With plaster of paris, the previous form of base was apt to pull apart when the lamp was firmly screwed into the socket. The form of the base was therefore changed to that shown, which overcame these difficulties, and which has been used ever since. The lamp shown was standard for three years and is in the exhibit of Edison lamps in the Smithsonian Institution.] EDISON’S THREE-WIRE SYSTEM The distance at which current can be economically delivered at 110 volts pressure is limited, as will be seen from a study of Ohm’s law. The loss of power in the distributing wires is proportional to the square of the current flowing. If the voltage be doubled, the amount of current is halved, for a given amount of electric power delivered, so that the size of the distributing wires can then be reduced to one-quarter for a given loss in them. At that time (1881) it was impossible to make 220-volt lamps, and though they are now available, their use is uneconomical, as their efficiency is much poorer than that of 110-volt incandescent lamps. Edison invented a distributing system that had two 110-volt circuits, with one wire called the neutral, common to both circuits so that the pressure on the two outside wires was 220 volts. The neutral wire had only to be large enough to carry the difference between the currents flowing in the two circuits. As the load could be so arranged that it would be approximately equal at all times on both circuits, the neutral wire could be relatively small in size. Thus the three-wire system resulted in a saving of 60 per cent in copper over the two-wire system or, for the same amount of copper, the distance that current could be delivered was more than doubled. [Illustration: DIAGRAM OF EDISON’S THREE-WIRE SYSTEM, 1881. This system reduced the cost of copper in the multiple distributing system 60 per cent.] DEVELOPMENT OF THE ALTERNATING CURRENT CONSTANT POTENTIAL SYSTEM