How is the number of TV viewers measured?

[Sophia]

Hello,
OK, i have this stupid question that has haunted me for ages :)
How do they measure the number of viewers in TV? They always show these graphs with thousands of people and even the changes in time. Like x people left during commercials. Or y people did not watch to the end.
How is it measured? Like, I know this a reaaaaaaly stupid imagination, but do they spend more energy to broadcast to more people? or are there some changes in the signal? Or are there chips in most modern TVs and they send the info back?
As you've probably guessed, I have no Physics education :)

 

[Borg]

It's done through surveys. Obviously they can't interview everyone but, by interviewing a few hundreds, they can know what millions have viewed to within a few percent. The larger the sample size, the more accurate they can be.

 

[Silicon Waffle]

It's done through surveys. Obviously they can't interview everyone but, by interviewing a few hundreds, they can know what millions have viewed to within a few percent. The larger the sample size, the more accurate they can be.

How about internet TV ? You use it with your internet connection. Its signals are transmitted in the same cable with the internet but only different in wavelengths or frequencies.

 

[Borek]

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

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

 

[Sophia]

Thanks :)