We are a Current transformer* manufacturer.
We design and manufacturer Current Transformers in the traditional circular Toroidal core method, but also manufacture current transformers using U core and C core style magnetic cores.
Standard Current Transformers require a closed loop core to carry the winding. This core must fit around the primary conductor, which is being measured, which can be very difficult to achieve when the primary conductor is physically large or is difficult to access and this is why we also manufacture Current Transformers using U core and C core style magnetic cores, to solve this problem.
Also see Rogowski Coils which do not require a closed loop to carry the winding.
Current Transformers measure the current in a conductor (the primary) by comparing it to the current in a coil wound on a core (the secondary) which closes around the conductor. The current in the primary induces a current in the secondary. The size of this current is proportional to the turn ratio between the primary and secondary e.g. A 100 turns secondary will carry of 1/100th of the current that the single turn primary conductor carries. The secondary current can be measured and the primary current calculated.
This means that a large current in a physically large conductor can be measured without actually touching that conductor.