Question

How is a voltmeter connected in the circuit to measure the potential difference between two points?

Answer 1

Hint : To measure the potential difference between two points, the terminals of the voltmeter must each be on one point of the two points of interest. Voltmeters are made with very high internal resistance.

Formula used: In this solution we will be using the following formula;
$F = q\dfrac{V}{d}$ where $F$ is the force, $q$ is the charge, $V$ is the potential difference, and $d$ is the distance between the points of interest.

Complete step by step answer
Let us consider a resistor connected to a voltage source in a circuit (possible containing other resistors). In general, current will flow through the resistor. Now the potential difference across the resistor is a measure of the force which drives the current through the resistance, as we can see from the equation
$F = q\dfrac{V}{d}$ where $F$ is the force, $q$ is the charge, $V$ is the potential difference, and $d$ is the distance.
Hence, we see that the higher the potential difference, the higher the force, and the lower the potential difference the higher the force. Since the equation is dependent on the distance between the two points, the p.d. is measured across the two points of interest. Hence, each terminal of a voltmeter is connected to each end of the resistor.
Hence, the voltmeter terminals and the resistor terminals share the same point, which is exactly the nature of a parallel combination of components.
Thus, for a voltmeter to measure the potential difference between two points, it is placed parallel to the components (which could be a wire) attached to the ends.

Note
An alternative chain of reasoning is given thus: the presence of an additional component in a circuit affects the state of the circuit. To measure accurately, a voltmeter is designed to have a very large resistance. Hence, if this resistance is placed in series to a resistor, it effectively becomes an open current and thus cuts off current from the resistor. However, if the voltmeter is placed parallel to the resistor, the higher resistance prevents current from flowing through the voltmeter, but since elements connected in parallel has the same potential difference across them, hence, the voltmeter is at the same potential, it measures the potential of the resistor (or any other component).