Heat treating and inspection of metals Page: 52
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47-48 AIR CORPS
high a current value for the area of contact. A heavy copper gauze,
as shown in figure 10, or special contact blocks serve to increase the
area of contact and reduce the danger. -
(2) When the part is magnetized by placing it in a coil or solenoid,
the operation is termed bipolar magnetization. In this method no cur-
rent is passed through the part; the magnetism being induced by the
current flow through the bipolar or solenoid coil. The magnetic lines
2. Contact heads.
FIGURE 12.-Circular magnetization of a propeller blade.
of force are parallel to the axis of the piece, forming a magnetic field
similar to that of a bar magnet. Whenever possible the bipolar coil
assembly should be used (fig. 13). However, for parts too large to fit
inside the bipolar coil assembly, a coil of 3 to 6 turns may be formed
around the part by the use of the auxiliary cable (fig. 14). The ter-
minals of this auxiliary cable are attached to the contact heads, and
the test conducted in the same manner as when using the bipolar coil
48. Application of magnaflux method.-a. The following is
an outline of the steps necessary in a complete inspection. Although
not required in every case, both the circular and bipolar methods of
magnetization are included in order to make the procedure complete.
(1) Clean the part thoroughly to insure freedom from grease, dirt,
pencil marks, fingerprints, etc.
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United States. War Department. Heat treating and inspection of metals, book, September 10, 1941; Washington D.C.. (digital.library.unt.edu/ark:/67531/metadc96657/m1/54/: accessed July 21, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.