Zzzap contained by a Faraday cage

He shot toward the door and there was a deafening crack. Zzzap flailed in the air, then rushed the door again. There was a second report, and the wraith was hurled away a second time. His outline blurred for a moment, then pulled back to a crisp silhouette.
The doctor polished his glasses on his shirt sleeve and balanced them back on his nose. “I’m sure you’re familiar with the concept of a Faraday cage, Mr. Burke,” he said. “They were very popular with scientists and espionage agencies because they block out all outside signals and interference. One as well built as the one around this chamber can block any type of electromagnetic signal. Cell phones, television, radio waves—it can keep all of it out.”
The rumpled old man smiled at the gleaming wraith.
“Which also means it can keep anything in.”

Excerpt From Ex-Patriots, pg 341-342
Peter Clines
https://books.apple.com/us/book/ex-patriots/id585738393