On 03/01/2013 13:48, Dave Plowman (News) wrote:
In article
,
Man at B&Q wrote:
On Jan 3, 11:05 am, "Dave Plowman (News)"
wrote:
In article ,
alan wrote:

As long as the connecting leads can handle the current and are "low"
resistance, the resistance of the leads makes no difference to the
measurement.

Irrelevant. The meter is measuring the *VOLTAGE* dropped across the
reference resistor. No current at all flows in the meter leads.

OK a few fA or less must flow but with modern FET front ends even that
is probably an exaggeration. The voltage dropped in the meter leads
would not be measurable even with state of the art kit.

Swapping leads on my various DVMs - using what appears to be decent
quality leads - will give different resistance readings for very low
values. Try it.

But there the meter leads are both supplying the current and trying to
measure the voltage drop in the load. Four terminal measurement does not
have that problem - that is exactly why it is done that way!

The connecting leads referred to are clearly those to the shunt, hence
the reference to the current rating. They make no difference to the
reading across the shunt for a given set of test leads and DVM.

Well, yes. But my point is in practice they will make a difference.

No it shouldn't if the DVM is measuring voltage. Modern meters have such
a high input impedance that they do not affect the measurement.

Exactly where you measure the voltage between could do, but provided
that the system is a four terminal shunt then the measured voltage
should be stable and reproducible no matter what test leads are used on
the meter.

--
Regards,
Martin Brown