A null measurement be more accurate than one using standard voltmeters and ammeters because standard measurements of voltage and current alter circuits. There are numerical uncertainties in standard measurement of voltage and current. Current flow reduced by an ammeters and voltmeter draws more current.
In null measurement circuit does not alter. In null measurement voltage balance and no current passing through the measuring device.
factors which are affects the null measurement:
-Resistance of wiring'
-Change in resistance due to change in temperature
-current sensitivity of measuring device.