (usually none) constants required for calculation. The MKS system seems to fit the bill quite well, although others could work too.
 
Old 11th Sep 2017, 8:02 pm   #26
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Yes, and SI is pretty good there. Not many constants - though folks like us do have our microfarads, nanofarads, picofarads but very few farads!
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Old 11th Sep 2017, 8:16 pm   #27
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Err... I'm not sure about that last bit. As Mass and Length are quite different concepts (as is Time), how can we say whether the units are the same order of magnitude or not?
Since the gram and the metre are related in the metric system (a gram of water is that which occupies a one-centimetre cube) I think the metric system has a number of magnitude odditites.
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Old 11th Sep 2017, 8:18 pm   #28
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I've done this for ages. Can't remember where I heard about it (it may well have been UKCOL's thread!) but it works for easy mental arithmetic. And mine is VERY mental!
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Old 12th Sep 2017, 5:53 am   #29
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I think it's a matter of understanding why it works, rather than just using it blindly.
Exactly so.
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Old 12th Sep 2017, 10:30 am   #30
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Agreed, the reason is works is not because of some arbitrary coincidence, it works for a valid reason that is not difficult to understand.

V = IR,

or in base units

Volts = Amperes * Ohms

A potential difference of 1 Volt will cause 1 Amp to flow in a resistance of 1 Ohm.

It is equally valid to say that a potential difference of 1 volt will cause one thousandth of an Amp (one times ten to the minus three Amps) to flow in one thousand Ohms (one times ten to the three Ohms).

That is, a potential difference of 1 volt will cause 1mA to flow in 1k ohm.

These units are much more convenient to use when working with a vintage radio because it allows us when confronted with an unknown I, R or V, when we know the other two, to do the calculation in our heads without having to juggle with powers of ten.

Yes, when you are doing more complex things than the very simple calculations above use your calculator and scientific notation.
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Old 12th Sep 2017, 12:57 pm   #31
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Quote:
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Err... I'm not sure about that last bit. As Mass and Length are quite different concepts (as is Time), how can we say whether the units are the same order of magnitude or not?
Since the gram and the metre are related in the metric system (a gram of water is that which occupies a one-centimeter cube) I think the metric system has a number of magnitude odditites.
Ah. But the metre and the kilogram are similarly related - a kilogram of air is (nearly) that which occupies a one-metre cube.

Choice of water (for detail, at a specific temperature); air (at a specific temperature and pressure), or anything else (mercury? liquid helium?) is purely arbitrary. I'm not suggesting that anyone should propose anything so sensitive as a gas as a mass standard (particularly when the pressure has to be specified, in derivatives of the units we're trying to define!), but it could have been. Water was just a practical choice, because it's plentiful, cheap, easy to purify, and holds its temperature well.
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Old 12th Sep 2017, 1:02 pm   #32
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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.
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Old 12th Sep 2017, 2:16 pm   #33
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Curious then that Americans still measure their antennas in feet. 468/f seems to be one formula they use. Yet they (reluctantly?) measure wavelength in metres.
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Old 12th Sep 2017, 2:41 pm   #34
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I don't think Americans use metric units for everyday shopping, measuring etc, only for science and engineering purposes.
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Old 12th Sep 2017, 6:22 pm   #35
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Ah. But the metre and the kilogram are similarly related
Yes, they are. They are of a similar order of magnitude.
Unfortunately, the Kilogram is not a basic unit. It is a multiple of the Gram.
The base unit of length ought to have been in the same order of magnitude as the base unit of mass. And it isn't. The Gram is tiny, compared to the Metre.
Very poorly thought-out.
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Old 12th Sep 2017, 7:32 pm   #36
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Unfortunately, the Kilogram is not a basic unit. It is a multiple of the Gram.
The kilogram is one of the seven SI base units. To quote from SI Brochure: The International System of Units (SI) [8th edition, 2006; updated in 2014]

Section 1.4: Coherent units, derived units with special names, and the SI prefixes
As an exception, the name of the kilogram, which is the base unit of mass, includes the prefix kilo, for historical reasons. It is nonetheless taken to be a base unit of the SI.
http://www.bipm.org/en/publications/.../chapter1.html

Appendix 2 Practical realization of the definition of the kilogram
The unit of mass, the kilogram, is the mass of the international prototype of the kilogram kept in air under three bell jars at the BIPM. It is a cylinder made of an alloy for which the mass fraction of platinum is 90 % and the mass fraction of iridium is 10 %.
http://www.bipm.org/en/publications/.../kilogram.html
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Old 12th Sep 2017, 8:57 pm   #37
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The non physical kilo is being worked on, all other units can be determined from basic principals. I think there are three fundemental things (I may be wrong) the speed of light, the mass of fundamental particals and the charge of an electron. And of course time. The rest are derived.
 
Old 12th Sep 2017, 9:24 pm   #38
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In my earlier message, I mentioned all the different systems met during education. In later working life I came upon many conversions which I used all the time, but today Kalee gave me a new one. I never knew the kg/per cu M for air. OK, I don't learn something new every day, but I WILL remember that.
I can still recall more general formulae than most people can shake a stick at, but have not needed many of them in the last 40 years.
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Old 12th Sep 2017, 9:28 pm   #39
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Within just a few generations the speed of light has been assumed to vary, measured to be constant (somewhat surprisingly at first), assumed to be constant and now (or soon?) defined to be constant. It will just be the conversion factor between our traditional length and time scales (metres and seconds). Let us hope it really is constant, because measuring it will no longer be possible. I feel a little uneasy about taking our current understanding of physics and defining it to be true.
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Old 13th Sep 2017, 9:03 am   #40
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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.

Speaking of 'grams', I wonder how many millikilograms my Collaro pickup is set to track at?
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