Most of these early magnetrons were glass vacuum tubes with multiple anodes. Some magnetrons have ceramic insulators with beryllium oxide (beryllia) added. Research on magnetrons began at Kharkov University in Ukraine during the mid-1920s. This was effective against radars using magnetrons, which cannot change their operational frequency. Led by Aleksandr S. Usikov, a number of advanced segmented-anode magnetrons evolved. All magnetrons contain a small amount of thorium mixed with tungsten in their filament. Some tubes, such as magnetrons, traveling-wave tubes, carcinotrons, and klystrons, combine magnetic and electrostatic effects. Cavity magnetrons can be improperly used to create surface and internal burning. Japan Radio Company (JRC) had long worked with the NTRI in developing magnetrons. With this success, Okabe's research interest switched from magnetrons to VHF equipment for target detection. Controversy exists between the benefits of Klystrons, Gyrotrons, Magnetrons and solid state. These were known as "strapped magnetrons". The problem with existing magnetrons was not power, but efficiency. The company was initially a small supplier of magnetrons to the U.S. Army Signal Corps. Magnetrons and especially hydrogen thyratrons tend to be the worst offenders.