Has anyone here had experienced experimenting Millikan Oil drop using this site: http://webphysics.davidson.edu/applets/pqp_preview/contents/pqp_errata/cd_errata_fixes/section4_5.html ?

can someone help me please?
I tried catching the drop by adjusting the voltage but once i set the voltage the drop will still move. Is it supposed to be like that? Should I keep adjusting the voltage in order to suspend the drop at one position? Also, the rays will ionized the droplet right? so it means that my independent variable is not only the voltage but also the rays ?

Question

Has anyone here had experienced experimenting Millikan Oil drop using this site:  http://webphysics.davidson.edu/applets/pqp_preview/contents/pqp_errata/cd_errata_fixes/section4_5.html ? 

can someone help me please?
I tried catching the drop by adjusting the voltage but once i set the voltage the drop will still move. Is it supposed to be like that? Should I keep adjusting the voltage in order to suspend the drop at one position? Also, the rays will ionized the droplet right? so it means that my independent variable is not only the voltage but also the rays ?