It has sometimes been claimed that Edison's proposed elevated plates anticipated the subsequent invention by Marconi of the aerial wire or antenna, but it is particularly to be noticed that Edison employed no spark gap or means for creating electrical high frequency oscillations in these wires.
Employed for the production of the continuous trains of waves a high frequency alternator of his own invention (see The Electrician, 1907, 5 8, pp. 6 75, 701).
Unidirectional, or alternating, and the latter of high or of low frequency, amperemeters may first be divided into those (I) for continuous or direct currents, (2) for low frequency alternating currents, and (3) for high frequency alternating currents.
Leyden jars are now much employed for the production of the high frequency electric currents used in wireless telegraphy (see Telegraphy, Wireless).
To detect these currents some device has to be inserted in the antenna circuit or else inductively connected with it which is sensitive to high frequency currents.
The electric waves coming through space from the sending station strike against the receiving antenna and set up in it high frequency alternating electromotive forces.
Hence, when the coil at one fixed station was in action it generated high frequency alternating currents, which were propagated across the air gap between the ordinary telegraph wires and the metallic surfaces attached to one secondary terminal of the induction coil, and conveyed along the ordinary telegraph wires between station and moving train.
The instrument can be calibrated by continuous currents and may then be used for high frequency alternating currents.
In constructing a hot-wire instrument for the measurement of high frequency currents it is necessary to make the working wire of a number of fine wires placed in parallel and slightly separated from one another, and to rpass the whole of the current to be measured through this strand.
It very soon, therefore, became clear to inventors that a very great advantage would be gained if some means could be discovered of creating high frequency oscillations which were not intermittent but continuous.