I was going through one of the boxes from my collection and found this book: Units and Systems of Weights and Measures Their Origin, Development, and Present Status

I came across a photo of Willam F. Meggers looking though device from Mercury-198 bulb operating from radiofrequency (it has no electrodes) at page 9 of that book.

How the mercury content is maintained there, when the 180Hg has a halflife of barely 3 seconds?

Perhaps you can read better at https://cs.wikipedia.org/wiki/Izotopy_rtuti