Electric fields in the earth's surface can reach 100V/m you can take two large sheets of metal place them a meter apart vertically and a multimeter between them will read around 100 volts . I have never read up much on Tesla , sure he was a brilliant inventor , but some of the claims seem to far fetched , did he really understand the theory well enough or were most of his "experiments" let's toss this stuff together and see what's happens and make grand claims of free electricity , efficient turbines and long distance wireless transmission