What are those weird looking donut looking things you see on service wires

You see them on all kinds of commercial buildings where the service entrance wires come in from the electric power company they look like a donut with a hole thru it what purpose do they serve .
There called Current Transformers or CT 's as we call them in the trade.
There used to meter the power being used.

https://en.wikipedia.org/wiki/Current_transformer

In picture at left is a set of indoor current transformers you will find in commercial buildings electrical rooms  usually behind a locked cabinet with high voltage warning tags.

When measuring current being used the typical meter can not handle the 

amount of power coming thru it as in this case is 800 amps . So what is done instead is the main service wires are run thru the these donuts looking units which convert it down to a safe measurable current.

Ct's are used not only on electrical services but any where large amounts of current need measured. some are even used with small LED bulbs which turn on to show you power is running thru the line at that time and active or to turn on other items. They come in all types and sizes as well. depending on what the application calls for. One bad thing about CT's even after power is turned off they can some times still hold a powerful charge which needs discharged. This is why only properly qualified electricians should be handling and servicing them.