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The Cold War period was widely understood as one of bipolarity with the USA and the USSR as the world's two superpowers, whereas the end of the Cold War led to unipolarity with the US as the world's sole superpower in the 1990s and 2000s. Scholars have debated how to characterize the current international system. [2] [3] [4]
A block diagram of a bipolar HVDC transmission system, between two stations designated A and B. AC – represents an alternating current network CON – represents a converter valve, either rectifier or inverter, TR represents a power transformer, DCTL is the direct-current transmission line conductor, DCL is a direct-current filter inductor ...
Shorter time scales mostly arise from currents in the ionosphere (ionospheric dynamo region) and magnetosphere, and some changes can be traced to geomagnetic storms or daily variations in currents. Changes over time scales of a year or more mostly reflect changes in the Earth's interior , particularly the iron-rich core .
Like most electro-mechanical machines, a homopolar motor is reversible: if the conductor is turned mechanically, then it will operate as a homopolar generator, producing a direct current voltage between the two terminals of the conductor. The direct current produced is an effect of the homopolar nature of the design.
This current is carried by ions present in the atmosphere (generated mainly by cosmic rays in the free troposphere and above, and by radioactivity in the lowest 1km or so). The ions make the air weakly conductive; different locations, and meteorological conditions have different electrical conductivity .
The use of the bipolar option does not require the placement of a return electrode because the current only passes between tines of the forceps or other bipolar output device. Electrosurgery should only be performed by a physician who has received specific training in this field and who is familiar with the techniques used to prevent burns.
Each of them had a weight of 10.2 tonnes (22,500 lb), a length of 4.6 metres (15 ft), a width of 2.4 metres (8 ft) and a height of 4.0 metres (13 ft). Between 1992 and 1993 the mercury arc valves of Pole 1 were replaced with solid state thyristors from GEC Alsthom, increasing the maximum power and voltage of the line to its current levels.
In the height region between about 85 and 200 km altitude on Earth, the ionospheric plasma is electrically conducting. Atmospheric tidal winds due to differential solar heating or due to gravitational lunar forcing move the ionospheric plasma against the geomagnetic field lines thus generating electric fields and currents just like a dynamo coil moving against magnetic field lines.