Where the current splits
Check Kirchhoff’s current law without moving the meter into current mode.
Difficulty: Breadboard. Estimated build time: about 25 minutes. 5-line bill of materials. Compare supplier offers when available. Build two parallel branches with one shared sense resistor.
Prepare the bench
Use an isolated, regulated 5 V source with a 20 mA current limit and a digital voltmeter with approximately 10 MΩ input resistance. These are bench tools, not included in the shopping list. Disconnect power before changing connections. Confirm breadboard rail continuity and check for supply shorts before powering up.
Build the shared return
Connect a 10 kΩ branch from +5 V to node S and a 20 kΩ branch from +5 V to the same node S. Connect a 1 kΩ sense resistor from S to ground. S is not ground. Keep all three resistors distinct and check the common node before powering up.
Read volts, calculate currents
Measure S relative to ground: expect approximately 0.652 V. Total current is VS / 1 kΩ, about 0.652 mA. Measure the voltage from S to +5 V with black at S and red at +5 V: about 4.348 V. Divide that reading by 10 kΩ and 20 kΩ to get about 0.435 mA and 0.217 mA.
Balance the junction
Add the two branch currents and compare with the sense-resistor current. They should agree within component and meter uncertainty. The sense resistor raises S above ground and reduces both branch currents; without it, the branches would carry 0.5 mA and 0.25 mA. Leave the meter in voltage mode throughout.