Arc Current
in
Amperes.		 Height above
Road
in Feet.		 Distance
apart
in Feet.		 Horizontal Illumination
in Candle-Feet.
Maximum. Minimum.
10 20 120 1.85 0.12
10 25 120 1.17 0.15
10 40 120 0.5  0.28
 6.8 20 90  1.1  0.21
 6.8 40 120 0.3  0.17

As regards distance apart, a very usual practice is to place the lamps at spaces equal to six to ten times their height above the road surface. Blondel (Electrician, 35, p. 846) gives the following rule for the height (h) of the arc to afford the maximum illumination at a distance (d) from the foot of the lamp-post, the continuous current arc being employed:—

For naked arch = 0.95 d.
 ”  arc in rough glass globeh = 0.85 d.
 ”   ”   opaline globh =  ”
 ”   ”   opal globeh = 0.5 d.
 ”   ”   holophane globeh = 0.5 d.

These figures show that the distribution of light on the horizontal surface is greatly affected by the nature of the enclosing globe. For street illumination naked arcs, although sometimes employed in works and factory yards, are entirely unsuitable, since the result produced on the eye by the bright point of light is to paralyse a part of the retina and contract the pupil, hence rendering the eye less sensitive when directed on feebly illuminated surfaces. Accordingly, diffusing globes have to be employed. It is usual to place the arc in the interior of a globe of from 12 to 18 in. in diameter. This may be made of ground glass, opal glass, or be a dioptric globe such as the holophane. The former two are strongly absorptive, as may be seen from the results of experiments by Guthrie and Redhead. The following table shows the astonishing loss of light due to the use of opal globes:—

Naked
Arc.		Arc
in Clear
Globe.		Arc in
Rough
Glass
Globe.		Arc
in Opal
Globe.
Mean spherical c.p.319235160144
Mean hemispherical c.p.450326215138
Percentage value of transmitted light100 53 23 19
Percentage absorption 0 47 77 81

By using Trotter’s, Fredureau’s or the holophane globe, the light may be so diffused that the whole globe appears uniformly luminous, and yet not more than 20% of the light is absorbed. Taking the absorption of an ordinary opal globe into account, a 500-watt arc does not usually give more than 500 c.p. as a maximum candle-power. Even with a naked 500-watt arc the mean spherical candle-power is not generally more than 500 c.p., or at the rate of 1 c.p. per watt. The maximum candle-power for a given electrical power is, however, greatly dependent on the current density in the carbon, and to obtain the highest current density the carbons must be as thin as possible. (See T. Hesketh, “Notes on the Electric Arc,” Electrician, 39, p. 707.)

For the efficiency of arcs of various kinds, expressed by the mean hemispherical candle power per ampere and per watt expended in the arc, the following figures were given by L. Andrews (“Long-flame Arc Lamps,” Journal Inst. Elec. Eng., 1906, 37, p. 4).

Candle-power
per ampere.		Candle-power
per watt.
Ordinary open carbon arc 821.54
Enclosed carbon arc 550.77
Chemical carbon or flame arc2595.80
High voltage inclined carbon arc2002.24

It will be seen that the flame arc lamp has an enormous advantage over other types in the light yielded for a given electric power consumption.

The practical employment of the electric arc as a means of illumination is dependent upon mechanism for automatically keeping two suitable carbon rods in the proper position, and moving them so as to enable a steady arc to be Arc lamp mechanism. maintained. Means must be provided for holding the carbons in line, and when the lamp is not in operation they must fall together, or come together when the current is switched on, so as to start the arc. As soon as the current passes, they must be moved slightly apart, and gripped in position immediately the current reaches its right value, being moved farther apart if the current increases in strength, and brought together if it decreases. Moreover, it must be possible for a considerable length of carbon to be fed through the lamp as required.