- "Just a way of keeping an eye on you boys."
- —Doc Brown, after placing tracking bracelets on Jules and Verne.
When the Brown family traveled to September 21, 1939 on the Time Train to visit the New York World's Fair, Doc put one of these bracelets on Clara, Jules, Verne and Einstein so he would be able to find anyone who got lost. Doc himself wore a bracelet incorporating a tracking device for this purpose.
According to Doc, the bracelets' futuristic inner workings would allow him to track his family members anywhere on Earth.