inside the iron mast. (The pair of incandescent lamps halfway
down the standard are for use in the middle of the night,
when the arc lamp would give more light than is
required; they are lighted by an automatic
switch whenever the arc is extinguished.) The
lamp itself is generally enclosed in an opalescent
spherical globe, which is woven over with wire-netting
so that in case of fracture the pieces
may not cause damage. The necessary trimming,
that is, the replacement of carbons, is effected
either by lowering the lamp or, preferably, by
carrying round a portable ladder enabling the
trimmer to reach it. For the purpose of public
illumination it is very usual to employ a lamp
taking 10 amperes, and therefore absorbing
about 500 watts. Such a lamp is called a 500–watt
arc lamp, and it is found that a satisfactory
illumination is given for most street purposes
by placing 500–watt arc lamps at distances
varying from 40 to 100 yds., and at a height
of 20 to 25 ft. above the roadway. The maximum
candle-power of a 500–watt arc enclosed
in a roughened or ground-glass globe will not
exceed 1500 candles, and that of a 6.8–ampere
arc (continuous) about 900 candles. If, however,
the arc is an enclosed arc with double
globes, the absorption of light would reduce the
effective maximum to about 200 c.p. and 120
c.p. respectively. When arc lamps are placed
in public thoroughfares not less than 40 yds.
apart, the illumination anywhere on the street
surface is practically determined by the two
nearest ones. Hence the total illumination at
any point may be obtained by adding together
the illuminations due to each arc separately.
Given the photometric polar curves or illuminating-power
curves of each arc taken outside the
shade or globe, we can therefore draw a curve
representing the resultant illumination on the
horizontal surface. It is obvious that the higher
the lamps are
placed, the more
uniform is the
street surface illumination,
but
the less its average
value; thus
two 10-ampere
arcs placed on
masts 20 ft. above
the road surface
and 100 ft. apart
will give a maximum illumination of about 1.1 and a minimum of
about 0.15 candle-feet in the interspace (fig 12). If the lamps are
raised on 40-ft. posts the maximum illumination will fall to 0.3, and
the minimum will rise to 0.2. For this reason masts have been employed
as high as 90 ft. In docks and railway yards high masts (50
ft.) are an advantage, because the strong contrasts due to shadows
of trucks, carts, &c., then become less marked, but for street
illumination they should not exceed 30 to 35 ft. in height. Taking
the case of 10-ampere and 6.8–ampere arc lamps in ordinary
opal shades, the following figures have been given by Trotter as
indicating the nature of the resultant horizontal illumination:—
| 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 arc | h = 0.95 d. |
| ” arc in rough glass globe | h = 0.85 d. |
| ” ” opaline glob | h = ” |
| ” ” opal globe | h = 0.5 d. |
| ” ” holophane globe | h = 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. | 319 | 235 | 160 | 144 |
| Mean hemispherical c.p. | 450 | 326 | 215 | 138 |
| Percentage value of transmitted light | 100 | 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 | 82 | 1.54 |
| Enclosed carbon arc | 55 | 0.77 |
| Chemical carbon or flame arc | 259 | 5.80 |
| High voltage inclined carbon arc | 200 | 2.24 |
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 maintained. Means must be provided for holding Arc lamp mechanism. 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
