Most metals have a positive temperature coefficient of resistance. This means that the higher the temperature the higher the resistance. You did know that I hope?
Measure the resistance of a 100 Watt lamp when cold and then calculate the current flow when connected to a 230 Volt supply. Bet you did not get under half an amp?
So what does that have to do with the price of eggs or indeed calculation of R1+R2 with respect to Zs?
When using a low value ohmmeter the wiring, the supply will be dead. This means that there will be no heating of the conductors, since no current is flowing, so the resistance obtained is actual less than that which would be measured.
There is a calculation that can be carried out providing that temperature measurements can be taken but it is accepted practice to apply the 3/4 approximation.
Hang on though why is Zs all important when it comes to electrical testing anyway? More in the next posting.
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