I am watching the Smithsonian channel and there is film of a Boeing 737 flying and there is an object trailing from the top of the tail.
It looks like the thing is on a wire and is about 6 feet behind the plane.
Does anyone know what this is?
Did it look like the thing in this picture? Reddit says it’s a “trailing cone,” used for pressure measurements that confirm the pitot tubes (for measuring airspeed) work properly.
Subsequent Google search for 737 trailing cone brings up lots of results.
Yes, that’s it. Thanks for your help. I didn’t even know what it was called.
FWIW, I didn’t know either. Based on your description I did a google-images search for “737 drogue chute,” even though I figured you weren’t actually describing a drogue chute. But that’s how I found the image and Reddit article, which then enabled a search for the real terms.
So I learned something today too. Thanks! 
Might be important to note that the trailing cone is only installed for flight testing, you won’t find it installed on an airplane in regular service.
There are several pieces of test gear installed on prototype aircraft to take air data measurements. The trailing cone is one. A long nose-mounted pitot tube is another.
The sensors used for the normal production instruments are all pretty small and subject to distortions in the airflow field around them. This is accounted for in the design as best they can. But they need to verify the compensations are accurate.
So they take a separate set of measurements with long probe pitots & trailing cones & such. Which inputs are driven into engineering quality test instruments and recorders. Meantime the inputs from the production sensors are recorded as well.
Back in the shop they construct compensation curves from the differences between the two data sets. And install them into the production instrument systems so the final indications sent to the pilots, autopilots, flight data recorders, etc. are as accurate as possible.
Since all the built aircraft are identical, these tests are only run at the beginning of a new model’s prototype flying then the resulting compensation curves are installed in all the rest of the fleet as they’re being built.
The process gets repeated for new sub models only when the aerodynamics near the sensor ports changes significantly.