My mom had one of these when I was a child. I have been looking high and low for a soil tester for years.
This soil tester is great because it tests pH, moisture, and light. It is very difficult to find one that has a meter that does all three.
The probes work on indoor and outdoor plants and are about 7 inches long which will accomodate most if not all plants.
Just be sure to wipe the probes after each use and this will be a great tool for gardening.
I would highly recommend this product.