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way, but the mean of measurements at all angles above and below the horizontal. This gives the true average candle-power of the arc in all directions.

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180 170 160 150 145

130 Fig. 266. Light Distribution, Direct Current Open Arc. 4. The maximum candle-power found by making observations in all directions to ascertain the greatest candle-power. This is usually found at an angle of 40 degrees below the horizontal.

The term nominal candle-power” is often employed in commercial work, meaning a value arbitrarily agreed upon to correspond to a certain consumption of energy at the arc. Thus an arc consuming 450 watts is assumed to have 2,000 nominal candlepower, and one of 300 watts to have 1,200 candle-power ; although these figures greatly exceed the true spherical candle-power, they may come somewhere near the maximum under favorable conditions. As a matter of fact, the relation between watts and candlepower is quite variable, as shown later.

Of the various candle-powers the mean spherical candle-power is the most absolute and important, but unfortunately is the most difficult to determine. The mean hemispherical candle-power may be properly considered where illumination is required in one direction only, as in street-lighting, where the light is thrown outward and downward. The mean horizontal candle-power is of no special importance, the light given off in a horizontal plane being of no more value than that emitted in any other direction ; but it is quite easily measured, and often approximates closely to the mean spherical. This is, however, not always the case, and cannot be generally relied upon. The formula due to Gerard is sometimes employed to find the mean hemispherical candle-power by simply measuring the horizontal and maximum candle-power, the expression being :

Mean hemispherical candle-power = { mean horizontal + & maximum. This gives only approximate results, but can be used to save the trouble of making a large number of photometric determinations.

Relation of Light to Current. — Investigating the factors influencing the amount of light given off by the arc, Violle found that the quantity emitted by a unit surface of the crater was the same, whether the arc current be 10 or 1,000 amperes. This was to have been expected, since the crater cannot be heated beyond its point of volatilization and still remain in the solid state. The light is therefore roughly proportional to the area of the crater. The spherical candle-power is also approximately proportional to the total number of watts utilized in the arc, but is of course affected by anything that varies the efficiency.

Efficiency. – By the efficiency of the arc is meant the ratio of the luminous flux to the total heat and light radiation. Anything that tends to dissipate the energy at any place other than the crater of the positive carbon diminishes the efficiency.

The arc is the most efficient source of illumination known. The most generally accepted value for its efficiency is 13 per cent. The corresponding figures for the other sources of light, are for the candle 14 per cent, the gas-flame 1 per cent, the Welsbach light 24 per cent, and the magnesium light 12 per cent.

The efficiency of the incandescent electric light is about 5 per cent. These values are comparative but probably too high. Among the causes that modify the efficiency of the arc are these :

1. The structure, density, and composition of the carbons. These affect both the volatilization point, and hence the temperature, as well as the thermal conductivity upon which depends the amount of heat conducted away by the carbons, which is lost energy. Purity, softness, and evenness are desirable.

2. The cross-section of the carbons. Large carbons conduct and radiate more heat than small ones for equal currents, hence for a given current the efficiency diminishes about inversely as the diameter of the carbon increases.

3. The existence of a soft core reduces the temperature of the crater, and tends to lower the efficiency.

Current and Voltage. — The division of the watts at the arc into current and voltage is extremely important, depending on various factors not yet understood. Carhart found that with 450 watts at the arc, made up of 10 amperes and 45 volts, he got a maximum candle-power of 450; while with the same number of watts in 8.4 amperes and 54 volts he obtained 900 maximum candlepower, just twice as much. Blondel, however, finds the luminous flux greatest usually below 45 volts. The discrepancies are probably due to differences in size and quality of carbons, because there is naturally a certain current density for each carbon, which gives the best results.

