For most materials

where is the voltage **across** the object,
*I* is the current **through** the object, and *R* is a
proportionality constant called the resistance of the object.
Resistance is a function of the material and shape of the object, and
has SI units of ohms ( ).
It is more common to find units of k and M .
The inverse of resistivity is conductivity.

Resistor tolerances can be as bad as % for general-purpose resistors to % for ultra-precision resistors. Only wire-wound resistors are capable of ultra-precision applications.

The concept of *current through* and *potential across* are
key to the understanding of and sounding intelligent about electronics.

Now comes the most useful visual tool of this course.

Tue Jul 13 16:55:15 EDT 1999