I have heard that if a meter needle is centered in a permanently positioned scale AND its meter needle stays centered when power is off, it uses a Wheatstone bridge circuit and is not subject to devialtions caused by different voltage (alkaline cell).
That is correct. If the needle position for correct exposure is the same as the position the needle rests in with the power off and that position is the centre then it is a bridge circuit. As this circuit shows correct exposure when both sides of the meter are at the same voltage i.e. zero voltage difference across the meter then the actual voltage of the cell cannot alter this.
"People who say things won't work are a dime a dozen. People who figure out how to make things work are worth a fortune" - Dave Rat.
Agreed that alkaline cells are not suitable. Silver oxide have much more stable voltage over their life and I have found that the above works, well enough at least, in sunlight and for very low light shots.
Actually guys just picked up a Weston Master 2 off ebay for 99p so I think that will do the job
Cliffy The Weston Master 2 has no I.S.O. speeds ! it's only calibrated in Weston speeds which was their own way of rating film although you can get a conversion chart this is a very old meter from the 1940s. I f you buy a Weston buy a IV, V, or a Euro-master they are cheap enough these days, and make sure you get the Invercone incidental light attachment with it.
As I wrote before, one alternative is to go to www.zuiko.com and buy the MR-9 adapter, into which you insert a silver oxide 386 battery and then put the MR-9 into the battery compartment; it fits the spot any PX625 or PX13 would otherwise fit.
If anyone has a Canon EF although originally designed for 1.35 V Mercury cells you can quite happily use 1.5V alkaline or 1.55V Silver Oxide button cells in it because it has a self regulating circuit that will accommodate the voltage difference, but not batteries with rubber "O" rings round them because they need to make electrical contact all the way round.
See at www.smallbattery.company.org.uk based in London, they have the best and large options to replace the old outlawed PX625 1.35 mercury batteries, PX400, PX32, PX675, PX27 ecc ecc. Personally, i have bought some Kanto MR9 adapters to replace the PX625.