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138 CHAPTER FOUR
Emissions
Cells phones often drop out or have significant static on the line in the presence of inter-
fering appliances like computers. Car radios often buzz when we drive under power
lines. Computers can bomb completely if they get hit with a big static spark. These
occurrences are caused by electrical interference from outside the appliance. Thus, two
goals for the design of the robot spring to mind:

We should make the robot impervious to interference. This way, it will be more
reliable.
To be a good electric neighbor, we should design the robot so it does not create
interference that will be picked up by other appliances.
For reasons of symmetry, it turns out that these two goals are one and the same. If we
can keep interference from entering the robot, then interference cannot get out of the
robot either. To accomplish the goals, we will employ two basic methods:

Generation We will try not to generate interference within the robot. If we min-
imize the interference we generate, we will not have to struggle to keep it within
the robot.
Shielding We will try to put up sufficient shielding around the robot to help pre-
vent our interference from getting out. Further, these shields will help keep out-
side interference from getting in.
As a practical matter, we cannot be perfect in either endeavor. The robot will gener-
ate interference, and it will spread beyond the walls of the robot. We can use many tech-
niques to minimize interference and accomplish our goals.

GENERATION
To appreciate just what interference is, we should go back to the works of the master.
In 1873, James Clark Maxwell (see Figure 4-2) set out the very basic laws of physics
in his publication A Treatise on Electricity and Magnetism, including the formulas
known as Maxwell’s Equations (for more info, access www-gap.dcs.st-and.ac.uk/
history/Mathematicians/Maxwell.html).
The presence and movement of electrons creates electrostatic and electromagnetic
fields. These fields create action over a distance. A magnet, driven by force, near a wire
can move electrons in the wire to create a current (creating a generator). A current, mov-
ing in a wire near a magnet, can create force on the magnet (creating a motor). In both
cases, the fields involved are acting over a distance. So, too, electrons moving within