The consumption of the electrodes in an arc lamp depends on oxidation and volatilization. With carbon electrodes, in general, the consumption of the positive carbon is about twice that of the negative, because of the greater heating effect of the large positive crater. The consumption of both carbons increases with the current; while the lower carbon is not particularly affected, the upper carbon burns more rapidly with increasing arc voltage. Enclosure reduces the rate of con sumption of the carbons.
As the sun gives a colour tem perature of about 6,500° K., and the positive-carbon arc a colour temperature of about 3,700° K., it can readily be seen why the light of the arc is more yellow in colour. The A.C. carbon arc is more yellow than the D.C. arc because of the lower average tem perature of the craters, as each crater is a positive only half of the time. The addition of chemicals to the arc greatly extends the range of colours obtainable. In Table II. the best materials for producing various colours are given.
The conductivity of arc vapours increases very rapidly with the temperature. This is one of the factors that tend to force the current toward the centre of the arc and thereby further increase its temperature and con ductivity. The electrical conductivity of elements in the arc depends largely on the ease with which they give off electrons. In carbon arcs, it is common to use alkali salts, which are called arc supporters, for the purpose of improving the arc steadiness and the ease of its control.
The relation of arc voltage to arc length is that of a linear function. The minimum voltage below which the arc is inoperative is called the "starting arc voltage." For the carbon arc this is about 4o volts, for the flame arc about 20 volts. The great increase in the length of the flame arc over the pure carbon arc makes it less sensitive to mechanical deficiencies in the drawings of the arc.
With the pure carbon arc, on either direct or alternating current, the light increases with the current as the 1.4 power. With flame and magnetite arcs, the visual light increases with the current to the 1.6 power. With
flame arcs of the tungsten type, data indicate that the light in creases as the square of the current.
The ideal circuit for arc lamps is the constant-current or series circuit, in which all the power can be utilized in the arcs themselves. In American cities, series circuits operating with several thousand volts are common, while in Europe the multiple arrangement of arc lamps predominates. Low voltage of the arc has the advantage, sometimes of great practical value, of increasing the number of lamps per series circuit of fixed voltage.
The multiple arrangement of arcs for street lighting has been extensively used in Europe with flame arcs but has not been developed in America. The objection to the multiple arrangement is 'the loss of energy in the ballast of the arc lamps.
Since the arc is characterized by a marked decrease of its resistance with an increase in current, the arc voltage also decreases. Hence, the maintenance of a stable arc depends very much upon the kind and amount of external re sistance in series with it. This feature of the arc is an advantage from the standpoint of wide regulation but it is a disadvantage from the standpoint of the power lost externally to the arc in its current-regulating apparatus. On a multiple circuit the arc ballast may consume i to 50% of the total electrical energy used, while on a series circuit this loss is avoided because constant current is supplied by the generating apparatus.
Arc Ballast.—On direct current the arc ballast is usually a resistance wire with a low temperature coefficient of resistance, so that, as the resistance heats up, the arc regulation will not change. If the resistance is arranged on a magnetic core, the steadiness of the arc is improved. On alternating-current circuits, reactance may be, and usually is, employed instead of the steady ing resistance, and the waste of power is thereby greatly de creased. An important aspect in determining the kind of ballast which should be used in an arc is its effect on the efficiency of the arc itself. In the case of the carbon arc the use of a reactance decreases slightly the lumens per arc watt. The reverse results with the flame arc.