How to reverse the charge on Van de Graaff Generator?

oem7110

Referring to following link, Van de Graaff Generator generate positive charges on metal ball surface, I would like to use the same device to generate negative charges on metal ball surface, does anyone have any suggestions on what materials I need to replace to generate negative charges instead of positive charges on metal ball surface?
Thanks in advance for any suggestions

http://www.youtube.com/watch?NR=1&v=bUIPKeTdqJs