Well I have 4 Dabble A Batteries in a Pack that connects them in Series so I know the Batteries are 1.5 Volts each.
So my Battery Pack is giving me 6 Volts.
And I know my Led needs 3.5 Volts to work so my Voltage Drop must be 2.5 Volts.
And I can not find the Resistor I need without knowing the Current?
What if I just conect the 2 Leads coming off my Battery Pack to one another. And as the Current is moving through I usee my Voltmeter to see how many mA are moving.
Single LED's are basically current devices and usually operate between 1 and 50 mA. That's the ball park. Try a resistor for 1 mA, 5 mA, 10 ma, 20 mA or use a fixed resistor + a variable one. Make it so the adjust-ability is between 0.5 mA to 50 mA, Check the brightness of the LED.
There is a very wide range where the LED will operate.
Your LED probably isn't red. My guess is that it's a white LED?
What if I just conect the 2 Leads coming off my Battery Pack to one another. And as the Current is moving through I usee my Voltmeter to see how many mA are moving.
Yes. Now you know that the voltage across the resistor is 2.5 volts.
Now you need to decide how much current you want to flow through the resistor (same current flows through the LED).
As dougy83 already mentioned "If the current is 20mA, then the resistance would be 2.5/0.02 = 125 ohms"
20mA is usually the upper limit for leds, so I would use a resistor of 150 ohms or 180 ohms.
In another words. If you choose a 180 ohms resistor, And you know that the voltage across it is 2.5 V, then the current through it is:
2.5/180 = 0.014 A
(14mA)
And, because the resistor and the LED are in series, that same current flows through the LED.