The key differences between Ammeter and Voltmeter are discussed in this tutorial on the basis of certain important factors such as connection with appliances, uses, safety, resistance, ideal behavior, and measuring units.
Devices like ammeter and voltmeter are employed in order to measure electricity. Both of these devices are based on the galvanometer which is employed to detect small current. An ammeter is a device which is used to measure the “current” (in ampere, micro-ampere, and milli-ampere) through a conductor whereas a voltmeter is a device which is used to measure the “voltage” (in volts) between two points or terminals. Both the devices act very differently in terms of operation as well as circuit placement.
Difference between Ammeter and Voltmeter
Characteristics Ammeter Voltmeter
Use Measures current through an appliance Measures potential difference between two points
Measuring unit Measures current in amperes or even smaller units (mA, μ A) Potential difference is measured in volts.
Safety Low resistance to electricity so must not be connected straight across the supply High resistance to electricity so be able to be connected straight across the supply
Amount of current Large amount of current passes through Very small amount of current passes through
Connection with appliance 
Resistance Low resistance instrument High resistance instrument
Ideal behavior Resistance of an ideal ammeter is zero Resistance of an ideal voltmeter is infinity
Circuit Circuit must be disconnected from the power source in order to attach the ammeter Circuit does not need to be disconnected to attach the voltmeter
Representation 
How to connect ammeter in a circuit
An ammeter must be connected so that the current to be measured flows through the instrument. Consequently, Ammeters are always connected in series with the component in which current is to be measured, as shown in the figure.
An ammeter must have a very low resistance so that it does not affect the current level when connected to a circuit.
How to connect voltmeter in a circuit
A voltmeter must be connected so that the voltage to be measured appears at the voltmeter terminals. Consequently, A voltmeter is always connected in parallel with the component that is to have its voltage measured, as shown in the figure.
A voltmeter must have a very high resistance so that it does not affect the voltage level when connected to a circuit.