One of the more powerful arc transmitters constructed were the 1,000 watt units built for the U. By gradually scaling up the equipment Federal Telegraph finally produced a 30 k W unit that outperformed a powerful rotary spark transmitter at the Navy's Arlington station.
The navy wanted still more power and Elwell thought he could build a 60 k W unit by merely scaling up the parts again. Arc transmitters were gradually eliminated when the new vacuum tube transmitters came into use. Perhaps the last to be in operation on land were the stations operated by the Mackay Radio and Telegraph Company between cities on the Pacific coast.
Buzzers were used early on as a way to get ICW ( interrupted CW ) signals in early vacuum tube transmitters.
The buzzer would interrupt the CW at an audio rate, thus modulating the CW carrier.
But it was twenty four years later (eight years after Maxwell's death) that Heinrich Hertz (1857-1894) in Germany gave a practical demonstration of the accuracy of this theory.
He generated and detected electromagnetic waves across the length of his laboratory on a wavelength of approximately one metre.
They were complicated to operate and were infamous for exploding when an operator introduced too much alcohol into the chamber.
Arc transmitters were brought to the United States in 1909. In Java, a unit was rated at 3,000 W, the antenna was suspended over a mountain gorge.
Accounts of flags, mirrors and smoke signals appear in early history. By 1837 the system is improved and was demonstrated using 'lightning wires' and 'Morse code,' an electronic alphabet that could carry messages. A line was constructed between Baltimore and Washington and the first message, sent on May 24,1844, was 'What hath God wrought!
However, James Clerk Maxwell (1831-1879), Professor of Experimental Physics at Cambridge then came up with something entirely new.
By some elegant mathematics he had shown the probable existence of electromagnetic waves of radiation.
A synchronous rotary had the spark electrodes mounted on the shaft of the motor generator which feeds a HV step up transformer.
In this way, the spark would discharge the capacitor synchronously with the peak in the AC waveform.