by Chris Woodford. Last updated: July 1, 2017.
Fire was one of humankind's earliest and greatest discoveries—something like one or two million years ago. In our modern age of jet engines, space rockets, steel skyscrapers, and synthetic plastics, smoke and flames might seem positively prehistoric. But all four of those inventions—and dozens of others besides—rely on fire in one crucial way or another.
Fire is nothing less than brilliant, but it's not all that convenient. Sometimes it takes ages to get a fire going: coal-powered steam locomotives have to be fired up several hours before they need to pull trains, for example. Other times fire breaks out when you least expect it, threatening lives, buildings, and everything you hold dear. Wouldn't it be great if fire were as easy to control as electricity, so you could switch it on and off at a moment's notice? That's the basic idea behind heating elements. They're the "fire" inside such things as electric heaters, showers, toasters, stoves, hair dryers, clothes dryers, soldering irons and all kinds of other handy household appliances. Heating elements give us the power of fire with the convenience of electricity. Let's take a closer look at what they are and how they work!
Photo: A typical electric fire standing on a stone hearth. This one has three electric heating elements ("bars") mounted in white ceramic behind a silver-colored safety grid. You can see the elements right in the middle of the picture running horizontally and glowing red hot. At the top, there's a pretty unconvincing pile of plastic coal that serves no useful purpose, other than to remind us we're cave dwellers at heart. We still love the romance of fire, even if we prefer the convenience of electricity!
Making heat from electricity
In school we learn that some materials carry electricity well, others badly. The good carriers of electricity are called conductors, while the poor carriers are known as insulators. Conductors and insulators are often better described by talking about how much resistance they put up when an electric current flows through them. So conductors have a low resistance (electricity flows through them easily) while insulators have a much higher resistance (it's a real struggle for the electricity to get through). In an electric or electronic circuit, we can use devices called resistors to control how much current flows; using a dial to increase the resistance and lower the current in a loudspeaker circuit is a way of turning down the volume, for example.
Photo: A closeup of the coiled tungsten filament in an incandescent lamp, which makes light by making a great deal of heat. The amount of light a filament produces is directly related to how long it is: the longer the filament, the more light it gives off. That's why it's coiled: a coil packs more length (and light) into the same space.
Resistors work by converting electrical energy to heat energy; in other words, they get hot when electricity flows through them. But it's not just resistors that do this. Even a thin piece of wire will get hot if you force enough electricity through it. That's the basic idea behind incandescent lamps (old-fashioned, bulb-shaped lights). Inside the glass bulb, there's a very thin coil of wire called a filament. When enough electricity flows through it, it glows white hot, very brightly—so it's really making light by making heat. Around 95 percent of the energy a lamp like this uses is turned into heat and completely wasted (using an energy-saving fluorescent lamp is far more efficient, because most of the electricity the lamp consumes is converted into light with hardly any wasted heat).
Now forget the light—what if the heat were the thing we were really interested in? Suddenly, we find our wasteful incandescent lamp is actually very efficient, because it converts 95 percent of the energy we feed into it to heat. Fantastic! Only there's a problem. If you've ever got close to an incandescent lamp, you'll know it gets hot enough to burn you if you touch it (don't be tempted to try). But if you stand even a meter or so away, the heat from something like a 100-watt lamp is far too feeble to reach you.
So what if we wanted to build an electric heater broadly along the same lines as an electric lamp? We'd need something like a scaled-up lamp filament—maybe 20–30 times more powerful so we could really feel the heat. We'd need a fairly robust material (one that didn't melt and lasted a long time through repeated heating and cooling) and we'd need it to give off lots of heat at a reasonable temperature (maybe when it glowed red hot instead of white hot, so it didn't blind us). What we're talking about here is the essence of a heating element: a sturdy electrical component designed to throw out heat when a big electric current flows through it.
What is a heating element?
Photo: The heating element concealed inside a ceramic cooktop. This is one continuous element, beginning at the blue dot and curving around in a maze shape until it reaches the red dot. There's no point in this element being any other shape or size: it has to concentrate heat precisely underneath a cooking pan—and this is the most effective way to achieve that.
