http://www.allaboutcircuits.com/vol_3/chpt_2/5.html

The “marble in a tube” analogy to electron conduction in Figure below relates the movement of holes with the movement of electrons. The marble represent electrons in a conductor, the tube. The movement of electrons from left to right as in a wire or N-type semiconductor is explained by an electron entering the tube at the left forcing the exit of an electron at the right. Conduction of N-type electrons occurrs in the conduction band. Compare that with the movement of a hole in the valence band.

 

There may be some grammatical quirk about the word marble not having plural.
Can a native English speaker confirm?

 

timwhite is right.

"marble" should be changed to "marbles".

One marble
Two marbles

(I live in England and I am 100% certain)

 

So the phrase "marble in a tube" should be changed to "marbles in a tube", in the first sentence as well, right?

 

So the phrase "marble in a tube" should be changed to "marbles in a tube", in the first sentence as well, right?

Yes, well spotted. I missed that.

Change marble to marbles in both places.

 

Well, the word marble is mentioned 3 times in that page. It should be changed in all 3 appearances.

 

Thanks all, I guess I could have worded it like that instead of just highlighting it! Next time!