Cooking is one of the oldest of technologies—and for obvious reasons: humans would
never have survived (let alone thrived) without perfecting the art of
feeding themselves. The basic idea of cookery—heating food to kill
bacteria and make something nutritious and tasty—is fairly prehistoric: "food
plus fire equals cooked food" is roughly how it goes. There's not
an awful lot of difference between roasting a hunted animal on an open
outdoor fire, as our ancestors would have done, and cooking it with
electricity or gas in an oven, as we do today.
[1]
That's not to say there's been no progress in cooking technology. In the 20th century
alone, ingenious inventors came up with two brand new forms of
cooking. One, the microwave oven, uses high-energy
radio waves to
heat food quickly and efficiently in a fraction of the time you need with a conventional stove. The other,
induction cooking, uses electromagnetism to turn
cooking pans into cookers (creating heat energy inside the pan itself,
instead of firing it in from outside),
which cooks food more quickly and safely with less energy. Everyone
knows about microwaves these days, but induction cookers are much
less well understood. Let's take a closer look at exactly what
they are, how they work, and whether they're better or worse than
more familiar forms of cookery.
Photo: Induction cooktops, made from easy-to-clean toughened glass, look much the same as other ceramic cooktops. It's important to know that only cookware with an iron base will work properly with a cooktop like this. Most new pots and pans are very clearly labeled and it's relatively easy to find cooking products that are compatible.
Photo by Juhan Sonin published on
Flickr under a Creative Commons Licence.
Before you can understand induction cooking, you need to understand induction.
And the first thing you need to know is that "induction" is a
shortened way of saying "electromagnetic induction." In a
nutshell, induction means generating electricity using
magnetism. It stems from the simple fact that
electricity and magnetism aren't separate, unconnected things (as we
originally learn in school) but two different aspects of the same
underlying phenomenon: electromagnetism.
Electromagnetism—a quick recap
Photo: James Clerk Maxwell, who described the science of
electromagnetism in four equations. Public domain photo by courtesy of Wikimedia Commons.
A handful of brilliant European scientists figured out the science of
electromagnetism—the mysterious relationship between
electricity and magnetism—in a period of roughly 40 years spanning
the middle of the 19th century. Their findings have proved to be
among the most important discoveries ever made: scientists had known
about electricity since ancient times, but understanding the science (and
technology) of electromagnetism made it possible to power the world
with electricity for the first time.
It all started in 1820. A Danish physicist named Hans Christian Oersted found that when
an electric current flows down a wire, it creates an
invisible pattern of magnetism all around it (a magnetic field,
in other words). The next year, French physicist Andre-Marie Ampère
took this experiment a stage further: he found that two wires
carrying electric currents, placed near to one another,
will either attract or repel one another—a bit like two
magnets—because the magnetic fields they produce cause a force
between them.
So far, the emerging science of electromagnetism was completely theoretical: very
interesting, but not much use. Things took a much more practical
twist when the brilliant English physicist and chemist Michael
Faraday figured out how he could use electricity and magnetism to
develop a very primitive electric motor, also in 1821. He placed a
magnet near a piece of wire into which he fed an electric current. As
the current flowed through the wire, it generated a magnetic field
around it (in the way Oersted had found), pushing itself away from
the magnetic field that the permanent magnet generated. Other
inventors (notably Englishman William Sturgeon and American Joseph
Henry) went on to develop practical electric motors, while Faraday
continued to experiment with the science. In 1831, he pulled off the
opposite trick: he showed how rotating a coil of wire through a
magnetic field would make an electric current flow through
it—inventing the electricity generator that would soon (in the hands
of pioneers such as Thomas Edison) bring electric power to the world.
Animation: Move a magnet in and out of a coil, wired into a circuit, and you'll make electricity flow through it. This is a very simple example of electromagnetic induction—the basic principle behind
electricity generators.
The science of electromagnetism (how electricity can make magnetism and vice-versa)
was finally nailed down by Scottish physicist James Clerk Maxwell in
the 1860s. Maxwell summarized everything that was then known about
electricity and magnetism in four beautifully simple, crystal clear,
mathematical formulas. Maxwell's equations, as we now call
them, still form the foundations of electromagnetic science today.
You don't need to know much about electromagnetism to understand induction
cookery—simply that a changing electric current can make magnetism
and a changing magnetic field can make electricity. When you hear
someone talking about induction, or something that uses induction,
all it means is that magnetism is being used to generate electricity.
