by Chris Woodford. Last updated: September 17, 2020.
When you first learn about electricity, you discover that materials fall into two basic categories called conductors and insulators. Conductors (such as metals) let electricity flow through them; insulators (such as plastics and wood) generally do not. But nothing's quite so simple, is it? Any substance will conduct electricity if you put a big enough voltage across it: even air, which is normally an insulator, suddenly becomes a conductor when a powerful voltage builds up in the clouds—and that's what makes lightning. Rather than talking about conductors and insulators, it's often clearer to talk about resistance: the ease with which something will let electricity flow through it. A conductor has low resistance, while an insulator has much higher resistance. Devices called resistors let us introduce precisely controlled amounts of resistance into electrical circuits. Let's take a closer look at what they are and how they work!
Photo: Four typical resistors sitting side by side in an electronic circuit. A resistor works by converting electrical energy into heat, which is dissipated into the air.
What is resistance?
Electricity flows through a material carried by electrons, tiny charged particles inside atoms. Broadly speaking, materials that conduct electricity well are ones that allow electrons to flow freely through them. In metals, for example, the atoms are locked into a solid, crystalline structure (a bit like a metal climbing frame in a playground). Although most of the electrons inside these atoms are fixed in place, some can swarm through the structure carrying electricity with them. That's why metals are good conductors: a metal puts up relatively little resistance to electrons flowing through it.
Animation: Electrons have to flow through a material to carry electricity through it. The harder it is for electrons to flow, the more resistance there is. Metals generally have low resistance because electrons can flow through them quite easily.
Plastics are entirely different. Although often solid, they don't have the same crystalline structure. Their molecules (which are typically very long, repetitive chains called polymers) are bonded together in such a way that the electrons inside the atoms are fully occupied. There are, in short, no free electrons that can move about in plastics to carry an electric current. Plastics are good insulators: they put up a high resistance to electrons flowing through them.
This is all a little vague for a subject like electronics, which requires precise control of electric currents. That's why we define resistance more precisely as the voltage in volts required to make a current of 1 amp flow through a circuit. If it takes 500 volts to make 1 amp flow, the resistance is 500 ohms (written 500 Ω). You might see this relationship written out as a mathematical equation:
V = I × R
This is known as Ohm's Law for German physicist Georg Simon Ohm (1789–1854).
Photo: Using a multimeter like this one, you can automatically find the resistance of an electronic component; the meter feeds a known current through the component, measures the voltage across it, and uses Ohm's law to calculate the resistance. Although multimeters are reasonably accurate, you have to remember that the leads and probes also have resistance that will introduce an error into your measurements (the smaller the resistance you're measuring, the bigger the likely error). Here, I'm measuring the resistance of a loudspeaker in a telephone, which you can see, from the digital display, is 36.4 Ω. Inset: a switch on the multimeter lets me measure a range of different resistances (200 Ω, 2000 Ω, 20K = 20,000 Ω, 200K = 200,000 Ω, and 20M = 20 million Ω).
Resistance is useless?
How many times have you heard bad guys say that in movies? It's often true in science as well. If a material has a high resistance, it means electricity will struggle to get through it. The more the electricity has to struggle, the more energy is wasted. That sounds like a bad idea, but sometimes resistance is far from "useless" and actually very helpful.
Photo: The filament inside an old-style light bulb. It's a very thin wire with a moderate amount of resistance. It's designed to get hot so it glows brightly and gives off light.
In an old-style light bulb, for example, electricity is made to flow through an extremely thin piece of wire called a filament. The wire is so thin that the electricity really has to fight to get through it. That makes the wire extremely hot—so much so, in fact, that it gives off light. Without resistance, light bulbs like this wouldn't function. Of course the drawback is that we have to waste a huge amount of energy heating up the filament. Old-style light bulbs like this make light by making heat and that's why they're called incandescent lamps; newer energy-efficient light bulbs make light without making much heat through the entirely different process of fluorescence.
The heat that filaments make isn't always wasted energy. In appliances like electric kettles, electric radiators, electric showers, coffee makers, and toasters, there are bigger and more durable versions of filaments called heating elements. When an electric current flows through them, they get hot enough to boil your water or cook your bread. In heating elements, at least, resistance is far from useless.
Resistance is also useful in things like transistor radios and TV sets. Suppose you want to lower the volume on your TV. You turn the volume knob and the sound gets quieter—but how does that happen? The volume knob is actually part of an electronic component called a variable resistor. If you turn the volume down, you're actually turning up the resistance in an electrical circuit that drives the TV's loudspeaker. When you turn up the resistance, the electric current flowing through the circuit is reduced. With less current, there's less energy to power the loudspeaker—so it sounds much quieter.
Photo: "Variable resistor" is the very general name for a component whose resistance can be varied by moving a dial, lever, or control of some sort. More specific kinds of variable resistors include potentiometers (small electronic components with three terminals) and rheostats (usually much larger and made from multiple turns of coiled wire with a sliding contact that moves across the coils to "tap off" some fraction of the resistance). Photos: 1) A small variable resistor acting as the volume control in a transistor radio. 2) Two large rheostats from a power plant. You can see the dial controls that "tap off" more or less resistance. Photo by Jack Boucher from Historic American Engineering Record courtesy of US Library of Congress.