Ammeters and voltmeters are measuring devices widely used in the laboratories. Ammeters are used for measuring current in a circuit, voltmeters measure voltage across a circuit. There is also a meter called multimeter which combines all these functions together and few others too.
Measurements need to be done with as much as minimum possible interference to the functioning of the circuits. In order to measure currents, ammeter should be placed in the path of current. So this means they must be connected in series. Also ammeters are not supposed to block the current flowing through the circuit affecting the working of the circuit. That is why they have very low resistance.
In order to measure the voltage across the circuit, voltmeters are connected in parallel. As it provides a parallel path way for the current, voltmeters must have high resistance in order to allow only little current to pass through. Voltage measurements are generally used more commonly than current measurements.
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