1. THEVENIN’S THEOREM
Thevenin’s theorem permits the reduction of a two-terminal dc network with any number of resistors and sources (Complex Circuit) to one Equivalent circuit having only one source and one internal resistance in a series configuration shown belw:

2. Original and Thevenin Equivalent Circuit

3. Measuring & Calculating Vth by MultiSim:
Circuit above shows how to measure the Thevenin Voltage (Vth). That is measured 4V.
The Circuit below shows how to measure Thevinin’s Resistance (Rth):
Conclusion: To measure Vth, remove the load resistance (unknown), then measure the voltage across open circuit terminal
(Vo.c. = Vth).
The Circuit below, shows how to measure and calculate the Thevinin Resistance (Rth)

4. Original and circuit to measure Rth

5. Measured and Calculated Rth
To measure Rth, with load resistance removed, also remove and replace source voltage by a short (jumper wire), then measure and calculate the Thevenin resistance zacross the Open Circuit terminals:
Rth is measured 6 Ohms.
Considering the jumper wire, 6 and 3 Ohms resistors are in parallel, and result is in series with 4 Ohm resistor.
Rth (Cal.)= [(6 x 3)/(6+3) + 4] =2 +4 = 6 Ohms

6. Original & Thevenin Equivalent Circuit:

7. Using Ohm’s Law to Calculate Unknown Current
As you can see from this simple Tutorial Example, the load current for original and the Thevenin Equivalent Circuit are Identical.
This verifies our solution and demonstrate how to measure and calculate the unknown element current by using Ohm’s Law for Thevnin Equivalent Circuit:
IL = (Vth)/(Rth + RL) = (4V) / (6+6)
IL = 4/12 = A => Calculated!
Compare this with measured, they are exactly equal! Now you can apply this to other Problems.