I would like to measure a voltage drop across a resistor. The voltage over that resistor is 0,7V to 1.4V although the voltage at the resistor can vary between 10 to 14V (that's why the voltage drop is so interesting).
How can I do that?
Well it's pretty easy if you have a digital multimeter, you just place the volt meters leads across the two resistor leads and measure it's voltage drop directly. Or you could measure each end with respect to ground and subtract the difference, both would be the voltage drop across the resistor.
Now if you asking how you could have an arduino measure it when that is a different animal in that the voltage to be measured at either end of the resistor must always be less then +5vdc as that is the electrical limit the AI pin can be subjected to.
So what are you trying to accomplish or learn?