A voltmeter is a high resistance electrical device that is connected in parallel in a circuit and measures the potential difference across any load. It has very high resistance because it is connected in parallel across any component of a circuit; it draws a very small current from the main circuit and most of the current passes through the element. Hence, the potential difference across the component is not affected. If the resistance is small, the effective resistance is affected and most of the current passes through the voltmeter, and the potential difference reduces across the component and the voltmeter cannot measure accurately.