Differential Video - for those who wondered

[Akula69]

For those who ever wondered....I found this video and it is simplicity at its best:

 

https://www.youtube.com/watch?v=WKYfnkQMxv0

 

 

[Doc.]

That was a pretty nifty demonstration.

[87MJTIM]

That is just a small portion of a longer video.  I think the total length was 10 min.  Looks like it was made in the '30s.

 

The whole video is on youtube.

 

Here is the full length