Improved Screw Base Lamp, 1881.
The terminals of this base consisted of a cone shaped ring and a screw shell. At first wood was used for insulation, later plaster of paris which was also used to fasten the base to the bulb. This lamp is in the exhibit of Edison lamps in the Smithsonian Institution.
In the early part of the year 1881 the lamps were made “eight to the horsepower.” Each lamp, therefore, consumed a little less than 100 watts, and was designed to give 16 candlepower in a horizontal direction. The average candlepower (spherical) in all directions was about 77 per cent of this, hence as the modern term “lumen” is 12.57 spherical candlepower, these lamps had an initial efficiency of about 1.7 lumens per watt. The lamps blackened considerably during their life so that just before they burned out their candlepower was less than half that when new. Thus their mean efficiency throughout life was about 1.1 l-p-w (lumens per watt). These figures are interesting in comparison with the modern 100-watt gas-filled tungsten-filament lamp which has an initial efficiency of 12.9, and a mean efficiency of 11.8, l-p-w. In other words the equivalent (wattage) size of modern lamp gives over seven times when new, and eleven times on the average, as much light for the same energy consumption as Edison’s first commercial lamp. In the latter part of 1881 the efficiency was changed to “ten lamps per horsepower,” equivalent to 2¼ l-p-w initially. Two sizes of lamps were made: 16 cp for use on 110-volt circuits and 8 cp for use either direct on 55 volts or two in series on 110-volt circuits.
Final Form of Screw Base, 1881.
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.