A common use for induction is in electric toothbrushes,
which have one or two
rechargeable batteries packed inside. The trouble with electric
toothbrushes is that they get wet, so they need to have completely
sealed plastic cases to keep their mechanisms safe and dry. But that
creates a different problem: if they're completely sealed against water, how can you
get electricity inside to recharge them? A conventional charger
socket would be an open invitation to water as well. That's where
induction comes in. When your toothbrush battery runs flat, you sit
it on a little plastic charger unit to recharge it. Although there is
no direct electrical connection between the toothbrush and the
charger (both are made of plastic), electromagnetic energy flows from
the charger into the toothbrush battery by induction, straight
through the plastic that separates them: a coil of wire in the
charger produces a magnetic field that induces an electrical current
in a similar coil in the base of the toothbrush. You can find out more
(and see some diagrams of exactly how it all works) in
our main article on induction chargers.
Photos: Electric toothbrushes charge by induction: electromagnetic induction allows energy to flow from the (white) charger to the battery in the (dark blue) brush even though there is no direct electrical connection between them.
How does an induction cooktop work?
An induction cooktop (a cooktop is called a "hob" in European countries) is
simply an electromagnet you can cook with. Inside the glass cooktop,
there's an electronically controlled coil of
metal. When you turn on
the power, you make a current flow through the coil and it produces a
magnetic field all around it and (most importantly) directly above
it. Now a simple direct electric current (one that's always
flowing in the same direction) produces a constant magnetic field: one
of the laws of electromagnetism is that fluctuating magnetism is
produced only by a constantly changing electric current.
So you have to use an alternating
current (one that keeps reversing direction) to make a fluctuating magnetic
field that will, indirectly, produce heat. And that's all that an induction hob does: it generates
a constantly changing magnetic field. It does not generate heat directly. You can put your
hand on top of it and you won't feel a thing. (Warning: Don't ever
put your hand on a cooktop that has recently been used for cooking
because it may have become dangerously hot from the cooking pan
that's been standing on top of it.)
When you stand a suitable cooking pan on top of an induction cooktop that's powered
up, the magnetic field produced by the cooktop penetrates the metal
of the pan. So we have a fluctuating magnetic field moving around
inside a piece of metal (the base and sides of the pan)—and that makes an electric current flow
through the pan too (that's all that induction means). Now this is not quite the same as the electric
current that flows through a wire, carrying electrical energy in a
straight line from (say) a battery to a flashlight bulb. It's a kind
of whirling, swirling electric current with lots of energy but
nowhere to go; we call it an eddy current. As it swirls
around inside the metal's crystalline structure, it dissipates its energy,
making heat.
Eddy currents are one source of the heat in induction cooking.
There's another source of heat too. When you magnetize and demagnetize something, like an iron cooking pan,
the process isn't completely reversible: some energy is lost going round the cycle each time.
This is caused by something called magnetic hysteresis and it also generates heat.
It's a bit like when you stretch and release a rubber band lots of times and it gradually heats up
in the process (which is called elastic hysteresis).
So the metal pan gets hot and heats up whatever food is inside it, first by
conduction (it passes its heat energy directly to the food) but also by
convection (liquid food rises and falls in the pan carrying heat with
it). Read more about heat transfer in our main article about heat energy.
Photo: A typical induction cooktop. Photo by
Dennis Schroeder courtesy of US Department of Energy National Renewable Energy Laboratory
(NREL), image id # 68829.
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How induction cooking works
Let's summarize all this quickly and simply:
An induction cooker looks much the same as any other ceramic cooktop, usually with distinct zones where you can place your pots and pans. The cooking surface is usually made from tough, heat-resistant glass-ceramic such as Schott CERAN®.
Inside each cooking zone, there's a tightly wound coil of metal. When you turn on the power, an alternating current flows through the coil and produces an invisible, high-frequency, alternating magnetic field all around it. Unless there's a pan on the cooking zone, no heat is produced: the cooking zone remains cold. You might be wondering why we need a high frequency. Although your home power supply alternates at about 50–60Hz (50–60 times per second), an induction cooktop boosts this by about 500–1000 times (typically to 20–40kHz). Since that's well above the range most of us can hear, it stops any annoying, audible buzzing. No less importantly, it prevents magnetic forces from shifting the pan around on the cooktop.
Place a pan on the cooking zone and the magnetic field produced by the coil (shown here with blue lines) penetrates the iron inside it.
The alternating magnetic field induces whirling electrical (eddy) currents inside the pan, turning it into a heater (shown here in orange). Magnetic hysteresis
(energy loss during the repeated cycle of magnetizing and demagnetizing) also helps to heat the pan.
Heat from the pan flows directly into the food or water inside it (by conduction).
