Actually it was not Tesla invention. He just rediscovered some kind of very old technology from Egypt. If you investigate about the subject, the pyramids where made with materials and in a way to conduct electricity. If you look at youtube or on google you will see more about the topic.
The thing Tesla said that there the Earth had a sound of a ring bell. I am not sure but I think he said that there is sound and frequency. And that you could generate energy with sound.
Just to give you a hint, look at Michael Tellinger videos. He points out specific very ancient ruins which are over 400.000 years old in Africa, they are build in such a way that produce energy.
There are lots of info about that, he also talks about his investigations in the origins of humans, which is another topic.
Remember the people who say that is a scam, are either ignorant people who don`t know about electronics, psychics, and so on....