current will generate high voltage in the control windings. By connecting them in opposite phase, the two cancel each other, so that no current is induced in the control circuit. The alternate design shown above with the "8" shaped core accomplishes this same objective magnetically. Strengths The magnetic amplifier is a static device with no moving parts. It has no wear-out mechanism and has a good tolerance to mechanical shock and vibration. It requires no warm-up time. Multiple isolated signals may be summed by additional control windings on the magnetic cores. The windings of a magnetic amplifier have a higher tolerance to momentary overloads than comparable solid-state devices. The magnetic amplifier is also used as a transducer in applications such as current measurement and the flux gate compass. The reactor cores of magnetic amplifiers withstand neutron radiation extremely well. For this special reason magnetic amplifiers have been used in nuclear power applications. Limitations The gain available from a single stage is limited and low compared to electronic amplifiers. Frequency response of a high-gain amplifier is limited to about one-tenth the excitation frequency, although this is often mitigated by exciting magnetic amplifiers with currents at higher than utility frequency. Solid-state electronic amplifiers can be more compact and efficient than magnetic amplifiers. The bias and feedback windings are not unilateral and may couple energy back from the controlled circuit into the control circuit. This complicates the design of multistage amplifiers when compared with electronic devices. Magnetic amplifiers introduce substantial harmonic distortion to the output waveform consisting entirely of the odd harmonics. Unlike the silicon controlled rectifiers or TRIACs which replaced them, the magnitude of these harmonics decreases rapidly with frequency so interference with nearby electronic devices such as radio receivers is uncommon. Applications Magnetic amplifiers were important as modulation and control amplifiers in the early development of voice transmission by radio. A magnetic amplifier was used as voice modulator for a 2 kilowatt Alexanderson alternator, and magnetic amplifiers were used in the keying circuits of large high-frequency alternators used for radio communications. Magnetic amplifiers were also used to regulate the speed of Alexanderson alternators to maintain the accuracy of the transmitted radio frequency. Magnetic amplifiers were used to control large high-power alternators by turning them on and off for telegraphy or to vary the signal for voice modulation. The alternator's frequency limits were rather low to where a frequency multiplier had to