How does a triode work?
(Please note: treat this material with caution, I am revising the explanation.)
Armstrong's regenerative circuit fed back the input signal into the circuit over and over again, amplifying the signal far more than original designs, building great wireless and wireline transmission signal strength. The feedback circuit could also be overdriven, fed back so many times that supplying a small current to the circuit would develop in it an extremely high frequency, so high it could resonate at the frequency of a radio wave, letting the triode receive or detect signals, not just transmit them. You had a tunable electronic tuning fork, of sorts, a device which detected and amplified the rhythmic energy of the radio wave when set to the frequency desired.
In 1919 three firms came together to develop a wireless company that one day would reach around the world. Heavy equipment maker ASEA, boiler and gas equipment maker AGA, and telephone manufacturer LM Ericsson, formed SRA Radio, the forerunner of Ericsson's radio division. Svenska Radio Aktiebolaget, known simply as SRA, was formed to build radio receivers, broadcasting having just started in Scandinavia. (Aktiebolaget, by the way, is Swedish for a joint stock company or corporation.)
Much unregulated radio experimenting was happening world wide at this time with different services causing confusion and interference with each other. In many countries government regulation stepped in to develop order. In the United States the Radio Act of 1912 brought some order to the radio bands, requiring station and operator licenses and assigning some spectrum blocks to existing users. But since anyone who filed for an operating license got a permit many problems remained and others got worse.
In 1921 United States mobile radios began operating at 2 MHz, just above the present A.M. radio broadcast band. For the most part law enforcement used these frequencies. [Young] The first radio systems were one way, sometimes using Morse Code, with police getting out of their cars and then calling their station house on a wired telephone after being paged. As if to confirm this, a reader recently e-mailed me this paragraph. The reader did not include the author's name or any references, however, the content is quite similiar to Bowers in Communications for a Mobile Society, Sage Publications, Cornell University, Beverley Hills (1978):
"Until the 1920s, mobile radio communications mainly made use of Morse Code. In the early 1920s, under the leadership of William P. Rutledge, the Commissioner of Detroit Police Department, Detroit, Michigan police carried out pioneering experiments to broadcast radio messages to receivers in police cars. The Detroit police department installed the first land mobile radio telephone systems for police car dispatch in the year 1921. [With the call sign KOP!, ed.] This system was similar to the present day paging systems. It was one-way transmission only and the patrolmen had to stop at a wire-line telephone station to call back in. On April 7, 1928, the first voice based radio mobile system went operational. Although the system was still one-way, its effectiveness was immediate and dramatic."
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Resources:
Young, W.R. "Advanced Mobile Phone Service: Introduction, Background, and Objectives." Bell System Technical Journal January, 1979: 7