Gilbert found that a glass wand and a rubber wand produce

The personal physician of Queen Elizabeth I of England, William Gilbert found that the glass wand and the rubber wand rubbed with the piece of fur and silk handkerchief, respectively; that both of them have opposite charges.

Explanation:

Discovering the Electric Charges

When two substances carrying opposite charges are placed together, they either attract or repel each other. When William Gilbert, physician of England's Queen Elizabeth I experimented the nature of sphere balls against the rubber wand which was rubbed with a fur as well as the glass wand which was rubbed with silk, he found that the sphere balls repelled each other after getting touched by the rubber wand.

The same way, when the sphere balls were touched to the glass wand, rubbed with silk cloth, they attract each other. This signified that substance have two types of charges, opposite in nature to each other.