(some natural phenomenon of science)- when 2 objects are rubbed against each other  for example if a comb is rubbed with dry hair , how to find out that which object acquires which charge when rubbed against each other  ,in this case we don"t know that the comb has positive charge or the dry hair has ,similarly for negative charge.. which object out of the 2 has negative charge after being rubbed so how to find out the type of charge an object acquires after being rubbed