Psychrometer

The very first scientific instrument I ever used was a psychrometer.

That may sound like a gizmo to measure mental health, but (also known as a wet/dry bulb hygrometer) it is a tool used to measure relative humidity. 

It looked something like this:

One thermometer had a sock over the bulb, and to take measurements, you would wet the sock, then swing it around by a string attached to the top.  I was probably about eight years old, and “helping” my dad in his air-conditioning business, when he let me use it for the first time.

The back of the device was a specialized slide rule used to do the actual humidity calculation.

This type of hygrometer was invented by a German scientist named Richard Assmann.  Swinging it around was fun, but a big part of the attraction for an eight-year-old boy was being able to give the humidity reading according to the Assmann scale.