Heavy enameled copper wire is carefully wound on a tough phenolic core. The ends of the wires are brought out to brass binding posts on the rigid end plates. The Air Core Solenoid also can be used with our Current Balance or Mass of Electron Apparatus. The solenoid is 15 cm long and has an interior diameter slightly larger than 5 cm to fit around the cathode ray tube in the Mass of Electron Apparatus. It can carry 10 amps intermittently and 5 amps continuously without overheating.