I know that 4 - wires method uses 2 wires for driving current throught ammeter and measure current values. So, resistivity wires will be removed from the equation. Ammeter wires resistance has no effect on current measure because wires resistance and ammeter are also serially connected with the source supply.
Voltage on Subject Resistance is measured by a parallel connected voltmeter (2 wires more). As it has a big intern resistance it has no significant current passing through this device. So, voltimeter resistance wire will has no effect to the voltage measure, but potential contact with Subject Resistance terminals. These two potential appearing in contact vA and contact vB will be cancelled when source supply polarity changes and two values of ammeter and voltage will be took and added in a expression like this:
- V+ = vA + I·R -vB
- V- = -vA + I-·R +vB
- V- + V+ = (I- + I+)·R --> vA and vB has been cancelled because they are signed opposed
By the way that explanations I' ve found it seems to say that wire length doesn' t matter. But I' m almost sure that length and connector position does.
But the fact is that I don' t find explanations where to see whether this principle is only true when:
voltmeter and wires amperemeter have the same distance respect Resistance Subject. Does it works correctly if 4 wires have different length?
must the ammeter and voltmeter two both contacts be connected at the same place? Does it works if ammeter connector has a distance between ammeter connectors and voltmeter connectors?
Could there be some distance between the connectors and Subject Resistance terminals? or must they be connected close to Subject Resistance terminals?
Could anyone clarify when the 4 wires method will totally cancel contacts potential and wire resistance in these three prior indicated situation?
Some images as example of distances, wires and connector position possibilities:
If I need to extend subject R terminals by signle wire I will get this circuit with 2 new resistances -R10 and R9- at the ammeter net shared with voltmeter one too:
So they R9 and R10 will experiment a potential because of the current passing through the left net. So I think that being connector the more closer to the subject R terminals is a condition. If not, make me know.