be utilized to generate higher radio frequencies than the alternator was capable of producing. Even so, early magnetic amplifiers incorporating powdered-iron cores were incapable of producing radio frequencies above approximately 200 kHz. Other core materials, such as ferrite cores and oil-filled transformers, would have to be developed to allow the amplifier to produce higher frequencies. The ability to control large currents with small control power made magnetic amplifiers useful for control of lighting circuits, for stage lighting and for advertising signs. Saturable reactor amplifiers were used for control of power to industrial furnaces. Magnetic amplifiers as variable AC voltage controllers have been mostly replaced by silicon controlled rectifiers or TRIACs. Magnetic amplifiers are still used in some arc welders. Small magnetic amplifiers were used for radio tuning indicators, control of small motor and cooling fan speed, control of battery chargers. Magnetic amplifiers were used extensively as the switching element in early switched-mode (SMPS) power supplies, as well as in lighting control. Semiconductor-based solid-state switches have largely superseded them, though recently there has been some regained interest in using mag amps in compact and reliable switching power supplies. PC ATX power supplies often use mag amps for secondary side voltage regulation. Cores designed specifically for switch mode power supplies are currently manufactured by several large electromagnetics companies, including Metglas and Mag-Inc. Magnetic amplifiers were used by locomotives to detect wheel slip, until replaced by Hall Effect current transducers. The cables from two traction motors passed through the core of the device. During normal operation the resultant flux was zero as both currents were the same and in opposite directions. The currents would differ during wheel slip, producing a resultant flux that acted as the Control winding, developing a voltage across a resistor in series with the AC winding which was sent to the wheel slip correction circuits. Magnetic amplifiers can be used for measuring high DC-voltages without direct connection to the high voltage and are therefore still used in the HVDC-technique. The current to be measured is passed through the two cores, possibly by a solid bus bar. There is almost no voltage drop in this bus bar. The output signal, proportional to the ampere turns in the control current bus bar, is derived from the alternating excitation voltage of the magnetic amplifier, there is no voltage created or induced on the bus bar. The output signal has only a magnetic