William,
    The O2 sensor kind of acts like a fuel cell(a battery is a good
analogy). A voltage is produced (1 to 1.5 volts) when there is a difference
of oxygen levels between the inside of the sensor(the environment air) and
the outside of the sensor sitting in the exhaust stream . This indicates a
rich condition or to little O2.When there is O2 in the exhaust (lean), the
inside and outside of the sensor has less O2 differences and the voltage
output drops to zero. See: http://neksiwel.20m.com/photo5.html

Ken Lewis