Some electrical devices require dc and others ac. Scientists discovered, in 1870, that the naturally occurring ceramic galena(a chemical compound of lead and sulfur) was a semiconductor and could convert ac to dc. Essentially, the galena acted like a one-way street or tunnel. The ac electricity could enter and pass through the galena during the first half of the alternating cycle, but it couldn't reverse and come back when the ac cycle was reversed. The electricity entering the galena was ac and that exiting the galena was dc. This discovery allowed the scientists to invent new