The thyristor valve typically consists of 10-30 inverse-parallel-connected pairs of thyristors connected in series. The thyristor valve needs to contain enough thyristors in series to withstand this voltage safely. TRIACs are a subset of thyristors and are related to silicon controlled rectifiers (SCRs). Bipolar transistors and thyristors are currently developed. The term is particularly applicable to mostly semiconductor junctions, especially diodes and thyristors. These use solid state switching devices (thyristors and transistors) and have electronic power control. Cooling for the thyristors and snubber resistors is usually provided by deionised water. Reverse conducting thyristors are often used in frequency changers and inverters. J. Torkel Wallmark and Harwick Johnson (RCA) used both the thyristors and field-effect transistors. GTO thyristors capable of blocking reverse voltage are known as Symmetrical GTO thyristors, abbreviated S-GTO. The locomotives are notable for using thyristors instead of the older diode based system. The typical application for symmetrical GTO thyristors is in current source inverter. Converters using thyristors or mercury-arc valves are known as "line commutated converters". The inverter may use thyristors, IGCTs or IGBTs. Certain ratings, especially for large systems (many kilowatts) use thyristors (SCR).