When the welding electrode, which has been connected to the welding transformer, is brought near the surface of the job to be welded, an arc occurs the moment the electric field between the two become more than 3000V/mm, because at this electric field air ionizes , conducts electric current and arc occurs. Arc will be extinguished if the electrode touches the job. A good welder knows how to keep the electrode from touching the job. Air or some other gas must be present for arc welding.
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