1) Measuring instrument 
In electrical and electronic engineering, a current clamp or current probe is an electrical device having two jaws which open to allow clamping around an electrical conductor. This allows properties of the electric current in the conductor to be measured, without having to make physical contact with it, or to disconnect it for insertion through th...
feedback device used in electrophysiology experiments to maintain a constant electric current, usually across a membrane, such that changing impedance results in a corresponding change in voltage