Below is a list of Current Transformers manufacturers from St. Louis, Missouri. Current transformers are tools that are used to measure the current, or flow of electricity, in electrical transmission lines. Understanding a current transformer requires an understanding of electrical current. Electrical current is measured in amperes, and one ampere is equal to the movement of 6.241 × 1018 electrons passed a given point per second. While power transformers control voltage, current transformers can measure and control movement for measuring purposes and to help control the flow of electricity to electronic devices.