It is possible to generate electricity using small variations in the Earth's magnetic field but why haven't we done it? The Earth's field has an average strength of 0.5 Gauss that varies on the order of about 1.0 x 10-5 Gauss/second. How big a single stationary circular loop would you have to construct to generate a voltage equivalent to that of an AA-cell battery (about 1.5 V)? How fast would you have to spin a single circular loop with a radius of 1cm to get the same voltage?