Part 16

VARIOUS SERIES BASES IN USE, 1892.

The above six bases have been superseded by the “Large Edison,” now called the Mogul Screw base.]

THE EDISON “MUNICIPAL” STREET LIGHTING SYSTEM

High voltage direct current was generated, several circuits operating in multiple, three ampere lamps burning in series on each circuit. Photograph courtesy of Association of Edison Illuminating Companies.]

The arc lamp could not practically be made in a unit smaller than the so-called “1200 candlepower” (6.6 ampere) or “half” size, which really gave about 350 spherical candlepower. A demand therefore arose for a small street lighting unit, and Edison designed his “Municipal” street lighting system to fill this requirement. His experience in the making of dynamos enabled him to make a direct current bipolar constant potential machine that would deliver 1000 volts which later was increased to 1200 volts. They were first made in two sizes having an output of 12 and 30 amperes respectively. Incandescent lamps were made for 3 amperes in several sizes from 16 to 50 candlepower. These lamps were burned in series on the 1200-volt direct current system. Thus the 12-ampere machine had a capacity for four series circuits, each taking 3 amperes, the series circuits being connected in multiple across the 1200 volts. The number of lamps on each series circuit depended upon their size, as the voltage of each lamp was different for each size, being about 1½ volts per cp.

A popular size was the 32-candlepower unit, which therefore required about 45 volts and hence at 3 amperes consumed about 135 watts. Allowing 5 per cent loss in the wires of each circuit, there was therefore 1140 of the 1200 volts left for the lamps. Hence about 25 32-candlepower or 50 16-candlepower lamps could be put on each series circuit. Different sizes of lamps could also be put on the same circuit, the number depending upon the aggregate voltage of the lamps.

Inside the base was an arrangement by which the lamp was automatically short circuited when it burned out.]

A device was put in the base of each lamp to short circuit the lamp when it burned out so as to prevent all the other lamps on that circuit from going out. This device consisted of a piece of wire put inside the lamp bulb between the two ends of the filament. Connected to this wire was a very thin wire inside the base which held a piece of metal compressed against a spring. The spring was connected to one terminal of the base. Should the lamp burn out, current would jump from the filament to the wire in the bulb, and the current then flowed through the thin wire to the other terminal of the base. The thin wire was melted by the current, and the spring pushed the piece of metal up short circuiting the terminals of the base. This scheme was later simplified by omitting the wire, spring, etc., and substituting a piece of metal which was prevented from short circuiting the terminals of the base by a thin piece of paper. When the lamp burned out the entire 1200 volts was impressed across this piece of paper, puncturing it and so short circuiting the base terminals. Should one or more lamps go out on a circuit, the increase in current above the normal 3 amperes was prevented by an adjustable resistance, or an extra lot of lamps which could be turned on one at a time, connected to each circuit and located in the power station under the control of the operator. This system disappeared from use with the advent of the constant current transformer.

THE SHUNT BOX SYSTEM FOR SERIES INCANDESCENT LAMPS

Lamps were burned in series on a high voltage alternating current, and when a lamp burned out all the current then went through its “shunt box,” a reactance coil in multiple with each lamp.]

Soon after the commercial development of the alternating current constant potential system, a scheme was developed to permit the use of lamps in series on such circuits without the necessity for short circuiting a lamp should it burn out. A reactance, called a “shunt box” and consisting of a coil of wire wound on an iron core, was connected across each lamp. The shunt box consumed but little current while the lamp was burning. Should one lamp go out, the entire current would flow through its shunt box and so maintain the current approximately constant. It had the difficulty, however, that if several lamps went out, the current would be materially increased tending to burn out the remaining lamps on the circuit. This system also disappeared from use with the development of the constant current transformer.

THE ENCLOSED ARC LAMP

Up to 1893 the carbons of an arc lamp operated in the open air, so that they were rapidly consumed, lasting from eight to sixteen hours depending on their length and thickness. Louis B. Marks, an American, found that by placing a tight fitting globe about the arc, the life of the carbons was increased ten to twelve times. This was due to the restricted amount of oxygen of the air in the presence of the hot carbon tips and thus retarded their consumption. The amount of light was somewhat decreased, but this was more than offset by the lesser expense of trimming which also justified the use of more expensive better quality carbons. Satisfactory operation required that the arc voltage be increased to about 80 volts.

