Lectures on Physics has been derived from Benjamin Crowell's Light and Matter series of free introductory textbooks on physics. See the editorial for more information....

The Loop and Junction Rules


  • DC power supply
  • multimeter
  • resistors

1. The junction rule

Construct a circuit like this one, using the power supply as your voltage source. To make things more interesting, don't use equal resistors. Use nice big resistors (say 100 kΩ to 1 MΩ) - this will ensure that you don't burn up the resistors, and that the multimeter's small internal resistance when used as an ammeter is negligible in comparison.

Insert your multimeter in the circuit to measure all three currents that you need in order to test the junction rule.

2. The loop rule

Now come up with a circuit to test the loop rule. Since the loop rule is always supposed to be true, it's hard to go wrong here! Make sure you have at least three resistors in a loop, and make sure you hook in the power supply in a way that creates non-zero voltage differences across all the resistors. Measure the voltage differences you need to measure to test the loop rule. Here it is best to use fairly small resistances, so that the multimeter's large internal resistance when used in parallel as a voltmeter will not significantly reduce the resistance of the circuit. Do not use resistances of less than about 100 Ω, however, or you may blow a fuse or burn up a resistor.

Last Update: 2010-11-11