What is a "gravitational cell"? I am not a physicist, and I don't understand the details of electromagnetism. Anyhow, I was looking for how the batteries work in Google. So, I came across this article: "How batteries work: A gravitational analog" by Dana Roberts. 
In the abstract, she said something about "gravitational cell". Then she gave a lot of technical words that I could not understand. I was going to read the whole article, but I don't have account for AJP. So, I don't have access for this article.
My question here is: What is the "gravitational cell"? Please, explain it to me in detail with some illustrations.
EDIT:
Just to let you know, I found the full article here
 A: I got the access but I'm too sleepy to understand. Sorry. I will just throw a picture here. Hope I didn't violate any sort of copyright law. yawn

DO NOT COPY OR SPREAD THIS PICTURE. PLEASE DELETE THIS ANSWER IF THERE ARE ANY VIOLATION OF THE LAW. I'm a Chinese. I know no copyright thingy.
A: The article is referring to the Gravity Cell which is stype of Daniell Cell. ffor more information refer to - http://en.wikipedia.org/wiki/Daniell_cell#Gravity_cell. It was termed so because the zinc sulphate sat on top of a layer of copper sulphate due to the difference in Specific Gravity - http://en.wikipedia.org/wiki/Specific_gravity.
A: Both the title of the article ("How batteries work: a Gravitational Analog") and the abstract clearly state that the gravitational cell is used as an analogy. This is used to explain the movement of electrons through diffusion under osmotic pressure. 
In other words, "gravitational cell" is only used to help understand normal batteries. It does not mean that there is anything like a "gravitational battery"
