Why does the pieces of copper has the same density for the bar of copper ?
replace them. I would say once a year. if you are concious of the cost you can buy a roll of just the blade rubber and fit it in the wiper yourself.
If you go to a place like AutoZone, they will sell you the correct wipers and help you learn to put them on.
Because, for any particular substance (e.g. copper), the density of that substance is a FUNDAMENTAL CHARACTERISTIC which is indirectly determined by its chemistry. Other such fundamental characteristics would be the melting point, the crystal structure, the hardness etc.
replace every 3-6 mos. or use rain-x once a month.
