Franklin decided that sparks given off by an object charged by a glass rod (vitreous electricity) looked more like fluid leaking out than did the sparks from an object charged by a rubber rod (resinous electricity). Thus he decided that glass had an excess of electrical fluid.

Today we know that electric charge is mostly carried by electrons. Electrons are charged the same way that rubber or plastic is (negatively). Thus we say that they have a negative charge. Because they are transferred much more easily than are the more massive positively charged nuclei, when there is an excess of electrons the object is negatively charged; when there is a lack of electrons it is positively charged. So even though Franklin made the wrong choice, we still follow his convention.