Methods for Obtaining Resistance

Resistance to the flow of electricity is greater when the wire or other conductor is very narrow, similar to the flow of water in a very narrow pipe. Electric current is really the motion of billions of electrons within the wire, and these all have negative charges, so they repel each other. The voltage is only sufficient to force a relatively small fraction of the mutually repelling electrons into a very narrow space at the same time, so the others must wait their turn, and this delay decreases the overall flow rate. The narrower the space, the slower the overall flow, at any given voltage. On the other hand, a higher voltage can force more electrons together into the narrow space, so that will tend to increase the flow.

The electrons hit various imperfections (including chemical impurities) in the wire and bounce off to the side to some degree, further slowing the overall forward motion. At higher temperature, the atoms of the conductive material are moving around randomly and are therefore not in their usual positions within the material, and this also slows the net forward motion to some degree. Also, the electrons tend to collide with each other, which also causes deviation from a straight path. Taken together, these effects are called "scattering," and that raises the electrical resistance that we measure with an ohmmeter. Examples of scattering effects are (1) nickel metal wire with 20% chromium mixed randomly into it, which...