Backstay insulator
Backstay insulators, when used as a pair, are devices which allow for the electrical isolation of a section of wire on a yacht (e.g. the backstay) so that it can be used as an antenna for a single sideband (SSB) radio.
Since these insulators form part of the rigging, not only must they not leak current, but they must also be strong and durable enough not to fail mechanically under the sometimes tough load conditions experienced in sailing.