Current decrease in a lamp due to the change in scale of an analog ammeter
To change the scale of an ammeter, you are changing a resistor in parallel with the meter not in series with it.  The full scale voltage will be the same for all ranges.  So for a 1 Amp scale on a 1V meter you would put a resistor that gives you 0.1V across the meter.  For a 100mA scale, you would put a resistor that gives you 1v across the meter.  Ra should be across the meter.