Originally Posted by cao
The point of a simple balanced bridge circuit is that at the null point (ie correct exposure indication) the two arms of the bridge across which the meter is connected are at the same potential, regardless of what that is. Consequently no current flows and the meter gives a null or centre indication. Whether the Spotmatic meter system is in fact a balanced bridge circuit is another matter (I've not seen a circuit diagram), but the fact that the needle defaults to the underexposed end of its travel when the meter is switched off and doesn't naturally fall to the centre suggests that it does need current flowing and that although it is a bridge, it isn't a balanced bridge but an unbalanced one, indicating a null when a current of 3uA flows through the meter winding.
My feeling is that a different battery voltage will theoretically give a different null point but that the difference of 0.15 volts between the original EPX400 and a 1.5v cell is sod-all (to use a technical term) in the overall run of things and within the tolerances of the mechanical shutter, fim latitude, etc. won't make a blind bit of difference.
All that theory and pontification aside, I still have a Spotmatic working happily with an EPX400 cell (don't tell the Green Police) so will buy a silver oxide cell, stick it in and see what transpires (ie whether it blows all the above out of the water!)
Good fun, this analogue photography!