I am doing simple ohm's low using sinometer digital power supply and potentiometer 50 watt when i begin to increase the current the voltage from the power supply decreased that's okay but when i measured the voltage on the potentiometer using my voltmeter it give a value lower than that displayed on the power supply and this value decreased more and more when increasing the current , so what is the reason for this accuracy error ? is it my voltmeter needs the battery to be changed ?could the battery weakness do that?or the error is in the power supply