The Federal Judicial Committee has published proposed Model Jury Instructions (re "The Use of Electronic Technology to Conduct Research on or Communicate about a Case") [link opens 2-page PDF]. I honed in on the following language, to be given before trial and at the close of the case: "I expect you will inform me as soon as you become aware of another juror’s violation of these instructions."
As well meaning as this language may be, is it a realistic strategy? Never mind the fact that it's next-to-impossible to know what that person across the room is tinkering with on his or her smartphone.
As Roy Scheider (Brody) said in the 1975 movie, Jaws: