Technical Notes:
Page 2

With a similar dipole set up some meters away, sparks also jump.
Since This "receiver" is very insensitive, a mechanism was sought for with better magnetic wave reception.

Edouard Branly 1844 - 1940 observes the fact that fine iron filings in a glass tube become electrically conductive if an electromagnetic waves hits it. He calls this detector a "coherer" (Ger. Kohörer).

Ferdinand Braun 1850 - 1918 uses a lead sulfide crystal (Galena) for a detector (of electromagnetic emissions), which is influenced by a very thin wire (Cat's whisker). This crystal detector is still frequently used, to this day beginners learning radio engineering make their first radio receivers by the building of crystal detector receiver sets.(See notes on receivers)

Despite improvements in spark transmitters the detectors were not sensitive enough, in order to communicate over large distances. This is one of the reasons improvement was sought for the detectors.
The German glass blower Heinrich Geissler melts two electrodes in glass tubes and pumps the air out. When applying a high voltage he observes that the electric current appears in the form of a bright cloud between the electrodes. Depending upon the types of gas used, it displays different colors (like a neon sign, lighting tubes). Crookes and Geissler Tubes

Sir Joseph John Thomson 1856 - 1940 recognizes in 1897 that this electric current exists in the vacuum in the form of small particles, which he first calls Corpuscles, later electrons.

Thomas Alva Edison 1847 - 1931, the inventor of the lamp, always worked with direct current. For some reason he built a metal plate into one of his lamps. This metal plate was connected by a current measuring instrument with the positive pole of the Voltage (110V), which fed the lamp. He observed that between the filament and the metal plate a current flowed through the vacuum of the lamp. Although Edison did not know to begin anything with this discovery, he applied for a patent. When later different people struck much money from this discovery, Edison easily received much money. What did the others do with Edison's discovery? If one puts an alternating current to the lamp instead of direct current, then only if the metal plate is positive, one calls it the anode, electrons flow through the vacuum. So one can make direct current of alternating current very simply. Thus the electric rectifier was invented. Instead of the lamp, now the heater elements will be surrounded by a metal tube and the whole thing is melted into a spherical or oblong glass bulb melted and provided with metal pins or wires.