If you can’t answer the riddle, don’t feel bad. Metal conductors usually conduct electricity and heat. Usually, that’s true, but researchers at the Department of Energy’s Lawrence Berkeley National Laboratory and at the University of California, Berkeley, have found that vanadium dioxide can conduct electricity without conducting heat.
The Wiedemann-Franz Law states that good conductors of electricity are also good conductors of heat. Vanadium dioxide not only switches from an insulator to a conductor at 67C (152F), but it appears that it also doesn’t conduct as much heat as that law predicts while it is in its electrically conductive phase.
[Ryan] designed a PCB that lets you easily take readings from a commercial electrical conductivity probe over I2C. Conductivity measurements are great for measuring the salinity of a solution, which is useful for applications like hydroponics. While the probes themselves are a bit pricey (on the order of $50 from eBay), they are very accurate and last a long time.
Commercial conductivity probes contain platinum electrodes to prevent corrosion. The electrodes are excited with an AC signal, which prevents polarization of the solution and avoids chemical reactions at the electrodes. The voltage across the two electrodes is measured while the electrodes are being excited, which is proportional to the conductivity of the solution
[Ryan]’s board generates +/-5v and uses a Wien bridge oscillator to generate a sine wave which excites the outermost electrodes. The voltage across the electrodes is amplified and fed into a MCP3221, an inexpensive 12-bit ADC with an I2C interface. [Ryan] also wrote an Arduino library for the MCP3221 so you can easily get your probe up and running.
Hardware modding can take you to some strange places: sometimes, you think of really useful improvements to your most practical devices. Other times, you turn a cup of tea into a theremin. This is true at least for [Kyle McDonald], who immersed the L/R leads of an 1/8″ cable in a cup of tea, and connected it to a computer running Max/MSP with a special patch. The result is a working, miniature theremin you can drink after you’re done playing. This process should work with any liquid, so feel free to use beer, cola, water, or anything you think will taste better after you’ve run a small charge through it.