47 AIR CORPS
will be interrupted and the strongest indication of a given defect
will be shown. The failure to obtain flux lines other than parallel
to the defects may result in failure to locate them. It'lollows, there-
fore, that parts should be magnetized in several directions and the
powder applied after each magnetization, as a second magnetization
I ri4 J
1. Large unit. 2. Small unit.
FIGURE 10.-Demagnetizing apparatus.
tends to neutralize the first. Parts to be inspected should be free
of grease, loose grit, and dirt. Paint, lacquer, plating, and other
protective coatings, if dry, need not be removed.
b. Magnetizing methods may be divided into two general groups:
the residual method, and the continuous method.
(1) In the residual method of magnetization, the residual mag-
netism of the material being inspected is relied upon to attract the
particles. The residual magnetism is imparted to the object by means
of an initial charge induced by the magnetizing apparatus. This
method is not considered as sensitive as the continuous method al-
though it may be used to produce satisfactory results in many cases.
United States. War Dept. Heat treating and inspection of metals. Washington, D.C.. UNT Digital Library. http://digital.library.unt.edu/ark:/67531/metadc96657/. Accessed December 7, 2013.