Brand New Battery!
To count out any inconsistencies due to my woefully dead battery, here's a new experiment with a brand new 6V lantern battery, with a 100 ohm and a 16 ohm resistor removed:
1) Measured the battery source voltage 6.47Vs
2) Measured the new resistor 215 ohms
3) Hooked up the circuit, LED alight and measured 4.01Vresistor
4) Measured the LED voltage 2.39Vled
5) Calculated the current and LED voltage drop:
a) Current:
I = Vresistor / R
I = 4.01 / 215
I = 0.0186511627906977A
I = 18.65mA
b) LED voltage drop:
Vf = Vs - Vresistor
Vf = 6.47 - 4.01
Vf = 2.46
(see attached filmstrip of the experiment)
From the previous experiment with the same LED:
Vf = Vs - Vresistor
Vf = 7.82 - 5.09
Vf = 2.73
I then repeated the experiment with a 12 volt wall wart power supply and a 330 ohm resistor added (I went out and bought 100 pieces 330 ohm pack the other day
) to give 536 ohms:
1) Measured the source voltage 12.15Vs (fluctuated a few decimal points up and down)
2) Measured the voltage across resistor 9.77Vresistor
3) Calculated LED forward voltage:
Vf = Vs - Vresistor
Vf = 12.15 - 9.77
Vf = 2.38
Given the fluctuations in the source voltage I'm more inclined to trust the 6V battery answer - the LED Vf = 2.46. This value is consistent with the 12V supply results due to the fluctuations in Vs.
So I guess you were correct audioguru - the dead battery was leading us in the wrong direction because it was unstable?