Exactly right material density as the battery losses charge it losses density. Does not work on rechargeables batteries. Heard of this on one of the radio forums that I frequent a few years ago. Guess I thought that it was common knowledge or would have posted earlier. I still use a multimeter to check the voltage of the batteries that I use before I even put them in any devise.
Papa Your right on that a invaluable tool. I use it for so many task. Can help you find so many problem and correct them quicker than any other tool that I have around the house. Most of the time I grab my cheap one from Harbor Freight. Think I bought it on sale one time for under $4,00. Just the batteries for it are getting higher than I would like to pay. I went on and bought the rechargeable seems to still work ok I use my solar charger to charge them. It only take 2 hours to get them to full charge. Initial cost is More than four new 9 volt batteries but after you charge them 5 time the cost goes down fast.
That's how the wife test me to see if it bedtime. She drops me on my head and if I fall over she covers me. Works all the time but I no heart to tell her she is knocking me out!!!!!!!