Archived posting to the Leica Users Group, 1999/06/21
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Michael Dienert wrote: > hello everybody, > Snip: > most light-meters using CdS cells use a simple electronic > (bridge-) circuit to transform the light dependent resistance of the CdS > cell to a proportional voltage which is measured with an analog meter. > thus the meter's readout is dependent on the voltage one uses to power > the circuitry. I'll have to disagree with you there. If a bridge circuit is used, then the battery voltage is not important. The meter of the bridge will show a zero position, when the light reading is correct. In other words, when the bridge is in balance. No matter what voltage is used. The notable difference with a lower or a higher voltage is the amount the meter deflects away from the zero (balance) position , when the meter is out of balance. The voltage independence is the big advantage of the bridge. The problem with many light meters was the reliance of the voltage accuracy of the Mercury battery. This constant battery voltage made a bridge circuit unnecessary. (less components and simpler). But the voltage had to be constant to the end of the battery life. Mercury batteries had this property. they where even used as voltage reference in earlier pieces of electronic equipment ., Varta in Germany makes still Mercury batteries. I get my Pentax Spotmatic Batteries from there. The reason why the Germans still manufacture batteries containing Mercury is, because they have a proper disposal system for batteries. (that's any type of battery). They large bins, together with bins for glass and paper, located at strategic points in every town. The normal rubbish collection in germany is very costly. this encourages the people to drop the glass, paper and battery rubbish in to these bins. Regards, Horst Schmidt >