If you don't want to read through this entire narrative, I will cut to the chase with a spoiler:
The red wire on a K type thermocouple will not stick to a magnet, and the yellow wire will stick to a magnet!
Short and sweet. File this in your brain because you will need it someday.
OK, and Now, the Story of This Discovery:
One of my current projects is to build a digital controller for an old Cress ceramics kiln I picked up on CraigsList. The old Cress kiln had the old-school analog dials and controls for speed of heating and time of heating, and a little mechanical device that shuts off the kiln when it reaches a certain temperature. I really just wanted to be able to select a certain temperature and not mess with dials and cones and guess work.
So, of course, I Arduino'ed it, like I seem to do with all my projects.
This system I built for the kiln is made up of an Arduino Pro Mini 8MHz 3.3v (these are $2.50 each on eBay, and I buy them by the dozen), an LED driver transformer that takes 115 to 240 VAC and puts out 12 VDC at 1 A (these are $1 on eBay and great to power line-powered projects), an adjustable buck converter that takes the 12 VDC and drops it down to 3.3 VDC to power the Arduino, 31855 board, the 5110 LCD and the solid state relays (these buck converters are around $1 too), a 31855 thermocouple amplifier breakout board from Adafruit, a Nokia 5110 LCD display (these are less than $1 on eBay), and a K type thermocouple. Oh yeah, there are two momentary push buttons, and LED, 2 solid state relays, and a 2-pole 30 amp wall switch from Lowe's. To use it, you simply enter your target temperature with the up and down buttons, and the kiln heats up to that temperature and stays there. It works great!
The next iteration of the project will be to add ramp and soak time controls, so you could tell the kiln to heat up to say 1,000 degrees F over 5 hours, then hold at 1,000 degrees F for 2 hours, then heat up to 2,000 degrees F and hold for 30 minutes, then drop back to room temperature over 6 hours. I plan to include a web server and interface it through a phone or tablet. There are now $34 7" Android tablets on Amazon and I think that would make a great dedicated color touch interface for any Arduino project!
Anyway, the controller project and the ramp and soak addition will have to wait to be another Instructable I will write some other time.
Back to Thermocouples:
A thermocouple is a junction of two dissimilar metals which, when heated, will produce some power, which can then be measured in millivolts. Then you (or your microcontroller) look up the temperature for that amount of millivolts and you know how hot your thermocouple is.
The most common type is the K type thermocouple which uses chromel and alumel metal alloys for the two types of metal. Well, it turns out that most grades of chromel used in thermocouples. specifically chromel "C", (also known to us makers as "nichrome wire"), usually contains 24% iron and is therefore magnetic. Here is a link to the Wikipedia entry for K type thermocouples.
The problem I kept having in my project was that I was not able to tell which wire that was coming out of the thermocouple was the red one, and which was the yellow one. It matters, because thermocouples are DC, so there is a polarity. On the thermocouples I had, some were marked with red, but when I hooked them up to my multimeter or a thermocouple amplifier, the voltage signal would sometimes be reversed (the value would go down when the thermocouple was heated), and I would have to flip it. I figured out that the chromel wire which is used for the yellow or non-red connection is magnetic, and the alumel wire used for the red connection is not magnetic! Problem solved.
Further Information on Thermocouple Usage and Wiring
Then as I learned more about thermocouples, it became clear to me that I really should eliminate all the other potential thermocouple junctions. Because of the nature of the way thermocouples work, ANY junction of two dissimilar metals will produce some small voltage. ALL of them! There is at least one Instructable about how to make your own thermocouple from scratch. Because you can make a thermocouple from any two dissimilar metals, it means that if you use any wires to connect your thermocouple with the meter, amplifier chip, etc., you will have thermocouple junctions at every connection between wires or connectors! Even jacks, connections, solder joints, plugs, multimeter probe connections to wires, and all connections will produce a voltage, so you won't be able to tell how many millivolts is actually being produced by your real thermocouple!
Well that's lame...I guess there's no way to use a thermocouple then, right? Well, there is a solution. You have to use the exact same metal in your wires to connect all the way from your thermocouple all the way to your meter. For a K type thermocouple, that means you have to use K type thermocouple extension wire which is made out of...you guessed it, chromel and alumel! If you use that type of wire, then one side of your thermocouple uses chromel all the way from the junction down to your meter or amplifier board or chip, and the other side uses alumel. If you hook it up this way, then there will be no junctions of dissimilar metals in your wiring except the thermocouple itself (which is the only one we want), and of course where the chromel and alumel wires ultimately connect to your chip, but this is really unavoidable. Fortunately the 31855 thermocouple amplifier chip reads its own internal temperature and is able to calculate how many millivolts are being produced by the local connections between the wires and the chip leads which are acting as thermocouples, and cancel those voltages out of the overall reading.
(See diagram image of thermocouple wiring.)
The thermocouple I use for my kiln has a ceramic terminal block, but the metal barrel connectors in it are made from just some random metal. So I didn't use the terminal block to connect the wires because that would have created 4 more thermocouple junctions! Instead, I twisted the thermocouple extension wire around the legs of the thermocouple and tightened that into one of the barrel connectors.
So What About the Magnet?
When you grab a piece of thermocouple extension wire, it may or may not be color coded with red and yellow insulation. Most of mine was not. Also, a few of the thermocouples I have are not marked as to which lead is the red coded lead, and some are actually marked incorrectly! So how do you tell if A.) you have thermocouple wire, and B.) which wire needs to connect to which wires coming out of the thermocouple? Easy...with a magnet. If you touch a magnet to each wire on a thermocouple or thermocouple extension wire, you will find that the chromel side sticks to the magnet, and the alumel side does not. If you have a pair of wires in which one is magnetic and one is not, then it is likely to be thermocouple wire. To connect the chromel wire to your thermocouple, just find the magnetic wire on your extension wire and connect it to the magnetic wire on your thermocouple, and then connect the non-magnetic wire to the non-magnetic wire on your thermocouple, and you are good to go! The magnetic wire will be the one traditionally color coded as yellow. The non-magnetic wire will be the one color coded as red.
The moral of the story is: Do not use just any old wire to connect a thermocouple to your circuit or you WILL get extra voltages created by the additional junctions between the different types of metal, and it will be impossible for you to determine the voltage being produced by your primary thermocouple.