The transmitter power lost in the ground resistance, and so the efficiency of the antenna, depends on the soil conductivity. This varies widely; marshy ground or ponds, particularly salt water, provide the lowest resistance ground, while dry rocky or sandy soil are the highest. The power loss per square meter in the ground is proportional to the square of the transmitter current density flowing in the earth. The current density, and power dissipated, increases the closer one gets to the ground terminal at the base of the antenna, so the radial ground system can be thought of as providing a higher conductivity medium, copper, for the ground current to flow through, in the parts of the ground carrying high current density, to reduce power losses.