## 3 Experiments and Analysis
### Observation of a Hidden Object  
In this experiment, an object (a 9 V dry-cell battery) was located on a top of a book stack. We used a B-Camera with a computer-based PDU. The microcontroller (Arduino) was connected to the USB port of the computer where a PDU program was running. We easily obtained a better color contrast by adjusting the range of the colormap, which enhanced the display of the magnetic field distribution. The purpose of this experiment was to show that a hidden object separated by a thick wall could be detected (and imaged) by the B-Camera.  
Figure 4 shows (a) the experiment setting (test-range), (b) the field distribution before an object was placed in the array and (c) the field distribution after the object was placed on top of a book stack. The ruler in Figure 4 indicates that the distance between the sensor array to the object was around 12 cm. By comparing the field distribution without the object in Figure 4a with the one with an object shown in Figure 4b, we can see that the field changed and therefore the B-camera was capable of sensing the field of an object located behind a non-magnetic thick wall.  
Figure 4: Experiments to show the capability of the B-Camera to image a hidden object. (**a**) A dry-cell battery, the cover of which was made of iron, was located on top of a book stack to simulate a 12 cm wall hiding an object. (**b**) Magnetic field distribution with subtracted background without an object. (**c**) Field distribution after the object was put on top of the book stack. Color bar in (**b**) indicates magnetic flux density in gauss units. By comparing figure (**b**) to (**c**), we observe that the field distribution changed. This shows that the device was capable of detecting (and imaging) ferromagnetic objects hidden behind a thick non-magnetic wall.  
Figure 3: Position of an object in the B-Camera’s array and corresponding magneto-photograph. The array was designed to be used downward; thus, the left–right position of the image has been inverted. The object was a RP.1000 nickel coin: (**a**) positioned at the center of the the array, (**b**) located in the corner of the array. Images on at the right part of (**a**,**b**) show the consistency of the field distribution with their corresponding locations at the left part.  
Experiments using large sensor arrays are shown in Figure 5. First, we measured the flux density around a C-clamp made of cast iron that was placed 3 cm above the sensor array, separated by air. The maximum flux density around 0.3 G was measured near the thick parts of the clamp, which was located close to the sensors. The second experiment used an X-shaped lug wrench made of stainless steel. Although the wrench was placed on top of a 5 cm-thick stack of non-magnetic papers, the measured fields were greater than those found in the clamp experiment. The difference in field magnitude measured around the rods was due to the different lug nut sizes (from 19 to 23 mm) and their distance to the sensors. Figure 5 shows the measured flux density in a normalized vectorial format (\(B_{x}/\left|B\right|\), \(B_{y}/\left|B\right|\)) superimposed on the magnitude \(\left|B\right|\) in gauss, shown in colors. Shorter arrows indicate that the fluxes were oriented more vertically (to/from the sensor array), which usually corresponded to a greater field magnitude due to nearby magnetic objects. The images show the capability of the device to provide rough information of the shape of both objects.