When a body charged with an _excess_ of, or _positive_, electricity, is connected with the earth, electricity is transferred _from_ the charged body to the earth; and similarly when a body is charged with a _deficiency_ of, or _negative_, electricity, is connected with the earth, electricity is transferred _from_ the earth to the body. Generally whenever two conductors in different electrical conditions are put in contact, electricity will flow from one to the other. That which determines the direction of the transfer is the relative _potential_ of the two conductors. Electricity always flows from a body at _higher potential_ to one at _lower potential_, when the two are in contact, or connected by a conductor. When no transfer of electricity takes place under these conditions, the bodies are said to be at the _same potential_, which may be either _high_ or _low_. The _potential_ of the earth is assumed to be _zero_. _Definition of Potential._--"The _potential of a body or point, is the difference between the potential of the body or point, and the potential of the earth_." Difference of potential for electricity is analogous to difference of level for water. Now, since, when a metal is placed in a vessel containing a liquid, electricity is produced, the liquid becomes of a different potential to the metal, each being electrified in an opposite way; and therefore, as above stated, there being a _difference_ of potentials, electricity will tend to flow from one to the other. This is evidence of a _force_ being in action, for there can be no motion without some force to produce it. _Electro-motive Force._--_Electro-motive force_ is the name given to a peculiar force to which is due the property of producing a difference of potentials. When it is said that zinc and water produce a definite electro-motive force, what is meant is, that by their contact a certain definite difference of potentials is produced. The _electro-motive force_ of a Voltaic element may be termed its _working_ power, in the same way as the pressure of steam is the working power of a steam engine, though this is not to be considered as the real source of power, which, as will be seen, is uncertain. Due to the difference of potential of the metal and the liquid, a current of electricity will flow from one to the other, causing the chemical decomposition of the liquid, and the reaction may be taken as the origin of the power employed. But while the expenditure of energy (which is necessary to produce a _force_) is accounted for by taking the chemical action as the source of power, the preceding cause of this chemical action, viz. the flowing of the current of electricity due to the difference of potential of the metal and the liquid, must also have first involved the expenditure of energy; thus the real source of power is very uncertain.