Sorry if this has already been asked. I tried searching the forums first, but could not find an answer. I could have been searching for the wrong thing.
I recently purchased the Advanced Arduino Starter Pack from the Maker Shed. In the pack, there is a thermistor. It has the number 503 printed on it. I'm guessing this is an NTC thermistor?
Does anyone know where to find the datasheet for it or is there a good tutorial on how to use it?
There is a page in the playgrounds:
http://www.arduino.cc/playground/ComponentLib/Thermistor2
That is for 10k thermistors. It worked fine on mine right out of the box (and was accurate within one degree), but it may not for yours because I did not get mine from the Maker shed.
Good luck!
I tried that tutorial and I'm using the thermistor that came in the pack, but I'm getting values like -50 degrees. It's 68 in my house right now, so... yeah. The only thing I changed was the pin, which I have as pin 3. Otherwise, the thermistor connects to 5v, then to a row on the protoshield breadboard. The other resistor also connects to that row, then to ground, and a jumper wire from the row to pin 3. As far as I can tell, that should be correct.
Is the math.h library one of the included libraries? I thought maybe I needed to download that, but I can't find any information on that anywhere. It's not listed as an included library and I don't know anything about programming, so I don't know if I'm just supposed to know that it's included.
When I did a straight read where I had it print the regular value without all the math, it showed up at around 1000 (positive). What am I doing wrong?
The thermistor and resistor form a voltage divider. Have you measured the resistance of the thermistor? What value resistor are you using?
When I turn everything on (plug in the Arduino), the thermistor registers about 56.4 with the meter set at 200k (so is that 56.4k?). The resistance drops when I unplug it from the board. I was using a 1k resistor, apparently, so I swapped that to the correct 10k resistor. Now I am getting readings around 12 from the serial monitor. It should be about 68 degrees in my house.
Did you get this figured out? I'm having the same problems 
I long since gave up on thermistors for measuring temperature, there are just too many variants to have a nice equation to convert the readings to anything resembling a temperature. I use Dallas DS1820 or DS18S20 and get direct readings. Or you could try the cheaper LM35 but watch the supply voltage if you're using USB.
Thermistors are usually rated in terms of resistance at some standard temperature, such as 10K@25C. The one you have has a 503 on it so I would guess it's 50K@25C. The resistance may not be proportional to the temperature and it may change with or inversely to the change in temperature. Your best bet is to try using a 50K resistor in conjunction with that thermistor and see if you get a reasonable reading. I usually use a potentiometer rather than a fixed resistor so I can sort of calibrate the thermistor.
The resistance may not be proportional to the temperature
Read as "The resistance will not be proportional to the temperature" 