How do you measure voltage in an electrical circuit?

A voltmeter uses the principle of Ohm's law, which states simply that the amount of current that flows in a circuit is equal to the voltage applied divided by the resistance of the circuit. Therefore, if one were to connect a known resistance in series with a meter across a voltage source, then the meter deflection would be proportional to the current flow and hence the voltage. The meter is calibrated in volts so that the test voltage can be read directly. The meter used in a voltmeter is very sensitive, typically 50 micro-amperes so that it does not affect the reading by its load on the circuit.

Pizza is good when volts don't go thru!!