My folks and I visited the Edison Historical Site. Very enjoyable. We saw the laboratory, library, and a museum displaying several of his inventions.
I remember looking at a large switch which carried heavy DC current. The contact brush was a couple inches across, consisting of several copper plates held tightly together. Their edges built up to a large surface which slid into a large groove in a copper plate.
Through repeated operation the edges wore into the groove and made contact over a large area. This sort of cumbersome design was necessary, back when they were lighting up a city block with DC.
---------------------------------------------
Over the years I had heard different reports of the number of trials Edison performed on light bulb filaments, before finding a substance which could be commercially viable. I think the largest I heard was 10,000.
So I asked the guide "What is the correct number?" She said 3,000.