Problems
The question I am about to make is either too stupid or hasn't bothered anyone because its obvious because I can't really find the answer anywhere.
I am currently studying electricity and magnetism and my book starts by telling that matter consists of atoms that are made of a heavy positively charged nucleus and a cloud of light negatively charged electrons orbiting around it and it bases the rest of the book on these facts.
However electricity, positive and negative charges were studied before the atomic theory was confirmed. Actually at that time it wasn't even remotely supported. Benjamin Franklin for example thought that electrical charge was some kind of cloud/gas. When there was abundance of this cloud the charge was positive while otherwise it was negative.
Questions
Firstly, I suppose that classically we define charge as the physical property that objects have to have in order to interact electrically – at least for now.
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How did physicists back then know about the existence of two charges, positive and negative? Sure, if you bring two glass rods close after rubbing them with silk they repel each other, while if you bring glass rod and one plastic rod they attract each other, but is that really enough?
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Moreover, how did they know that opposite charges attract while same charges repel each other? You can't arbitrarily choose to be so, since electrons really do repel each other. Was there a way to tell that a glass rod had abundance of that cloud after the rubbing while the plastic had a deficit?
In my book it is said that Franklin, after a series of experiments, determined that there are two kinds of charges without elaborating. I cannot find those experiments anywhere. All I get is about the famous kite experiment.
Best Answer
Get together a collection of charges. As many different ways to generate a charge as you can think of. Go ahead and invite your friends so they can think of some more. (As a practical matter you make static charges just before you use them, but still...)
Now, test them pair wise to see if they attract or repel one-another. Keep careful records.
Find the largest set that are all mutually attractive and the largest set that are all mutually repulsive.
You'll find that the attractive set has exactly two members (though you can make many different examples of this set) and the repulsive set consists of half (either half!) the charges you've created.
Ponder that for a while. It also gives you the answer to how like charges respond to one another (though you can get that directly by preparing two similar charges).