It’s a hygrometer. Nothing special, you might say, but the measuring method is pleasantly interesting. The instrument has two thermometers: one dry and the other wet. The difference in their temperatures is a measure of the relative humidity, which you can read from a table. If only everything were this simple.
I was reminded of this instrument by our preparations for a weather station project, which have been underway for a while now, and our work on a new design for the CO2 meter published in the January 2008 issue. We plan to upgrade the original design with a temperature sensor and a humidity sensor. Naturally, we intend to use a ready-made humidity sensor for this, but the old instrument still has a certain charm, perhaps due to its simplicity or the self-evident operating principle. It seems like you learn something from this instrument. The operating principle is somewhat similar to that of a differential amplifier.
Who knows, maybe one of you will put together an electronic version? Or am I simply making things unnecessarily complicated again?