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filling protects the iron not only against oxidation, but enhances the effect since the solubility of hydrogen in iron will increase when the temperature increases, resulting in the higher resistance.Iron-hydrogen[url=http://www.utsource.net/PT2422.html]PT2422[/url]
resistors were used in the early vacuum tube systems in series with the heaters of electron tubes to stabilize the heater current against fluctuating supply voltage.