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How The Ancients Measured Small Electrical Resistances
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#2
Posted to rec.audio.tubes
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How The Ancients Measured Small Electrical Resistances
On Mar 9, 7:21*am, John L Stewart John.L.Stewart.
wrote: Behold, The Kelvin Bridge- http://en.wikipedia.org/wiki/Kelvin_bridge Cheers, John -- John L Stewart With such an atrociously small amount of detail about the 2 ganged pots in the above Wikipedia link, and a hopelessly inadequate explanation for dummies, I expect nobody at r.a.t will build a Kelvin bridge any time soon. But don't worry, be happy, because anyone with brain will figure out how to do what the ancients of 1912 or anyone else did, ie, to measures low R **well enough** using *one) fixed R and a **good** millivoltmeter, and a Vdc supply - can be varaible to suit value of R so that Idc is not dangerously high. The R could also be a pack of switched R of known values, so that an ohm meter is not required, just the VM, and Ohm's Law. Some might use an additional pot, say 10k linear, and make a simple bridge, then null the voltage across the bridge and calculate the low R. If the Rin of the VM 9M, then it won't affect the measurements. But of course a Vdc supply is needed, and an ohm meter which does down to 10 ohms accurately. Once set up, one can measure low R quickly, because you damn well don't want to **** around all day just to measure R. With the simple bridge an accurate VM is not needed, because you are measuring the *absense* of a voltage, and a VM built with an opamp & NFB will sure be good to measure 0V or very close to it between two circuit points. But a 20,000 ohms/V analog meter might do. When the pot is adjsted to get the null there should be no current in the VM so it does not matter much what the input resistance of the meter is. Patrick Turner. |
#3
Posted to rec.audio.tubes
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How The Ancients Measured Small Electrical Resistances
On 03/09/11 14:42, Patrick Turner so witilly quipped:
But a 20,000 ohms/V analog meter might do. When the pot is adjsted to get the null there should be no current in the VM so it does not matter much what the input resistance of the meter is. Right - no current flow at the NULL point. 'Crappy ohm/volt' analog meters do fine for this. As an alternate kind of 'bridge', I have measured capacitors that are either unmarked or 'marked in a weird manner' by creating capacitor bridges - 60hz output from a 12.6V transformer, measure the AC volts across the known and the unknown cap. This works well with a 0.1 uF cap in series down to about 100pf (below which you can then use smaller series caps). In this case I measured total volts and 'across the unknown' (so my meter's impedence didn't throw anything off). yeah, lots of ways to skin cats / disrobe / measure electronic components. |
#4
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Quote:
http://www.answers.com/topic/kelvin-bridge Makes a lot of sense to me. Information only. The ancients did not have all the goodies we have. Pls thank them for bequeathing those goodies to us! Cheers, John |
#5
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Quote:
Cheers to all, John |
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