This electric pressure is called “potential,” and electricity tends to flow from a conductor of higher to one of lower potential. When we say that a place is so many feet above or below sea-level we are using the level of the sea as a zero level, and in estimating electric potential we take the potential of the earth’s surface as zero; and we regard a positively electrified body as one at a positive or relatively high potential, and a negatively electrified body as one at a negative or relatively low potential. This may be clearer if we think of temperature and the thermometer. Temperatures above zero are positive and represented by the sign +, and those below zero are negative and represented by the sign -. Thus we assume that an electric current flows from a positive to a negative conductor. [Illustration: PLATE I. _By permission of_ _Dick, Kerr & Co. Ltd._ HYDRO-ELECTRIC POWER STATION.] In a voltaic cell the plates are at different potentials, so that when they are connected by a wire a current flows, and we say that the current leaves the cell at the positive terminal, and enters it again at the negative terminal. As shown in Fig. 7, the current moves in opposite directions inside and outside the cell, making a complete round called a _circuit_, and if the circuit is broken anywhere the current ceases to flow. If the circuit is complete the current keeps on flowing, trying to equalize the electric pressure or potential, but it is unable to do this because the chemical action between the acid and the zinc maintains the difference of potential between the plates. This chemical action results in wasting of the zinc and weakening of the acid, and as long as it continues the current keeps on flowing. When we wish to stop the current we break the circuit by disconnecting the wire joining the terminals, and the cell then should be at rest; but owing to the impurities in ordinary commercial zinc chemical action still continues. In order to prevent wasting when the current is not required the surface of the zinc is coated with a thin film of mercury. The zinc is then said to be amalgamated, and it is not acted upon by the acid so long as the circuit remains broken.