In distributing electricity there is, as previously stated, more or less resistance to its passage through wires. In overcoming this resistance heat is developed and energy is lost by the friction caused by the electricity moving through the conductor. The resistance offered to an electrical current depends upon the material through which it passes, the length and sectional area of the circuit wires, and the surrounding conditions.
To illustrate: If 900 ft. of a certain wire offers a resistance of 2 ohms, the resistance of 450 ft. of the same wire is 1 ohm. If the diameter of the wire were one-half, the area would be one-quarter and the resistance four times as great, or 4 ohms. This is often expressed in mathematical language by stating: Resistance varies directly as the length and inversely as the square of the diameter of wire.
Since watts are the product of electromotive force and current, the question of furnishing 15,000 watts to a certain point from a power station might be settled by having either an electromotive force of 1500 volts and a current of 10 amperes, or 150 volts and 100 amperes. The loss due to heat increases with the strength of the current.
The size of wire necessary to transmit a given current is determined by the drop in voltage allowed between the generator and the point of application of the current, and the increase in temperature due to the current.
High voltages are used in long-distance transmissions to increase the carrying power of a given size of wire, in other words, to decrease the cost of line necessary to transmit a given amount of energy.