Ohm's law is : V = I * R or I = V / R
We have a piece of conductor or resistor for which we want to experimentally prove Ohm's law.
1. We should have some idea of how much resistance that could be. Then appropriately estimate the amount of current that flows through that resistance. Then use the ammeter that allows that much current.
2. Check the ratings and ranges of the voltmeter and ammeter. Try not to cross the ratings.
3. Use insulated static electricity proof footwear.
4. Do not handle electric instruments with wet hands.
5. Each material has a range of voltages and currents in which the law is valid. So Outside the range, it may not be valid. Also, note that non-ohmic substances or devices do not obey Ohm's law.
Start with the lowest voltage setting of the battery and lowest current in the the setup. Then increase that step by step.
6. Make sure to switch off the battery after each measurement. The resistor may get heated and then its resistance will vary due to change in temperature. Allow a little time duration for it to cool off, before taking another measurement.
7. Resistance may be altered if there are magnetic fields or electric fields near by. They may induce some emf in the conductor. That will alter measurements. Be sure to have no electromagnetic interference.
8. The measurement of resistances of gases and liquids may not exactly follow the same procedure as for solid wires.
9. Be careful not to short any device by wrong connections. Use Earthing where required for the devices used.
10. Take precautions as mentioned in the lab manual.