A typical heating element is usually a coil, ribbon (straight or corrugated), or strip of wire that gives off heat much like a lamp filament. When an electric current flows through it, it glows red hot and converts the electrical energy passing through it into heat, which it radiates out in all directions.
Heating elements are typically either nickel-based or iron-based. The nickel-based ones are usually nichrome, an alloy (a mixture of metals and sometimes other chemical elements) that consists of about 80 percent nickel and 20 percent chromium (other compositions of nichrome are available, but the 80–20 mix is the most common). There are various good reasons why nichrome is the most popular material for heating elements: it has a high melting point (about 1400°C or 2550°F), doesn't oxidize (even at high temperatures), doesn't expand too much when it heats up, and has a reasonable (not too low, not too high, and reasonably constant) resistance (it increases only by about 10 percent between room temperature and its maximum operating temperature).
Types of heating elements
There are lots of different kinds of heating elements. Sometimes the nichrome is used bare, as it is; other times it's embedded in a ceramic material to make it more robust and durable (ceramics are great at coping with high temperatures and don't mind lots of heating and cooling). The size and shape of a heating element is largely governed by the dimensions of the appliance it has to fit inside and the area over which it needs to produce heat. Hair curling tongs have short, coiled elements because they need to produce heat over a thin tube around which hair can be wrapped. Electric radiators have long bar elements because they need to throw heat out across the wide area of a room. Electric stoves have coiled heating elements just the right size to heat cooking pots and pans (often stove elements are covered by metal, glass, or ceramic plates so they're easier to clean).
Photo: Two kinds of heating elements. 1) The glowing nichrome ribbons inside a toaster. 2) You can clearly see the coiled electrical element at the bottom of this kettle. It never glows red hot in the same way as the toaster wires because it doesn't normally get hot enough. However, if you're foolish enough to switch your kettle on without any water inside (as I did once by accident), you'll discover that it is perfectly possible for a kettle element to glow red hot. That dangerous and disastrous episode permanently damaged my kettle and could have set fire to my kitchen.
In some appliances, the heating elements are very visible: in an electric toaster, it's easy to spot the ribbons of nichrome built into the toaster walls because they glow red hot. Electric radiators (like the one in our top photo) make heat with glowing red bars (essentially just coiled, wire heating elements that throw out heat by radiation), while electric convector heaters generally have concentric, circular heating elements positioned in front of electric fans (so they transport heat more quickly by convection). Some appliances have visible elements that work at lower temperatures and don't glow; electric kettles, which never need to operate above the boiling point of water (100°C or 212°F), are a good example. Other appliances have their heating elements completely concealed, usually for safety reasons. Electric showers and hair curling tongs have concealed elements so there's (hopefully) no risk of electrocution.
Designing heating elements
All this makes heating elements sound very simple and straightforward, but there are, in fact, many different factors that electrical engineers have to consider when they design them. In his excellent book on the subject (see references below), Thor Hegbom lists roughly 20–30 different factors that affect the performance of a typical heating element, including obvious things like the voltage and current, the length and diameter of the element, the type of material, and the operating temperature. There are also specific factors you need to consider for each different type of element. For example, with a coiled element made of round wire, the diameter of the wire and the form of the coils (diameter, length, pitch, stretch, and so on) are among the things that critically affect the performance. With a ribbon element, the ribbon thickness and width, surface area, and weight all have to be factored in.
And that's only part of the story, because a heating element doesn't work in isolation: you have to consider how it will fit into a bigger appliance and how it will behave during use (when it's used, or abused, in different ways). How, for example, will your element be supported inside its appliance by insulators? How big and thick will they need to be and will that affect the size of the appliance you're making? (For example, think about the different kinds of heating elements you'd need in a soldering iron, the size of a pen, and a large convector heater.) If you have an element "draped" between supporting insulators, what will happen to it as it gets hotter? Will it sag too much and will that cause problems? Do you need more insulators to stop that happening, or do you need to change the material or the element's dimensions? If you're designing something like an electric fire with multiple heating elements close together, what will happen when they're used individually and in combination? If you're designing a heating element that has air blown past it (in something like a convector heater or a hair dryer), can you generate enough airflow to stop the element overheating and dramatically shortening its life? All these factors have to be balanced against one another to make a product that's effective, economical, durable, and safe.