Arc on Constant Potential Circuits. When arcs can be run in series on circuits furnished by constant current machines they have the great advantage of having the current maintained as long as the arc is not cut out of the circuit, so that irregularities in the arc or mechanism produce only a variation in the intensity of the light, but the illumination, good or bad, is always maintained.

It is, however, often desirable to run arc lamps on constant potential mains at the usual 110 or 220 volts pressure, where the current is no longer constant unless a device is introduced to make it so. Every constant potential arc lamp has a mechanism of this kind contained in the case whose function is to separate the carbons when the current is too high, and bring them together when it is too low. If well made and adjusted, such a regulator may respond to current variations of five per cent either side of the value at which it is set, which might be expected to maintain a practically constant current. Such, however, is not the case. An arc whose carbons are fed by a mechanism of this kind, if connected directly to a constant potential main, will behave in the most erratic manner, even if it be started by hand regulation, and allowed to warm up before the test. The mechanism adjusted to respond to five per cent current variation now utterly fails to keep the current anywhere nearly constant, and the arc is very unsteady. This is true even if the voltage of the mains corresponds to the voltage desired at the arc.

The reason for it lies in the fact, shown by previous curves, that the resistance of an arc decreases as the current increases, which results in a tendency for the current to become almost infinite if constant potential is maintained across its terminals. Similarly, if the current begins to decrease, and so lessen the cross-section of the arc, the resistance rises and further chokes off the current, until the arc goes out. The arc when directly connected to constant potential mains is therefore in a state of unstable equilibrium, in which the current tends to drop to zero or surge toward infinity. This action, depending as it does only on the instantaneous cross-section of carbon vapor at any moment, is itself instantaneous. The mere inertia of a mechanism retards it so much that the arc is out before the regulator has perceptibly moved. The means used to counteract the instability of the arc must operate as fast as the current change can take place. Such an auxiliary

regulator, although an inefficient one, is made by the simple expedient of inserting a series resistance between either side of the arc and the mains.

The mechanism keeps the average current constant by increasing or decreasing the length of the arc. The resistance overcomes the tendency toward rapid fluctuation by automatically and instantaneously raising or lowering the voltage across the arc gaps as required. As an illustration, assume an arc to be connected to constant potential mains of 40 volts, and the regulating magnet to be wound to pass 10 amperes with a normal length of arc. If for some reason the current suddenly drops to 8 amperes the resistance of the arc rises, though its length may not have changed, and it requires more than 40 volts to bring the current back to 10 amperes and maintain it, therefore the arc goes out. If now we connect the same lamp in series with a one-ohm resistance to a 50-volt circuit, the current again 10 amperes, the lamp will have 40 volts at the terminals as before. Now let the current fall to 8 amperes. The drop through the resistance is only 8 volts, and we have 50 8 42 volts at the arc, which is sufficient to force more than 8 amperes through it, and so restore the current to the normal 10-ampere value. The resistance in series is sufficient when the rise of voltage at the arc, caused by less drop in the resistance, suffices to force the original current through it, in spite of its diminished cross-section. If too little resistance is used, a given decrease of current will not produce sufficient rise of voltage at the arc to maintain it, and it goes out. If too much resistance is employed, the rise of voltage at the arc is excessive for changes of current too small to move the mechanism, and the lamp tends to allow the current to surge beyond its proper limits. This regulating or steadying action of resistance is of course instantaneous, as it depends on electrical changes and not on inertia or mechanical motion. To a certain extent selfinduction may have a similar tendency to raise the voltage at the point of rupture or increase of resistance in an electric circuit.

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No Resistance in Series with Series Lamps. On series- or high-tension circuits resistance is not required, because the current is maintained, whatever the changes in the arc, by the inherent regulation of the dynamo. It is impossible, therefore, for the current to fail while the machine is in operation.

Constant Potential Lamps, Two in Series. The usual voltage of constant potential circuits being double that required for one open arc lamp with its resistance, being 110 volts or thereabouts, open arcs on these circuits are commonly connected two in series. If only one lamp is used, and the remaining excess of 60 or 70 volts taken up by resistance, the regulating action of the latter tends to make the light vary up and down slowly, as explained above. On circuits with a higher voltage than 110, more lamps are run in series, as for instance, 10 lamps in a string across a 500volt circuit. From what has been said, it will be evident that both current and voltage vary somewhat in the arc on constant potential or incandescent circuits, while voltage alone varies on a good series circuit. A carbon that will tend to produce a steady light, such as a cored carbon, is, therefore, advisable for constant potential lamps. Again, these lamps are commonly used for interior illumination, so that a better grade of carbon and one that produces a softer yellowish light is desirable.

