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Old 15th Jun 2012, 7:54 pm   #1
'LIVEWIRE?'
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Question The S.I. unit of electrical conductance is?

Many years ago I remember the 'In Your Workshop' feature in Radio Constructor having Smithy state that the unit of conductance was the 'mho'
Dick's reply included the immortal line 'two ohms is equal to "half a m(h)o!"
Now, as some members might have seen, the 'Eggheads' quiz in Radio Times
asks 'In physics, what is the SI unit for conductance?, the answer being, I thought, the Coulomb, but Kevin Ashman's answer is the Siemens.
Is he right, and, if so, when was this name adopted?
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Old 15th Jun 2012, 8:01 pm   #2
kalee20
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Default Re: The S.I. unit of electrical conductance is?

Yes, he's right.

It used to be the mho (or amps per volt). But in the 1970's the folks at SI decided to rename it the siemens (symbol S). Shame, I rather like the mho (the symbol being an upside-down omega - easy for handwriting but no a standard ASCII symbol!).

The coulomb is a unit of charge.

Last edited by kalee20; 15th Jun 2012 at 8:02 pm. Reason: Added the SI!
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Old 15th Jun 2012, 8:14 pm   #3
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Default Re: The S.I. unit of electrical conductance is?

Not to be confused with the unit of time: the second (s). 1ms is a short time; 1MS is a short-circuit!
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Old 15th Jun 2012, 9:00 pm   #4
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Default Re: The S.I. unit of electrical conductance is?

Quite so - it's really bad that s and S are symbols of units! Personally, I always write sec for seconds. (And I've yet to invoke the siemens).
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Old 15th Jun 2012, 9:43 pm   #5
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Default Re: The S.I. unit of electrical conductance is?

I tend to use s or sec, depending on who I think I am writing for.

We could write valve transconductance in mS, but I suspect that would confuse some people. Some FET datasheets use mS or S.
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Old 15th Jun 2012, 9:57 pm   #6
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Default Re: The S.I. unit of electrical conductance is?

Thank you for confirming about the siemens. My reaction to the quiz was "half a mo (mho)!" also. I hadn't picked up that name change either.
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Old 16th Jun 2012, 9:57 am   #7
Mike Phelan
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Default Re: The S.I. unit of electrical conductance is?

For clarification, the SI units (Système International) are alternatives for the Imperial ones we used to use.

Like millimetres vs inches. Thankfully, some remain the same - volts, amperes et al.
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Old 16th Jun 2012, 10:02 am   #8
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Default Re: The S.I. unit of electrical conductance is?

Thanks for refreshing my memory re; the Coulomb, folks. Only been working with electronics for 40+ years - should have known a coulomb is a unit of energy. Did the good folks at S.I. think about the possible confusion between s.(second) and S.(Siemens) when they adopted that name for a unit of Conductance?
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Old 16th Jun 2012, 10:08 am   #9
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Default Re: The S.I. unit of electrical conductance is?

The coulomb is a unit of electrical charge, which is not energy.

The dimensions of energy are Force (= mass x acceleration) x distance x time.

A proper dimensional analysis of these quantities will assist in a thorough understanding of the essential issues. I curl up when Govt. ministers confuse power and energy.

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Old 16th Jun 2012, 11:09 am   #10
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Default Re: The S.I. unit of electrical conductance is?

I just noticed my mistake, Leon. Meant to say 'unit of charge'!!
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Old 16th Jun 2012, 1:43 pm   #11
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Default Re: The S.I. unit of electrical conductance is?

Leon - I tend to "curl up" when government ministers mention anything at all...!

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Old 16th Jun 2012, 2:25 pm   #12
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Default Re: The S.I. unit of electrical conductance is?

Confusion between s and S (units) is less likely than between m and M (multiplier prefixes), as context will usually make it clear what is meant even when the writer (or typesetter) is confused.

By the way, the usual convention is that the unit name has no initial capital even if it is named after a person. So power is measured in watts (W), energy in joules (J), and conductance in siemens (S). I guess that can help clarify when we are talking about a person and when we are talking about a quantity derived from their name.
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