This video shows a simple circuit consisting of a light bulb and a variable resistor (rheostat). The voltmeter measures the voltage across the terminals of the battery: V = emf - Ir, where emf is the electromotive force of the battery, I is the electric current in the circuit and r is the internal resistance of the battery.  When the switch is open, I = 0 and V = emf. When the switch is closed, a current flows through the circuit and V = emf - Ir. V can also be calculated as IR where R is the total resistance of the circuit. If R is increased, I decreases and V increases (as the product Ir gets smaller and V = emf - Ir).