The resistance of an ammeter should be low. An ammeter is used to measure the current in a circuit and it should always be connected in series. Resistance means the obstruction caused to the movements of electrons. If resistance is more, current will be less. So, if an ammeter has a high resistance then the current measured will be quite less as compared to the actual current which is undesirable. If an ammeter has zero resistance, then we will get the exact current in the circuit. But this is not possible as every material in this world has some internal resistance.