So I have to do a lab, where I rank common household materials based on electronegativity. I am told to acquire two plastic combs and two pieces of string with which to hang them freely off a table. Besides that, I am to acquire common household materials such as rubber, wool, cotton, glass, plastic wrap, etc. The explanation for procedure is kind of limited. From what I gather, I am to tie together the piece of string with the comb and to suspend it from the edge of a table. Then I rub it in wool to give it a negative charge and thereafter use it as a testing device: when I bring charged substances near the comb (without touching), whether they are attracted or repelled will allow me to determine their electronegativity or electropositivity. The degree of attraction or repulsion will allow me to rank them. So, as far as that goes, I think I have things right. But I do not know why I need two plastic combs. Is it necessary to use the second plastic comb to charge the household materials? In other words, do I go about rubbing, say, cotton against the plastic comb to charge it? Do I assume that these materials are to some extent charged before doing anything to them? If someone could please clarify how to make this experiment work, I would be very grateful.