With a lower frequency, such as: 60Hz, this circuit does not produce enough negative impedance to suck the current from the grounding rod (labeled: EarthSource on the schematic) unless a sufficiently higher voltage is utilized to carry this frequency, such as using a voltage which is standard at American wall outlets of 120V. But at a higher frequency, such as at 1kHz, it becomes easier to use less voltage to drive this pump to extract current from the ground at this circuit's EarthSource location within the schematic to the left.