1. An ammeter is used to measure the current in an electrical circuit. It is connected in series with the circuit and the resistance of an ammeter is to be made as low possible, as it is desired to measure currents in a electrical circuit without causing any appreciable potential drop. If it is connected in parallel with a circuit then it would cause a short circuit since the resistance of the ammeter is very low which will damage the ammeter or the circuit.
2. A voltmeter measures the potential difference across two points of an electrical circuit. It is connected in parallel to the branch across which the potential difference is to be measured. If it is connected in series, then the potential difference between the two points at the same line is zero and hence, the readings will always be zero.
Also, the resistance of a voltmeter must be high so that a small magnitude of current that flows through the voltmeter does not alter the current conditions in the branch, across which it is to measure the potential difference.
4. Semiconductor diodes, vacuum tubes, transistors, etc does not obey ohm's law. For such non-ohmic devices, the value of R is not constant.
Ohm's law fails when V/I is not a constant.
i) As per ohm's law, the current varies uniformly with potential difference. The curve drawn between potential and current must be a straight line.
But in case of conductors, due to rise in temperature, with increase in current the resistance changes and hence, the V-I graph will be slightly
curved. Ohm's law is obeyed only when the temperature of the conductor remains constant.
ii) In semicondutor diodes, the increase in current for the same change in voltage during reverse bias is smaller than that during forward bias. This
indicates, that the variation of the current with the potential difference depends upon the sign of the potential difference applied. Dependence of V
on current I is non-linear in such a case.
iii) In a thyristor, the current decreases with increase in the potential difference.
iv) In electrolytes, the back emf developed causes a variation in the V-I graph.
Note: Kindly ask the other questions as a separate query.