Using a Multimeter to Identify Positive/Negative Charge

A multimeter is used to determine the sign of the charge on an electrophorus.

Discussion

An electrophorus is a device used to transfer electric charge. There are two parts to an electrophorus, a metal disk or plate with a non-conducting handle, and a base made of an insulating material such as plastic.

In this movie the plastic base of the electrophorus is given a static electric charge by rubbing with a piece of fur. Next the electrophorus disk is set on the base, and charged by induction (as explained in the next movie). Two metal plates mounted on an insulating base are connected to each other through a multimeter set to measure electrical current, with the plate on the left connected to the positive (red) jack of the multimeter and the plate on the right to the negative (black) jack of the multimeter. When the charged electrophorus is brought near the right hand plate, the meter shows a transient positive current flow. This tells us that electrons are moving from the right hand plate (nearest the charged electrophorus) through the multimeter to the left hand plate. When the charged electrophorus is removed, there is again a transient current flow, but now the meter reading is negative, which means that electrons are now moving from the left hand plate through the multimeter to the right hand plate. From these electron flows we can conclude that the electrophorus is negatively charged, and that when it is brought near the right hand plate, this negative charge drives electrons from the right hand plate to the left hand plate, leaving the right hand plate with a positive charge and the left hand plate with an equal negative charge. When the electrophorus is removed, electrons flow from the left hand plate to the right hand plate until both plates are electrically neutral.

It should be noted that the sign of the charge on an electrophorus disk depends on the composition of the base. Some materials produce a negatively charged electrophorus disk, as here; others produce a positively charged disk.