Diagram of Constant Current Series System.
This, in 1878, was the only method of distributing electric current.
Diagram of Edison’s Multiple System, 1879.
Edison invented the multiple system of distributing electric current, now universally used.
When Edison proposed a very low resistance armature so that the dynamo would have an efficiency of 90 per cent at full load, he was ridiculed. Nevertheless he went ahead and made one which attained this. The armature consisted of drum-wound insulated copper rods, the armature core having circular sheets of iron with paper between to reduce the eddy currents. There were two vertical fields above and connected in shunt with the armature. It generated electricity at about a hundred volts constant pressure and could supply current up to about 60 amperes at this pressure. It therefore had a capacity, in the present terminology, of about 6 kilowatts (or 8 horsepower).
Edison Dynamo, 1879.
Edison made a dynamo that was 90 per cent efficient which scientists said was impossible. This dynamo is in the collection of the Smithsonian Institution and was one of the machines on the steamship Columbia, the first commercial installation of the Edison lamp.
A multiple system of distribution would make each lamp independent of every other and with a dynamo made for such a system, the next thing was to design a lamp for it. Having a pressure of about a hundred volts to contend with, the lamp, in order to take a small amount of current, must, to comply with Ohm’s law, have a high resistance. He therefore wound many feet of fine platinum wire on a spool of pipe clay and made his first high resistance lamp. He used his diaphragm thermostat to protect the platinum from melting, and, as now seems obvious but was not to all so-called electricians at that time, the thermostat was arranged to open circuit instead of short circuit the burner when it became too hot. This lamp apparently solved the problem, and, in order to protect the platinum from the oxygen of the air, he coated it with oxide of zirconium. Unfortunately zirconia, while an insulator at ordinary temperatures, becomes, as is now known, a conductor of electricity when heated, so that the lamp short circuited itself when it was lighted.