Early movie projectors and theater lights used carbon arc lamps to generate light. These lamps required operators to adjust the carbon rods to maintain the arc generating the light. As the lamp is operated, the rods get shorter, much like welding. These maintenance intensive lamps fell out of favor in the 60s and 70s. Decades later, notes remain on the walls of the projection booth at the abandoned Everett Square theater. I assume these notes on the walls were to help the projectionists know when to change the carbon rods. Additionally, a cabinet still holds spare parts for projectors removed long ago.