Advantages of induction cooktops
Photo: Gas burners are easy to control, but waste energy by heating the surrounding air and the cooktop as well as the food in the pan. Since they're naked flames, they're more likely to cause a fire than any electric method of cooking.
If you can easily cook with an electric ring or a gas-powered stove, why use an
induction cooktop at all? There are quite a few good reasons.
Efficiency and speed
A traditional cooker generates heat energy some distance from the cooking pot or pan
and attempts to transport as much of that energy into the food as possible—with
varying degrees of success. If you've ever cooked food on a campfire,
you'll know that it's great fun but takes forever.
The main reason is that a huge amount of the energy you produce on an open fire is radiated out into
the atmosphere; great for ambience, but very slow and inefficient.
Even cooking at home can be quite inefficient: you're wasting energy
heating the cooktop and (in the case of a stove with a roaring gas
flame) the air all around your pots and pans. With induction cooking,
the heat is produced in the pan, not the cooktop, and much more of
the energy goes into the food. That's why induction cooking is more
energy efficient than most other methods (around 85 percent compared
to 71 percent for a traditional electric cooktop)—and 70 percent
more efficient than cooking with natural gas.
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Induction cooking also gets
energy to the food more quickly, because pans that get hotter faster cook faster.
Typically, depending on what you're cooking, it's 25–50 percent faster than other methods,
which can be a big plus for restaurants if it helps get dishes
to the table more quickly.
[3]
Convenience, control, and safety
Induction cookers are usually built into ceramic or
glass cooktops (similar to halogen cooktops), which
are very easy to keep clean with just a quick wipe. The magnetic fields they produce make heat appear in the pan almost instantly—and they can make it disappear
instantly too. That's very different from traditionally heated pans,
which take a while to get hot, so there's a greater risk of burning your
food if you don't pay attention!
Photo: This model has a touch-sensitive control pad built into the cooktop itself.
Each cooking zone is operated by a 0–9 control. There are also low-power, fast-boil, and double
controls to expand or reduce the cooking area to suit the pan size. Photo by Dennis Schroeder
courtesy of US Department of Energy National Renewable Energy Laboratory
(NREL), image id # 68931.
You can turn the heat up or down with as much speed
and control as a gas cooker (unlike a traditional electric cooktop,
which takes some time to heat up or cool down). Even so, it's a
different form of cooking and it does take some getting used to:
you have to learn which numeric value on the dial corresponds to
the amount of heat you need, and that takes practice
(to be fair, that applies to any new form of cooking you might try).
On the other hand, induction cooktops are easy to switch on or off automatically,
so some feature built-in timers, built-in temperature sensors, and
even remote control from simple smartphone apps.
There's no open flame on an induction cooktop and (until there's a cooking pan actually
present) no heat to burn you. Heat appears only when the cooking pan
is in place—and the cooktop itself can never get any hotter than the pan sitting on top of it.
Electronically controlled cooktops can detect whether pans are standing on them and how much heat they're producing, and most will cut the power out automatically if they're left on by
mistake or if a pan starts to boil dry. Induction cookers built into
ceramic cooktops are only a couple of inches thick so they can be
fitted at any height (good for disabled people in wheelchairs who
might want a low-level kitchen).
Photo: Ceramic cooktops are strong, durable, and easy to wipe clean in seconds
(burned-on food can be gently and carefully removed with a shallow blade). This one is made from a glass-ceramic called Schott CERAN®, widely used in cooktops since it was first introduced in 1971. It's heat resistant (up to at least 700°C or 1300°F), capable of surviving sudden temperature changes, and highly energy efficient (carrying over 80 percent of the heat from the induction coil underneath it to the cooking pan above).
Less pollution
Combustion-type cooking (natural gas flames or even, in developing countries, open fires) makes significant amounts of
indoor air pollution. Gas stoves, for example,
generate surprising amounts of nitrogen oxides, gases more commonly associated with diesel engines and outdoor smog. Although electric cooking
can still give off "particulates" (unhealthy fine particles) of air pollution, particularly if you're doing
things like frying, it's generally cleaner and better for your health.
[4]
Drawbacks of induction cooktops
Until recently, cost was the biggest disadvantage: a typical induction cooktop could be two or three times
more expensive than an ordinary electric or gas cooktop and, even
though you'd save energy, the energy savings weren't usually
significant enough to pay back the difference. The price of induction cooktops has now
fallen significantly and there's much less difference in cost compared to ordinary ceramic cooktops.
