The difference of potential at the terminals of a cell when it is on open circuit is its electro-motive force, or E. M. F. When the cell is sending a current, the difference of potential at its terminals is less, - by an amount called the loss of potential in the cell. In Fig. 1 the cell with terminals A and Dis sending a current through the circuit from A to B, C, D, and back to A. The sum of the differences of potential between A and B, B and C, and C and D, is equal to the difference of potential between A and D; and if to this we add the loss of potential in the cell, the sum is equal to the E. M. F. of the cell. The practical unit of E. M. F. and also of potential difference, is the volt.
When the two poles of a cell are connected by a conductor, as in Fig. 1, a current is said to flow from the + to the -. The practical unit of current is the ampere.
If we connect the poles of the cell by a short, heavy wire, and measure the current that passes, and if we then connect the poles with a long, thin wire, and again measure the current, we find that the greater current is passing in the first case. We see, therefore, that the wire has a certain property that affects the amount of current passing through it. This property is called resistance, the unit of which is the ohm.
The current of electricity passing along a wire is somewhat analogous to the flow of water in a pipe. The current or amount of water that flows through a pipe depends upon two things: (1) the difference between the water levels at the ends of the pipe, or the "head " of water; and (2) the size and smoothness of the inside of the pipe. The "head" or difference of water pressure at the two ends of the water pipe may be compared to the difference of potential, or difference of electrical pressure, at the ends of a wire; the resistance to the flow of water in the pipe, depending on size and roughness, may be compared to the resistance to the electrical current, depending on the cross section and material of a wire; and the resulting current of water may be compared to the electrical current.
