The human body consists largely of salt water. Pure water is not a terribly good conductor, but salt water, with its ${\text{Na}}^{+}$ and ${\text{Cl}}^{-}$ ions, is. Consequently, and occasionally tragically, humans are reasonably good conductors. This fact allows us to understand how it is that touching a charged object discharges it, as we observed in Experiment 10. As FIGURE 22.9 shows, the net effect of touching a charged metal is that it and the conducting human together become a much larger conductor than the metal alone. Any excess charge that was initially confined to the metal can now spread over the larger $\text{metal }+\text{ human}$ conductor. This may not entirely discharge the metal, but in typical circumstances, where the human is much larger than the metal, the residual charge remaining on the metal is much reduced from the original charge. The metal, for most practical purposes, is discharged. In essence, two conductors in contact “share” the charge that was originally on just one of them.

FIGURE 22.9 Touching a charged metal discharges it.

Moist air is a conductor, although a rather poor one. Charged objects in air slowly lose their charge as the object shares its charge with the air. The earth itself is a giant conductor because of its water, moist soil, and a variety of ions. Any object that is physically connected to the earth through a conductor is said to be grounded. The effect of being grounded is that the object shares any excess charge it has with the entire earth! But the earth is so enormous that any conductor attached to the earth will be completely discharged.

The purpose of grounding objects, such as circuits and appliances, is to prevent the buildup of any charge on the objects. The third prong on appliances and electronics that have a three-prong plug is the ground connection. The building wiring physically connects that third wire deep into the ground somewhere just outside the building, often by attaching it to a metal water pipe that goes underground.