Thermcouples - Excellent Circuit Cellar article on thermocouples, uncovered by John Baas. Get it from my local copy or the original copy.

Humidity Sensors - Documentation from Honeywell, mostly related to their HIH line of sensors.

• Theory of operation
• Chemical resistivity
• Psychrometry
• Application guide

Hall Sensors - An application note from Melexis, who principally make automotive parts.

4-20 mA Loops - Overview of theory and use of 4-20 mA current loops for instrumentation interfaces. Found by SF Cillari.

Field Effect Transistors (FETs) - From the guys who are the front-line developers of these parts, International Rectifier, research provided by Dennis Novotny.

• IR Archives - A list of application notes and white papers on power switching devices.
• Choose Wisely - IR's white paper discussing the application areas and relative merits of MOSFETs and IGBTs.
• AN-969 - Economic, High Performance, High Efficiency Electronic Ignition with Avalanche Rated HEXFETs. For us those interested in firing ignition coils.

Analog to Digital Conversion (ADC) - Here is a Motorola/Freecale paper on ADC theory, very complete and informative.

Be sure to look at Electronic Component Documentation, too.