Troubles in the Arc Proper. The chief troubles found in direct current arcs not caused by the mechanism are these:

Flaming, from too long an arc, or impure carbons, or half-baked carbons containing unexpelled gases. The flame usually runs up the side of the positive, accompanied by a drop in resistance and loss of light.

Hissing, due to too short an arc or too vigorous vaporization or too coarse-grained carbons. This is attended with loss of light, low resistance, and an objectionable hissing noise.

Sputtering, from impurities in the carbon, or loose-grained

carbons.

Whistling, occasioned, as in a Chicago installation, by electrostatic induction between the underground conductors and their metal sheaths. These current vibrations reproduce themselves in variations in the volume of the arc stream, producing a shrill noise.

Traveling of the arc around the carbons producing unequal illumination and flicker. This arises from the tendency of the arc to continually seek the path of least resistance, which wandering increases with the area of the carbon over which the arc may travel, in other words, the area of the end. This may be remedied by the use of smaller carbons or of cored carbons, in

which latter case the soft core vaporizes first, and the arc is confined to the inner surface of the crater thus produced.

Bucking of arcs connected in series, owing to the mechanism of all the lamps endeavoring to correct a change of current due to the improper working of one particular lamp. This is frequently very marked on incandescent circuits, where only two lamps are in series. If one sticks, it frequently consumes all of the energy, leaving the other nearly dark. To overcome this, both proper design of the mechanism and proper adjustment are required.

Alternating Arcs. When arcs are fed by alternating current the arc is no longer a continuous flame, but is lighted and extinguished at every reversal of the current. When these follow one another faster than 100 per second, corresponding to a frequency of 50 periods, the flicker is not apparent to most eyes. Owing to the reversal of the current each carbon acts as a positive at every other alternation. There is, therefore, no crater, both carbons remaining pointed, but the upper one wastes away 8 or 10 per cent faster than the negative, due to receiving the ascending heat.

Voltage and Current. Under commercial conditions, using cored carbons, open alternating arcs consume about 15 amperes at 30 to 35 volts. This would seem to be unaccountably less than that required for a continuous current arc using the same carbons ; but it must be borne in mind that an effective alternating voltage of 35 has a maximum potential of about 50 at the top of the wave.

Function of Core and Object of Heavy Current. When the carbons are separated, it would appear that the first extinguishment of the arc as the current passed through zero would put out the light; but a continuous path is provided for the current by the bridge of incandescent carbon vapor that persists until the voltage acquires a substantial value in an opposite direction. To obtain this effect the current used in alternating arcs must be larger than in continuous current arcs, and the carbons are always cored, to insure a sufficient supply of carbon vapor.

Power Factor. If the current and E.M.F. are in phase the power at the arc in watts is the product of the volts and amperes. Steinmetz, however, has shown that since the apparent resistance varies with the current there must be a lag of current behind the electromotive force. Experiments show that the true power in

the open alternating arc is about 85 per cent of the apparent

watts.

Wave Form. The efficiency of the alternating arc increases slightly with the number of alternations, and is considerably affected by the form of current wave, which is largely determined by the shape of the wave of E.M.F.; a flat-top wave producing a higher efficiency than a peaked one. This is because the flat-top wave creates less interruption in the flow of current than the sharp pointed wave, the latter allowing the carbon a considerable interval between maximum values in which to cool off.

Hum. A peculiarity of the alternating arc is its hum, corresponding in pitch to the alternations. It arises from the expansions and contractions of the arc stream with the current, producing corresponding vibrations of the adjacent air. This has nothing to do with the hissing sound that may occur from very short arcs as with the continuous current. When the alternating arc hisses, the voltage falls, but less abruptly than with the continuous arc, while the current lag is increased until the true watts are only 75 per cent of the apparent watts. The light emission of the alternating arc at any instant lags a little behind the curve of instantaneous value of the real watts. It never passes through zero, owing to the retention of heat by the carbons.

Efficiency. With the same energy and carbons the mean spherical candlepower of the alternating open arc is about one-half that of the continuous current open arc.

Vertical Axis of Carbons

10

20

50

60

70

80

Horizontal Plane

90

H

--100

-110

120

-130

140

150

160

Distribution. The distribution of light is nearly equal above and below the horizontal, as shown by Fig. 267. Therefore the light going upward, which is nearly one-half, would be wasted were it not for the white reflector usually employed immediately above the arc to throw the light down.

Fig. 267. Light Distribution, Alternating Current Open Arc.

Focussing Mechanism. The more equal consumption of upper and lower carbons in alternating lamps necessitates a mechanism