01-04-2013, 08:09 PM
Sod's or anyone else's I know not, but it is certainly a strange law. 
Scale reading: over the scale range 0 to 80, it looks like an inverse-square law: at scale = 70, coil current = 1.4 mA. But at scale = 80, the curve suddenly straightens out (well, nearly straightens) and continues in that fashion up to the scale end = 100, where coil current = 1.8 mA.
Inside the meter itself, hand-written on the magnet: fsd = 1.8 mA. Well, that's re-assuring to some degree. Inside the case, there is a small resistor in series with the coil - presumably to bring the fsd voltage up to some convenient numerical figure: I haven't measured any resistances yet.
As for the current versus scale reading, I am beginning to wonder if the magnet has lost some of its magnetism. The movement itself is also substantially under-damped: a shunt R will easily fix that.
Al.
Scale reading: over the scale range 0 to 80, it looks like an inverse-square law: at scale = 70, coil current = 1.4 mA. But at scale = 80, the curve suddenly straightens out (well, nearly straightens) and continues in that fashion up to the scale end = 100, where coil current = 1.8 mA.
Inside the meter itself, hand-written on the magnet: fsd = 1.8 mA. Well, that's re-assuring to some degree. Inside the case, there is a small resistor in series with the coil - presumably to bring the fsd voltage up to some convenient numerical figure: I haven't measured any resistances yet.
As for the current versus scale reading, I am beginning to wonder if the magnet has lost some of its magnetism. The movement itself is also substantially under-damped: a shunt R will easily fix that.
Al.
