Alonso's resonant gold leaf cavity SAW detector. Kind of tuning fork for Tesla's telluric currentas or phenomenon currents.
Phenomenon cause syrface acoustic waves in resonant cavity beat of 11Hz gold leaf resonant signal.
How to use SAW for detection small vibration of golden leaf
http://www2.nkfust.edu.tw/~jcyu/Pape..._for_the_d.pdf