Even so, don't buy an induction cooker with the expectation that you'll see your energy bills fall:
cooking represents only a small fraction of the total energy most
people use at home and any savings you do make (though welcome and
important for the environment) will be modest.
[5]
Another drawback is that induction cooking only works properly with
stainless steel cooking pans containing
iron—the only practical, everyday metal that
efficiently produces electrical (eddy) currents and has enough resistance to generate heat as
they circulate. Copper and aluminum pans
also produce eddy currents and conduct electricity very well,
but that also means they have much lower resistance, so they don't generate
useful heat the same way.
Glass doesn't conduct electricity so
it doesn't work at all.
[6]
Iron-based pots and pans compatible with induction cooktops are
widely available, so the cookware issue is only really a problem if
you have a large collection of existing, unsuitable cookware you're
not prepared to replace. Indeed, some people even see it as an opportunity to upgrade.
If you are going to replace your cookware, you could investigate "cool-touch" pots and pans made specifically
for induction. Some have insulated outer bodies (made
from ceramics or heatproof plastics) that stay relatively cool to
the touch, with lumps of stainless steel or iron embedded in them
to pick up the magnetic field from the cooktop and turn it into heat.
Some have built-in temperature sensors that help the cooktop to
regulate the power it needs to supply, which also enables
automatic, remote control from things like smartphone apps.
Artwork: Some induction cookers use smart pans with built-in sensors.
Here's an example of how one works. 1) The coil in the cooktop generates a magnetic field. 2) A piece of
iron or steel embedded inside the pan picks up the field and converts it into heat. 3) A
thermocouple (electrical temperature sensor) directly underneath continually monitors how hot the pan is getting. 4) A separate induction coil inside the pan picks up energy from the cooktop and converts it into just enough electrical power
to drive a small radio transmitter. 5) The transmitter sends information from the temperature sensor back to the cooktop (6). The cooktop picks up the radio signal (7) and raises or lowers its power as necessary.
Two other minor issues worth noting are that induction cooktops can produce a small
amount of noise (from built-in cooling fans) and radio-frequency
interference that might pose a risk for people wearing
certain types of heart pacemakers.
[7]
Should you buy an induction cooktop?
If you like the speed and control of gas, but prefer the wipe-clean convenience of a
ceramic cooktop, and the relatively high initial purchase cost is not
an issue, induction cooking might be worth considering. Don't buy to
make savings through energy efficiency; you probably won't. Check
your existing cookware before you buy; if you have to purchase an
entire new set of quality pots and pans, that could add significantly
to the outlay of switching to induction cooking.
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Don't want to read our articles? Try listening instead
The Freedom Cooktop by Charlie Sorrel. Wired, 9 January 2012. A brief review of one of the latest induction cooktops.
Induction Seduction by Matthew Fort. The Guardian, 13 September 2010. A food writer raves about his new induction cooktop.
Is Induction Cooking Ready to Go Mainstream? by Kim Severson. The New York Times. April 6, 2010. Explores the pros and cons of induction cooktops and asks whether its adoption by professional chefs will prompt greater mainstream uptake.
Induction Cooktops: Health and safety: Swiss Federal Office of Public Health, 8 November 2011. Are induction cooktops safe? This authoritative review should reassure, although it does point out that there are potential concerns if you have a heart pacemaker or implanted defibrillator. [Archived page served via the Wayback Machine.]
Videos
Induction cooker demonstration by Matt Hodnett of Fulton Innovations demonstrates induction cooking and wireless power to a Guardian blogger.
Patents
For much deeper technical detail, patents are always worth a look. Here's a selection of a few early designs and one very modern, cutting-edge cooktop from Bose:
Handbook of Induction Heating by Valery Rudnev et al, CRC Press, 2002. A detailed guide to all kinds of induction heating (not just household cooking).
Electricity by Chris Woodford. Rosen. 2013 (issued previously by Blackbirch, 2004). One of my own books, this volume explains how scientists have figured out the mysteries of electricity and magnetism, from ancient times to the present day. Ages 9–12.
↑ For more on why cooking was such a radical step forward for humankind, see Richard Wrangham's Catching Fire: How Cooking Made us Human.
New York, Basic Books: 2010.
↑ Compared to space heating, air conditioning, and refrigeration, cooking represents such a minor part of home energy use that it's not even separately detailed in the US Department of Energy's Residential Energy Consumption surveys.
For homes reliant on natural gas, detailed energy use consumption charts suggest cooking uses about 4 percent as much energy as space heating.
↑ I've simplified this discussion of different metals somewhat. There's also the skin effect to take into consideration, but explaining that is a bit beyond what most readers will want or need to know.
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