Electromagnets: Two Forces Team Up
Resistive electromagnets, another type of temporary magnet, combine the powers of electricity and of magnetism to enormous effect. Although superconducting electromagnets, which we’ll discuss soon, offer many advantages, certain laws of physics prevent them from reaching the field strengths electromagnets are capable of.
Some of the world’s most powerful resistive magnets here at the MagLab; they can generate a sustained magnetic field of 35 tesla. (We’re not counting here our world-record hybrid magnet or the stronger, short-lived fields of pulsed magnets, both of which we’ll explain shortly.) Scientists use these high fields to do a wide range of experiments concerning physical, chemical and biological processes of different types of matter.
But electromagnets don’t have to be hugely powerful. Whether in your toaster or TV, even small ones can be quite useful. To make one, you start out with some wire.
Now, plug that wire in so that a current runs through it. The result? A magnetic field (depicted in blue) – and a temporary magnet that you switched on with the current. Think of this as the simplest kind of electromagnet.
Current in a wire.
Turns out, there’s a magnetic field circling every electrical wire, a fact famously stumbled upon by an astounded Dane named Hans Christian Ørsted in 1819, who noticed the magnetic needle of a compass align itself perpendicularly to a nearby current-carrying wire.
Ever hear of “animal magnetism”? Some doctors once believed magnetic fields ran through the human body, and that applying magnets to them could cure various ills. Some people today believe magnets have special healing properties.
That’s because electrical current – the flow of electrons through a wire – always generates a magnetic field. You can tell which way the invisible lines of that field are traveling: If you point the thumb of your right hand in the direction of the current, the magnetic field travels in the direction that your fingers, curved as if clasping a rod, are pointing. (This web site features an interactive tutorial on the magnetic field around a wire, if you’d like to learn more.)
The magnetic field around a household electrical wire, and household appliances, is pretty small. About a foot away from a whirring ceiling fan, for example, the magnetic field measures just a few milligauss. To create a much more powerful magnet, we need to take it to the next level. Let’s see what happens when we take that same wire and coil it around once in a circle.
Coiled wire and magnetic fields.
What do you know? With that simple step, all the smaller, separate magnetic fields circling the wire have joined forces to create a far, far greater magnetic field.
If that’s the effect of creating a single coil, what will happen if we make a bunch of them ‘ 10, let’s say? Let’s try it and find out.
Very cool! Our new magnetic field is about 10 times more powerful. This special type of coil, by the way, is called a solenoid, and the magnetic field it produces increases proportionately to the number of coils you include. Coiling the wire also makes the field more uniform, a property important to scientists testing the effects of magnetic fields on different materials.
We can improve the magnetic field power of this solenoid even more by inserting an iron alloy core in the middle. Remember our iron atoms of earlier, and how each was a tiny little magnet? Well, when you put the iron alloy core into a magnetic field, all the atoms in the iron align with it and, in so doing, boost the magnet’s field strength significantly ‘ yet without using more electricity! Speaking of electricity, the more power you add to your solenoid, the greater your magnetic field.
Bitter plate and a coil from an electromagnet.
You can improve on this basic solenoid by replacing the coils with specially designed Bitter plates that better withstand both the pressure resulting from high magnetic fields and the heat resulting from electrical current. These plates were first invented by a fellow named Francis Bitter in 1936. In the 1990s MagLab scientists greatly improved on his design, inventing the Florida Bitter plate, which enabled the creation of more powerful magnets. Most large electromagnets used for research use Bitter plates, which is why they are sometimes called “Bitter magnets.”
These electromagnets are called “resistive” magnets because, as in any machine running on normal electricity, the electrons that make up the current encounter resistance as they bump into atoms and other electrons along their journey. This inefficiency costs when it comes time to pay the electric bill; in fact, the Magnet Lab uses about 7 percent of the electricity consumed in Tallahassee, a city of about 160,000 residents! If we didn’t have magnets to operate, our on-site power source could light up some half-million 100-watt light bulbs ‘ at the same time! The MagLab also uses a considerable amount of water, which is forced through the holes of the Bitter plates to prevent the magnets from getting too hot.
So resistive magnets eat lots of electricity and drink lots of water, two expensive habits. This is where superconducting magnets, the next type of temporary magnet we will explore, offer some advantages.