Enclosing the arc lengthened the life of the carbons, thereby greatly reducing the cost of maintenance.]

This lamp rapidly displaced the series open arc. An enclosed arc lamp for use on 110-volt constant potential circuits was also developed. A resistance was put in series with the arc for use on 110-volt direct current circuits, to act as a ballast in order to prevent the arc from taking too much current and also to use up the difference between the arc voltage (80) and the line voltage (110). On alternating current, a reactance was used in place of the resistance.

The efficiencies in lumens per watt of these arcs (with clear glassware), all of which have now disappeared from the market, were about as follows:

6.6 ampere 510 watt direct current (D.C.) series arc, 8¼ l-p-w. 5.0 ampere 550 watt direct current multiple (110-volt) arc, 4½ l-p-w. 7.5 ampere 540 watt alternating current (A.C.) multiple (110-volt) arc, 4¼ l-p-w.

Certain salts impregnated in the carbons produced a brilliantly luminous flame in the arc stream which enormously increased the efficiency of the lamp.]

By condensing the smoke from the arc in a cooling chamber it was practical to enclose the flame arc, thereby increasing the life of the carbons.]

The reason for the big difference in efficiency between the series and multiple direct-current arc is that in the latter a large amount of electrical energy (watts) is lost in the ballast resistance. While there is a considerable difference between the inherent efficiencies of the D. C. and A. C. arcs themselves, this difference is reduced in the multiple D. C. and A. C. arc lamps as more watts are lost in the resistance ballast of the multiple D. C. lamp than are lost in the reactance ballast of the multiple A. C. lamp.

This reactance gives the A. C. lamp what is called a “power-factor.” The product of the amperes (7.5) times the volts (110) does not give the true wattage (540) of the lamp, so that the actual volt-amperes (825) has to be multiplied by a power factor, in this case about 65 per cent, to obtain the actual power (watts) consumed. The reason is that the instantaneous varying values of the alternating current and pressure, if multiplied and averaged throughout the complete alternating cycle, do not equal the average amperes (measured by an ammeter) multiplied by the average voltage (measured by a volt-meter). That is, the maximum value of the current flowing (amperes) does not occur at the same instant that the maximum pressure (voltage) is on the circuit.

THE FLAME ARC LAMP

About 1844 Bunsen investigated the effect of introducing various chemicals in the carbon arc. Nothing was done, however, until Bremer, a German, experimented with various salts impregnated in the carbon electrodes. In 1898 he produced the so-called flame arc, which consisted of carbons impregnated with calcium fluoride. This gave a brilliant yellow light most of which came from the arc flame, and practically none from the carbon tips. The arc operated in the open air and produced smoke which condensed into a white powder.

The two carbons were inclined downward at about a 30-degree angle with each other, and were of small diameter but long, 18 to 30 inches, having a life of about 12 to 15 hours. The tips of the carbons projected through an inverted earthenware cup, called the “economizer,” the white powder condensing on this and acting not only as an excellent reflector but making a dead air space above the arc. The arc was maintained at the tips of the carbons by an electro-magnet whose magnetic field “blew” the arc down.

Two flame arc lamps were burned in series on 110-volt circuits. They consumed 550 watts each, giving an efficiency of about 35 lumens per watt on direct current. On alternating current the efficiency was about 30 l-p-w. By use of barium salts impregnated in the carbons, a white light was obtained, giving an efficiency of about 18 l-p-w on direct current and about 15½ on alternating current. These figures cover lamps equipped with clear glassware. Using strontium salts in the carbons, a red light was obtained at a considerably lower efficiency, such arcs on account of their color being used only to a limited extent for advertising purposes.

This converted alternating current of constant voltage into constant current, for use on series circuits.]

These arcs were remarkably efficient but their maintenance expense was high. Later, about 1908, enclosed flame arcs with vertical carbons were made which increased the life of the carbons, the smoke being condensed in cooling chambers. However, their maintenance expense was still high. They have now disappeared from the market, having been displaced by the very efficient gas-filled tungsten filament incandescent lamp which appeared in 1913.

THE CONSTANT CURRENT TRANSFORMER FOR SERIES CIRCUITS