First, how can I record the open circuit voltage and short circuit using digital multimeter?

What is the procedure?

Even if I change the resistors ranging from 200-8500 ohm, the voltage reading did not vary much. 

Is the voltage obtained can straight be used in the calculation of current by ohm law? Because the calculated current is different from the one I recorded using multi meter,  shouldn't it be the same? 

Lastly, the mineralization of pollutant should be done simultaneously with the voltage measurements? Because if do it together, will the varying resistors affect the photodegradation?

Note: it'll be better if the explanation attached with literature. 

Thank you