Easy to work out using basic circuit analysis principles - just draw out the circuit with lumped component values and apply the Ohm's Law to get the equivalent resistance.
Resistance for a 2.5mm^2 @70degC ~18 milli-ohms / metre and note the lower the temperature the lower the resistance will be due to the temp coefficient.
But, does the OP understand why it's (r1+r2)/4? IMO he should. And I don't mean "because it's a ring", I mean does he understand how to calculate the equivalent value of resistances in series and parallel combinations. That level of understanding should be a requirement for all sparks, IMO.
sime27: does any of this information help you or do you require further detailed explanation - as pointed out it's a fundamental requirement to understand the basic principles, lumped circuit model, nodal analysis, measured / calculated results and what affect they have on the circuit.
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