Among two ammeters or voltmeters which one should be taken as test ammeter/voltmeter and which one as standard ammeter/voltmeter? Among the two, the ammeter/voltmeter with higher range should be taken as standard and the ammeter/voltmeter with a lower range should be taken as test. For example, if two ammeters have ranges as DC Ammeter 0-2.5 A (DC) and 0-5 A (DC) then the first one should be the test ammeter and the second one should be the standard. This is so chosen to calibrate the instrument properly when the error is negative. If the pointer of the meter deflects in the opposite direction (downscale) what is to be done? As the experiment is performed under DC excitation, if proper polarity of the supply is not given to any meter it can show deflection in the opposite direction (i.e., downscale). If such incident occurs then the terminals of that instrument are to be interchanged. Nothing else!!! How to calculate the percentage error? A common mistake for calculating the percentage error is to use the formula (say, for ammeter): `./view/images/1.png` However, for indicating instruments the error should always be calculated as % of Full-scale. Thus the proper way to calculate the error is `./view/images/2.png` and `./view/images/3.png`