He majored in Greek and after taking one undergraduate course in physics, graduated from Yale University. He taught languages at The Albany Academy before returning to Yale, to take a doctorate in physics. He then undertook research on photoelectricity whilst teaching physics for five years at the Worcester Polytechnic Institute.
During 1916, Hull began investigation into the use of magnetic control of thermionic valves (vacuum tubes) as an alternative to grid or electrostatic control and he had tested successfully magnetic control by applying a magnetic field parallel to the axis of the tube.
Initially, Hull's work on these novel electron tubes was part of an effort at General Electric to develop amplifiers and oscillators that might be used to circumvent the vacuum- tube triode patents of Lee de Forest and Edwin Armstrong.
He invented the dynatron which had three electrodes: a thermionic cathode, a perforated anode, and a supplementary anode or plate. In normal operation the supplementary anode was maintained at a lower positive voltage than the perforated anode. The secondary emission of electrons from the plate made the dynatron behave as a true negative resistance and so the tube could generate oscillations over a wide range of frequencies or be used as an amplifier. When a control grid was added between the cathode and the perforated anode, the device was called a "pliodynatron."
By 1920 his research led to his invention of the magnetron. This took the form of a coaxial cylindrical anode and cathode with an axial magnetic field produced by an external coil. The Hull magnetron was tested as an amplifier in radio receivers and also as a low-frequency oscillator. It was reported in 1925 that a magnetron made at GERL could generate a power of 15 kW at a frequency of 20 kHz. At the time Hull anticipated that the magnetron would find greater use as a power converter than in communication applications.
The Magnetron was later used by Percy Spencer to create the microwave and the British military to create radar.
During the 1920s, Hull also was a major contributor to the development of gas-filled electron tubes at the GERL. He discovered how to protect thermionic cathodes from rapid disintegration under ion bombardment. This discovery enabled the successful development of hot-cathode thyratrons (gaseous triodes) and phanotrons (gaseous diodes).
In the 1918 issue of the Proceedings of the IRE he published a paper on the dynatron vacuum tube which he had invented .
Hull was promoted to assistant director of the GERL in 1928 and was known for his collegial management style with minimal interference with the research agenda of his staff members.
He was awarded the 1930 IEEE Morris N. Liebmann Memorial Award and the IRE Medal of Honor in 1958 by the Institute of Radio Engineers (IRE) "For outstanding scientific achievement and pioneering inventions and development in the field of electron tubes."
He did consulting work and served on an advisory committee of the Army Ballistics Research Laboratories after retirement from General Electric and was elected a member of the National Academy of Sciences. He also served as president of the American Physical Society in 1942.