Ammeter: In series connection, the same current flows through all the components. Ammeter aims at measuring the current flowing through the circuit and hence, it is connected in series: so that the same current that is there in the circuit flows through it and gets measured.
Ammeter has a low resistance, because we do not want to change the current that is flowing through the circuit.
Voltmeter: A voltmeter measures voltage change between two points in a circuit. So u have to place the meter probes at these two points, without removing the portion of circuit between the probes (or else you change the circuit). This then, by definition, places the voltmeter in parallel with the section of circuit between the probes.
When you place the meter probes at the two points you want to measure, a tiny portion of the current in the circuit will now go thru the meter and give you a reading. We make the voltmeter a very high resistance instrument. So when the current splits into the meter the high resistance allows only a tiny amount of original current to split into the meter
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