To demonstrate V=IR.
Ohm's law demonstrator, overhead projector.
This setup, including variable voltage power supply, voltmeter, ammeter, and plug for resistor, sits on an overhead projector; the circuit is clearly visible. Changing the voltage produces an appropriate change of current. Several resistors can be used.
Steven A. Waltner and Thomas A. Lehman, When is Ohm's Law Valid?, TPT 31, 102-103 (1993). ◙J. D. Jackson, Surface charges on circuit wires and resistors play three roles, AJP 64, 855-870 (1996).