This is a chart of the electromagnetic spectrum, published by the W.M. Welch Scientific Company in 1944, and posted online about three years ago. It covers electromagnetic radiation from simple diffraction to cosmic rays, and shows the frequency allocations and their historic use across the spectrum. (full resolution image is here)

When Nikola Tesla established his first laboratory on Grand St. in New York in 1889, he began to experiment with high frequency electrical phenomena, initially to reproduce the frequencies of sunlight. He invented for this purpose a ‘Tesla Coil’, a high frequency transformer that takes the 60-cycle frequency of alternating current electricity, steps it up to hundreds of thousands of cycles per second, and generates extremely high voltages. With these experiments he made a radical departure from conventional physics. Tesla had discovered a different form of energy which he called ‘compressed voltage’ or the ‘aether’.

Tesla presented the results of his initial experiments with his new invention in a lecture / demonstration at Columbia College in 1891, stating that the energy produced by his coil was not electromagnetic.
