Electroscopes ???

I came across the name of this instrument, device, etc. and haven't the foggiest idea of what it is ?? Also would like to know if any of our forum members are familiar with this device. Can explain what it is and have some hands on experience with this unit.

Can it be used, in conjunction with metal detectors and be an effective tool?? I Hope I'm not out of line here? If this does not belong here just tell me where to go? Thank you very much as any an all your feedback will be appreciated. SP USAF 62-66

I had to Google "Electroscopes" because I had never heard of one! I'll bet if they could be used for metal detecting, someone would have figured out how by now.