Pearson Electronics of the United States began designing and producing monitoring transformers in 1958. After decades of development, its products are widely used in U.S. military, medical, aviation and other fields, and have long served in extremely complex transient signal and periodic frequency signal environments, establishing its requirements for precision and high quality. The current test ring, also known domestically as a current transformer or current sensor, operates equivalently to a transformer with its secondary side short-circuited. Ignoring the excitation current, the ampere-turns are equal: I1N1=I2N2. The ratio of the primary winding current I1 to the secondary winding current I2 of the current transformer is called the actual current ratio I1/I2=N2/N1=k.