material, such as metal. Air causes the magnetic fi eld to weaken. Arc blow is greatest at the ends of the weld joint when DC is used. See Figure 12-4. The magnetic fi eld is strongest in the metal. This causes the fi ller metal to blow toward the center of the joint. See Figure 12-5. Arc blow that occurs at the beginning of a joint is forward arc blow. Backward arc blow Goodheart-Willcox Publisher Figure 12-3. The magnetic field around a wire with DCEN (DCSP) rotates in the opposite direction from the magnetic field around a wire carrying DCEP (DCRP). The magnetic field is indicated by the arrows around the wire, and the current is indicated by the end arrows. Goodheart-Willcox Publisher Figure 12-4. The magnetic field around the electrode is deflected at the ends of a weld joint (A and C). The field attempts to flow in the metal and not through the air. The concentration of magnetic flux at the ends of the metal forces the arc toward the center of the base metal. The arc “blows away” from the area directly under the electrode at the ends of the weld. Notice that the magnetic field is not distorted in the center areas of the weld joint at B. DCEN (DCSP) Direction of electron flow – + DCEP (DCRP) Direction of electron flow + – A B C Direction of weld + (–) (–) Workpiece lead (ground) 1/2" Base Metal Glancing or scratching motion Electrode Vertical Motion Base Metal Down-and-up or pecking motion A B Electrode Goodheart-Willcox Publisher Figure 12-2. A—In the scratching (glancing) method of striking an arc, the movement is very short. The electrode briefly touches the base metal. B—In the straight up-and-down or pecking method, the electrode quickly contacts the surface and is then raised. After striking the arc with either method, the end of the electrode must remain about 1/8″ (3.2 mm) above the base metal to maintain the arc. 170 Section 2 Shielded Metal Arc Welding Copyright Goodheart-Willcox Co., Inc.