I have to ask you a question, How are and what are you using to test for positive and negitive charges? What I,m saying is you have to have a scource charge of a known charge, such as a static generator or a electrophorus. With the generators you will need to use a proof-plane. The removable plate of the electrophorus will be your proof-plane. There is now on the market sold by science equipment companies that will tell you the polarity of a charge. I don't know the limit or range of such equipment. Hope this info helps.