How is a voltmeter usually connected to a circuit under test?

A voltmeter is designed to measure the difference in electrical potential, or voltage, between two distinct points in a circuit. To take this measurement without interrupting the normal operation of the circuit, the meter must be connected across, or in parallel with, the component being tested. Voltmeters are constructed with very high internal resistance. This ensures that they draw only a microscopic amount of current, preventing the meter itself from altering the voltage it is attempting to measure. Connecting a voltmeter in series would introduce massive resistance into the main path of the circuit, effectively blocking the flow of current and causing the circuit to stop functioning properly.