Yeah potential difference per unit of current is a good way of thinking about resistance IMO.
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This is about minimising the effect the measuring device on the quantity you're trying to measure
Ammeters are used by placing them in series at a point in the circuit where you want to measure the current flowing - but inserting a high resistance reduces the current flowing. zero resistance is ideal because it has no effect on the current flowing through it.
Voltmeters are used in parallel where you're interested in the PD between 2 points - but connecting a low resistance between two points reduces the effective resistance between those points (resistors in parallel rule) and effects the PD between them. infinite resistance is ideal because infinite resistance in parallel has zero effect on the effective resistance..
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Are you ok with some materials being better electrical conductors than others? if you wanted to make resistors with the same resistance out of wires that were the same diameter the resistor made out of a good conductor like copper would have to use a longer length of wire than a resistor the same value made out of a less good conductor like iron.
if you say something is a good conductor you're saying it has a high conductivity
resistivity is just 1/conductivity