② If the resistance is too large, you should choose to increase the gear, that is, the C gear;
Ohm zero, that is, a;
Re-test, that is, b;
(3) When the battery is not used for a long time, the internal resistance increases, the electromotive force remains basically unchanged, and it can still be zeroed normally;
Because the full bias current Ig is constant, it is given by the following formula:
In the range of Ig=ER, the internal resistance of the ohmmeter is constant in the range of r, and the measured value of the measured resistance is reflected by the pointer of the ammeter. From I=ER +Rx=Ig 1+RxR, it can be known that when r is constant, I is constant, and the pointer is unchanged compared with the original position, so the pointer of the ohmmeter is accurate.
So the answer is:
① 1500Ω;
② Cab;
3 accurate.