Resistance

As you are aware, conductors, such as copper, permit electric current to move through easily. On the other hand, insulators, such as glass, permit very little electron flow.

Although every material, even conductors, offers at least some resistance to electron flow, conductors are considered to have low resistance. Insulators have a high resistance.

In designing electric circuits, there is often the need to restrict the flow of current, so that it can be controlled. Varying amounts of current are required for different uses. What might be some methods of resisting the current flow?

One method would be to reduce the diameter of the wires in the circuit.

In an electric circuit, at a constant voltage, current flows with more resistance through a smaller wire than through a larger one. To reduce current flow, wires of a smaller diameter might be substituted for wires of a larger diameter, except that this approach would not allow precise control.

Another approach to controlling current through resistance would be to increase the length of the wires, since for a given diameter, increasing the total amount of material that the current must pass through increases the total resistance. Again, though, this approach would not allow precise control.

Figure 2.9

As you have probably guessed, the best method of precisely regulating current flow or establishing voltage levels in a circuit is to use a device called a resistor. There are two basic types of resistors. One type is the fixed resistor, which provides a specific resistance. Figure 2.9 displays an example of a fixed resistor, along with its graphic symbol.

Figure 2.10

The second type is known as the variable resistor, and provides a range of resistance from near 0% (so it acts like a conductor) to near 100% (so it acts like an insulator). Figure 2.10 displays an example of a variable resistor, along with its graphic symbol.

As you will recall, voltage is measured in volts and current is measured in amps. Resistance is measured in Ωs, after the man who developed Ohm's Law. A basic understanding of Ohm's Law is essential to learning about variable resistors, our major products.