
This is a switch for “constant current” electricity to go into a building, a “A modern commercial form of this switch,” is what the book calls it.
The book in question being Horstmann and Tousley’s Modern electrical construction; a reliable, practical guide for the beginner in electrical construction, showing the latest approved methods of installing work of all kinds according to the safety rules of the National Board of Fire Underwriters. You can peruse it over on archive.org.