Very sensitive electric method for AC measurement in SC coils

The authors present an air-core transformer whose mutual inductance can be smoothly changed in a very large interval giving a smoothly variable secondary voltage with suppressed extraneous noise and very small phase shift with regard to dI/dt in the primary. They also describe a very sensitive method of measuring AC losses in superconducting coils based on the use of this mutual inductance with a selective nanovoltmeter. This method eliminates the need for an integrator with small drift and phase shift error, a high resolution voltage divider with a sliding contact, and a shunt resistor with small phase shift, which are required by other analog electric methods. The experimental compensating coil and its parameters as well as AC loss measurements in different superconducting coils are also described.<>