Have students construct an investigation to repeat the test with different types of liquids, such as vinegar, lemon juice, rubbing alcohol, or carbonated water. (If you do this, the students will have to cover the jars with a plastic bag to have the fumes contained. Make sure they do not use volatile liquids like gasoline.) If they are testing different inks in different liquids, make sure that they test each type of ink separately in each type of liquid. Have them record their observations in a science log and compare their results with those of the rest of the class.
The charge around the conductor is measured using a proof proof plane is a small brass disc on an insulated handle. It is first Earthed to ensure there is no charge on the disc. The proof plane is then touched onto the surface of the conductor. The proportion of charge acquired by the disc when it is touched onto the surface of the conduction is proportional to the surface charge density of conductor. The charge can be measured by touching the brass disc of the proof plane on the inside of a metal can standing on the top of the cap of a gold leaf electroscope. The deflection is an indication of the charge at the surface at that point.