Magnetic field mapper locates imperfections in cadmium sulphide solar cells by detecting and displaying the variations of the normal component of the magnetic field resulting from current density variations. It can also inspect for nonuniformities in other electrically conductive materials

Masters, R. M.

Stenger, F. J.

English

NASA Technical Reports Server

September 1969	 Brief 69-10476 
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Magnetic Field Mapper
Test Fixture
Y^ 
Sensing Coil 
Mounted on X-Y 
Carriage 
Fig. 1. Block Diagram Magnetic Field Mapper 
A magnetic field mapper has been developed for 
locating imperfections in cadmium sulphide solar 
cells by detecting and displaying the variations of 
the normal component of the magnetic field resulting 
from current density variations. This same technique 
can be used to inspect for faults and other nonuni-
formities, such as thickness variations, in other elec-
trically conductive materials.
The apparatus, together with a block diagram of the 
circuitry, is shown in Figure 1. The principle of opera-
tion is based on the fact that a folded flat-loop con-
ductor has a very low external magnetic field as long 
as the current density in the conductor is uniform. 
A defect in the conductor, however, causes a non-
uniform current density and an external magnetic 
fringe field. This results in a nonuniform normal corn-
(continued overleal) 
This document was prepared under the sponsorship of the National	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States	 will be free from privately owned rights.
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Hole
. 
Trace Lines 
Sample Sheet 
ponent of the magnetic field near the defect. This 
nonuniform field can be detected with a sensing coil. 
In this particular instrument, the sensing coil consists 
of 200 turns of number 56 wire wound on a 1/8" 
bobbin. 
This coil is mounted on the pen carriage of an X-Y 
recorder which scans the sample area by receiving 
signals from an external X sweep and a Y step gen-
erator. Sensing coil signals are passed through a 
polarity sensing transformer which adds a bias signal 
to determine the direction of the normal magnetic 
field component. The combined signal is fed to a tuned 
amplifier which detects and amplifies signals of the 
same frequency as the input signal. This technique 
minimizes interference from stray electromagnetic 
fields. 
The signal output of the tuned amplifier is differen-
tiated with respect to the X axis. This results in a 
signal which varies across the sample conductor in a 
way similar to the nonuniform current causing the 
magnetic field. This modified signal is then fed to 
the Y axis input of an X-Y recorder which is slaved 
to the X sweep and Y step of the scanning recorder.
The resultant family of curves plotted by the X-Y re- 
corder provides a qualitative map or picture of the 
current distribution over the surface of the flat 
conductor under test. 
A typical family of curves illustrating the magnetic 
field sensed near a conducting sheet with a hole in 
it is shown in Figure 2. Magnetic field and hence cur-
rent density changes are easily observed as the trace 
moves above - or below the average field level. 
Note: 
No further documentation is available. Technical 
questions may be directed to: 
Technology Utilization Officer 
Lewis Research Center 
21000 Brookpark Road 
Cleveland, Ohio 44135 
Reference: B69-10476 
Patent status: 
No patent action is contemplated by NASA. 
Source: F. J. Stenger and R. M. Masters 
Lewis Research Center 
(LEW-10782) 
Fig. 2. Current Density Topographic Map
. 
Brief 69-10476	 Category 01


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