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.