Electrical telegraph

Electrical telegraphs were point-to-point text messaging systems, primarily used from the 1840s until the late 20th century. It was the first electrical telecommunications system and the most widely used of a number of early messaging systems called telegraphs, that were devised to communicate text messages quicker than physical transportation. Electrical telegraphy can be considered to be the first example of electrical engineering. Text telegraphy consisted of two or more geographically separated stations, called telegraph offices. The offices were connected by wires, usually supported overhead on utility poles. Many different electrical telegraph systems were invented, but the ones that became widespread fit into two broad categories. The first category consists of needle telegraphs in which a needle pointer is made to move electromagnetically with an electric current sent down the telegraph line. Early systems used multiple needles requiring multiple wires. The first commer

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Probe measurements of plasma potential nonuniformity due to edge asymmetry in large-area radio-frequency reactors: The telegraph effect

Lukas Derendinger, Christoph Hollenstein, Alan Howling, Laurent Fabien Hubert Sansonnens, Hannes Schmidt

In large-area radio-frequency (rf) capacitive reactors, the redistribution of rf current to maintain current continuity near asymmetric sidewalls causes a perturbation in rf plasma potential to propagate along the resistive plasma between capacitive sheaths. The damping length of the perturbation can be determined by a telegraph equation. Experiments are described using a surface array of unbiased electrostatic probes in the ground electrode to verify the theoretical model of the telegraph effect in Howling [J. Appl. Phys. 96, 5429 (2004)]. The measured spatial dependence of the plasma potential rf amplitude and circulating nonambipolar current agree well with two-dimensional numerical solutions of the telegraph equation. The rf plasma potential can be made uniform by using symmetric reactor sidewalls. (c) 2005 American Institute of Physics.

2005

Probe measurements of plasma potential nonuniformity due to edge asymmetry in large-area radio-frequency reactors: the telegraph effect

Lukas Derendinger, Christoph Hollenstein, Alan Howling, Laurent Fabien Hubert Sansonnens, Hannes Schmidt

2005

Electromagnetic field nonuniformities in large area, high-frequency capacitive plasma reactors, including electrode asymmetry effects

Christoph Hollenstein, Alan Howling, Laurent Fabien Hubert Sansonnens

Electromagnetic wave propagation effects can give rise to important limitations for processing uniformity in large area, radio-frequency (rf) capacitive plasma reactors. The electromagnetic wavefield solution is derived for a capacitive, high-frequency, cylindrical reactor with symmetric or asymmetric electrode areas containing a uniform plasma slab. It is shown that only two distinct electromagnetic modes are necessary and sufficient to determine the electromagnetic fields everywhere within the reactor except close to the sidewalls. The first mode gives rise to the interelectrode rf voltage standing wave effect associated with high frequencies in large area reactors, and the second mode gives rise to the telegraph effect associated with asymmetric electrode areas, which necessitates the redistribution of rf current along the plasma to maintain rf current continuity. This work gives a unified treatment of both effects which have previously been studied separately, experimentally and theoretically, in the literature. The equivalent circuit of each mode is also derived from its respective dispersion relation. Examples of this electromagnetic wavefield solution show that both modes can cause nonuniformity of the plasma rf potential, depending on the reactor geometry, excitation frequency and plasma permittivity and sheath width, which has consequences for large-area plasma processing.

2006