Schematic
    
    
    
    Current through a resistor is given by V/R, where V is the voltage across
    the resistor and R its resistance. In the diagram above, use a 1000 Ohm
    resistor as R2 so that the current can be calculated easily, it is just the
    voltage at A1 divided by 1000. 
    If R1 is unknown, we can calculate it from (PV1-A1)/I.