DC Null Indicator - a voltage or current meter which does not have to be very accurate, its only function is to tell when there is NO VOLTAGE (or current).
Resolution/Sensitivity
A 100μA meter is more sensitive that a 1mA meter is more sensitive than a 10mA meter, etc.
Consider a meter scaled 0 to 100mA, there are 100 scale divisions on the meter, that meter has a resolution of 1mA.
A silmilar meter scaled 0 to 100mA has 20 scale divisions. That meter has a resolution of 5mA.
Also be aware that resolution and accuracy are not the same thing.
Say the meter above with the 100 divisions had the scale drawn by a guy who was drunk at the time
when the pointer is on the 50 mark there is actually 52mA flowing therough the meter.
Yet the second meter with only 20 divisions, with the pointer on the 50 mark has 50.3mA flowing through it.
The second meter with the lower resolution is more accurate.
Where is the HP meter used?
Wherever it is useful for the job in hand!
That meter could be used as a null detector on the wheatstone bridge.
If it is on the bench and ready to use, you could use it to test the batteries in your radio/ipod/paddy thing/must have toy of the day!
JimB