| 摘要 |
<p>The gaseous medium, in which the arc is created, is maintained in a quasipermanent ionised state by superimposing an electric high frequency current (II) on the normal industrial frequency (I) employed for the arc. Current (II) pref. has a frequency so its spark creates an ultrasonic wave; and the esp. pref. frequency (II) is 30,000-100,000 Hz, with intermittent voltage only delivered during the periods of extinction of the arc. When a furnace uses a current (I) of 50 Hz, the cycle is 20 ms, in which the duration of current flow is ca. 12 ms, i.e. the arc is active for 60% of the time. The existence of current (II) lowers the restriking voltage of the arc as it acts as a pilot current and results in a much quieter sound wave.</p> |
