The following answer is from Quora:
I built a crude copy and found interesting things. It’s not a “cosmic energy collector.” Instead it’s an extremely sensitive detector. It picks up voltages (e-fields,) and is incredibly sensitive to ion flows in air. It’s a radio-wave detector from before catswhiskers or vacuum tubes. Also, it should easily detect ionizing radiation. Tesla’s actual patent shows it being used as a radiation alarm which rings whenever hit by beams of x-rays. So, it’s a Geiger counter, invented years before Hans Geiger.
The essence of Tesla’s invention is to “chop” any electrostatic signals coming from a capacitor plate, producing fairly enormous pulses, so they can be heard by human ears: as sound in headphones. In the decade before vacuum-tube amplifiers existed, Tesla’s idea harnesses exquisitely sensitive human hearing as a detector. Also, unlike detectors such as phosphor screens or foil-leaf electroscopes, the pulses can become part of electronic products, where a weak signal can trigger relay-coils (which Tesla calls “circuit controllers.”)
While building one of these, I realized that very obviously this device is based on the Branly Coherer radio detector. But in comparison, the Branly Coherer has low sensitivity. Tesla’s device, rather than depending on a fairly high voltage-threshold of iron particles being micro-welded, (and, rather than requiring a mechanical tapper to reset,) instead it directly harnesses the Coherer concept: the sudden closing of a switch to dump the charge created by a signal. But Tesla builds his version out of macro components, with no metal powder involved, and his continuous clockwork “tapper” is part of the detection, rather than a reset-device.
Since Tesla’s version needs no breakover-voltage (and so will operate all the way down to zero threshold,) and since it can be selectively slowed down to clicks-per-minute, it might be 10,000x more sensitive than Branly’s Coherer.
Along these lines, Tesla may have secretly intended this patent to be a radio detector, with a metal plate as an antenna, while Tesla carefully keeps this side-application secret, yet being able to patent the device. So, besides detecting x-rays, it also functions as a “Poulsen Tikker” -style of radio detector (which was independently invented a decade later,) and was a design fit for the VLF radio band which Tesla was using at the time.
If you experiment yourself, using metal foil and buzzing relay-contacts, you’ll find that the plate will be completely swamped by 60Hz environmental voltage signals. Build yourself a shielded room? Otherwise, the plate must be enclosed within a small grounded shield. To detect ions or hard radiation, put a big window in your little shield, covered with a few wires stretched across the opening, or with coarse metal screening.
That, or move your lab waaaay out in the country, far away from all AC power lines.
I conclude that most descriptions of this invention are loads of crap, with the main mistake being misinterpretation of Tesla’s term “Radiant Energy.” This term came from Tesla’s hero, Prof. W. Crookes, and “radiant energy” means the same as modern terms “ionizing radiation” or “nuclear radiation,” possibly also including ultraviolet light and radio waves.
In other words, the patent title actually means “Method for Detecting Ionizing Radiation.” Decades before the Geiger-Muller counter!
The second mistake people make is in not realizing that, at the time of Tesla’s patent, vacuum tubes and amplifiers didn’t exist. Tesla’s invention is a major advance compared to many other sensors of that era: relatively immense output, in the form of slow pulses.
As a radiation detector, Tesla’s invention was more sensitive than foil-leaf electroscopes or phosphor screens; the scintillators. In modern language, it’s the first Geiger Counter, but invented decades early. It’s also a voltage-detector like a foil-leaf electroscope, but automated, where its pulse signals are large enough to be used by electrical products …or, for extreme sensitivity, detected as faint audible clicking-sounds in headphones.
Also, this invention harnessed the photoelectric effect of metal surfaces, and would become charged when x-rays or hard UV would knock electrons out of the metal plate. Tesla accidentally discovered the Photoelectric Effect? And even patented it?!
Everyone today would recognize Tesla’s invention if his detector-plate was placed inside an argon-filled glass tube, adjacent to another electrode. It’s the Phototube or “electric eye,” but invented years before phototubes existed, and operated in open air with no glass tube. It’s the competition for the Selenium Cell “electric eye” already being used at the time. It’s the Geiger-Muller tube, but with no HV supply, and where its operating voltage is provided by the radiation striking its surface.
Is it an “energy” receiver? Of course. But, signal-level energy only. (Did Tesla say differently?) Photo-tubes don’t work as well as solar cells, but both are energy-receivers. Tesla’s device is more similar to the “Crookes Radiometer,” with its spinning black and white paddles, in that it’s not intended for directly powering motors. Instead it’s a Victorian Steampunk Geiger Counter. Or, it becomes the commands-receptor for drone robots controlled remotely by narrow directed communication beams (tesla x-ray beams, since microwave beams didn’t yet exist.)