Commercial Values of Voltage and Current. — The value of 45 to 47 volts at the arc, reached after years of commercial experience, is probably the best. At this point the efficiency is high, and the conditions are about half-way between hissing and the flaming points. At this voltage, too, as the curves show, the voltage at the arc is only slightly affected by Auctuations in the current strength. Ordinary current values range from 6.5 to 10

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amperes for long arcs in air. Greater voltage is inadvisable, as it reduces the number of arcs that can be placed in one series, increases carbon consumption, tends to produce Aaming, and introduces too much energy in a single-light unit.

Less current than 6.5 amperes gives too little light for a unit, used under conditions suitable for a series circuit. On constant potential systems small open arcs of low current have been attempted, but without much success. The low current arc has a large cooling surface for the energy used. If for some of the reasons given later in detail, the current in the arc falls, the proportion of the cooling surface to the energy is greatly increased, in fact to such an extent that the arc flickers violently or is put out by the chilling effect. Where arcs are inclosed in heat-retaining bulbs, the current may be greatly reduced before this effect takes place.

Composition of Light. — The composition of the light of the arc has been determined by Meyer to be as follows, where the intensity of the yellow light is expressed by unity. Red and orange 2.09, yellow 1.00, green 0.99, blue 0.87, indigo 1.03, and violet 1.21. Taking the intensity of red as 100, Abney gives for direct sunlight ; Red 100, green 193, violet 228; while for arc light his figures are: Red 100, green 203, and violet 250. For gas-light he found the values to be : Red 109, green 95, and violet 27.

The composition of the light of the arc may be, however, greatly changed by the hardness of the carbon, the material of the core, and by the current and voltage. Hardness usually determines the maximum temperature of the crater, while the current and voltage alter the proportions of the light fluxes coming from the yellow crater and from the violet arc stream. The vapor of the core acts to color the light as well as to determine the volatilization point of the crater.

The color of the arc light approaches very nearly to sunlight, and it has the remarkable quality of producing a similar sunburn. Great caution is necessary on this account in avoiding exposure of the eyes to the arc at close range, otherwise a painful sunburn of the eye, producing a tedious inflammation, is apt to result. This can be avoided only by protecting the eyes on all sides with leather goggles fitted with smoked glasses.

Short Arcs. — In the earlier days of arc lighting, so-called “short ” arcs were employed, taking 18 or 20 amperes at about 25 volts with an apparent arc length of t to do of an inch. The object of such a short arc was to increase the number of arcs that could be operated in series by a given voltage. Owing to the large current, the carbon consumption was high and the line drop excessive, so that these causes, combined with the frying sound and delicacy of regulation required, have brought about their abandonment. The long arc now employed in open arc lamps has roughly half the current and double the voltage. The carbon carried off the positive by electrolytic action is of course only half as great ; but the longer arc affords more opportunity for the air to oxidize the carbon, so that the carbon life in the long arc is not proportionately increased,

Unstable Arcs. – Between the condition of a short arc and a long arc there lies a zone of instability for which the probable analogy is the concussive boiling of water on the dividing line between the stage of rapid evaporation and quiet ebullition. After the long ‘arc is reached, it is necessary, in order to maintain a fixed voltage at the arc, to increase the distance between the electrodes as the current increases. The difference in the relative life of the carbons in the long arc and in the short arc is quite marked. In the short are the positive wastes away rapidly owing to the heavy current, while the deposition of carbon on the negative is almost sufficient to prevent waste; besides which the air currents have not sufficient room to form in the short arc.

Blowing Out of the Arc. — A peculiar feature of all arcs is their liability to be blown by a strong gust of air unless fed by a constant current machine which cannot fail to maintain the current. A magnet will also blow out an arc if the pole is brought sufficiently close. This magnetic action, as previously stated, causes the bow shape characteristic of the arc, and in the case of an alternating current causes the arc stream to rapidly bend from side to side across the earth's line of force. The blowing-out tendency of the magnet is frequently employed to direct the arc upon metals for the purpose of melting or heating them, as well as in various magnetic blow-out devices, in which the blowing-out effect rapidly extinguishes an arc formed between two contacts liable to be melted by the continued action of the current.

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