where [Sigma](Iab) is the sum of all the light actually falling on
the sphere surface, then
1
I0 = -- [Sigma](Iab)
2R
[Sigma](Iab)
= ------------ I_(max)
2RI_(max)
where I_(max) stands for the maximum candle-power of the arc. If,
then, we set off at b a line bH perpendicular to DE and in length
proportional to the candle-power of the arc in the direction AP, and
carry out the same construction for a number of different observed
candle-power readings at known angles above and below the horizon, the
summits of all ordinates such as bH will define a curve DHE. The mean
spherical candle-power of the arc is equal to the product of the
maximum candle-power (I_(max)), and a fraction equal to the ratio of
the area included by the curve DHE to its circumscribing rectangle
DFGE. The area of the curve DHE multiplied by 2[pi]/R gives us the
_total flux of light_ from the arc.
Owing to the inequality in the distribution of light from an electric
arc, it is impossible to define the illuminating power by a single
number in any other way than by stating the mean spherical
candle-power. All such commonly used expressions as "an arc lamp of
2000 candle-power" are, therefore, perfectly meaningless.
Photometry of arc.
The photometry of arc lamps presents particular difficulties, owing to
the great difference in quality between the light radiated by the arc
and that given by any of the ordinarily used light standards. (For
standards of light and photometers, see PHOTOMETER.) All photometry
depends on the principle that if we illuminate two white surfaces
respectively and exclusively by two separate sources of light, we can by
moving the lights bring the two surfaces into such a condition that
their _illumination_ or _brightness_ is the same without regard to any
small colour difference. The quantitative measurement depends on the
fact that the illumination produced upon a surface by a source of light
is inversely as the square of the distance of the source. The trained
eye is capable of making a comparison between two surfaces illuminated
by different sources of light, and pronouncing upon their equality or
otherwise in respect of brightness, apart from a certain colour
difference; but for this to be done with accuracy the two illuminated
surfaces, the brightness of which is to be compared, must be absolutely
contiguous and not separated by any harsh line. The process of comparing
the light from the arc directly with that of a candle or other similar
flame standard is exceedingly difficult, owing to the much greater
proportion and intensity of the violet rays in the arc. The most
convenient practical working standard is an incandescent lamp run at a
high temperature, that is, at an efficiency of about 2½ watts per
candle. If it has a sufficiently large bulb, and has been _aged_ by
being worked for some time previously, it will at a constant voltage
preserve a constancy in illuminating power sufficiently long to make the
necessary photometric comparisons, and it can itself be compared at
intervals with another standard incandescent lamp, or with a flame
standard such as a Harcourt pentane lamp.
[Illustration: FIG. 10.]
In measuring the candle-power of arc lamps it is necessary to have
some arrangement by which the brightness of the rays proceeding from
the arc in different directions can be measured. For this purpose the
lamp may be suspended from a support, and a radial arm arranged to
carry three mirrors, so that in whatever position the arm may be
placed, it gathers light proceeding at one particular angle above or
below the horizon from the arc, and this light is reflected out
finally in a constant horizontal direction. An easily-arranged
experiment enables us to determine the constant loss of light by
reflection at all the mirrors, since that reflection always takes
place at 45°. The ray thrown out horizontally can then be compared
with that from any standard source of light by means of a fixed
photometer, and by sweeping round the radial arm the photometric or
illuminating curve of the arc lamp can be obtained. From this we can
at once determine the nature of the illumination which would be
produced on a horizontal surface if the arc lamp were suspended at a
given distance above it. Let A (fig. 10) be an arc lamp placed at a
height h( = AB) above a horizontal plane. Let ACD be the illuminating
power curve of the arc, and hence AC the candle-power in a direction
AP. The illumination (I) or brightness on the horizontal plane at P is
equal to
AC cos APM/(AP)² = FC/(h² + x²), where x = BP.
Hence if the candle-power curve of the arc and its height above the
surface are known, we can describe a curve BMN, whose ordinate PM will
denote the brightness on the horizontal surface at any point P. It is
easily seen that this ordinate must have a maximum value at some
point. This brightness is best expressed in _candle-feet_, taking the
unit of illumination to be that given by a standard candle on a white
surface at a distance of 1 ft. If any number of arc lamps are placed
above a horizontal plane, the brightness at any point can be
calculated by adding together the illuminations due to each
respectively.
The process of delineating the photometric or polar curve of intensity
for an arc lamp is somewhat tedious, but the curve has the advantage
of showing exactly the distribution of light in different directions.
When only the mean spherical or mean hemispherical candle-power is
required the process can be shortened by employing an integrating
photometer such as that of C. P. Matthews (_Trans. Amer. Inst. Elec.
Eng._, 1903, 19, p. 1465), or the lumen-meter of A. E. Blondel which
enables us to determine at one observation the total flux of light
from the arc and therefore the mean spherical candle-power per watt.
[Illustration: FIG. 11.]
[Illustration: FIG. 12.]
Street arc lighting.
In the use of arc lamps for street and public lighting, the question of
the distribution of light on the horizontal surface is all-important. In
order that street surfaces may be well lighted, the minimum illumination
should not fall below 0.1 candle-foot, and in general, in well-lighted
streets, the maximum illumination will be 1 candle-foot and upwards. By
means of an illumination photometer, such as that of W. H. Preece and A.
P. Trotter, it is easy to measure the illumination in candle-feet at any
point in a street surface, and to plot out a number of contour lines of
equal illumination. Experience has shown that to obtain satisfactory
results the lamps must be placed on a high mast 20 or 25 ft. above the
roadway surface. These posts are now generally made of cast iron in
various ornamental forms (fig. 11), the necessary conductors for
conveying the current up to the lamp being taken 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:--
+-----------+------------+---------+------------------------+
| | | | Horizontal Illumination|
|Arc Current|Height above| Distance| in Candle-Feet. |
| in | Road | apart +-----------+------------+
| Amperes. | in Feet. | in 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:--
+--------------------------------------+-----+--------+-------+-------+
| | | Arc | Arc in| Arc |
| |Naked|in Clear| Rough |in Opal|
| | Arc.| Globe. | Glass | Globe.|
| | | | 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 Candle-power
per ampere. 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.
Arc lamp mechanism.
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 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.
[Illustration: FIG. 13]
[Illustration: FIG. 14]
One early devised form of arc-lamp mechanism was a system of clockwork
driven by a spring or weight, which was started and stopped by the
action of an electromagnet; in modern lighthouse lamps a similar
mechanism is still employed. W. E. Staite (1847), J. B. L. Foucault
(1849), V. L. M. Serrin (1857), J. Duboscq (1858), and a host of later
inventors, devised numerous forms of mechanical and clockwork lamps.
The modern self-regulating type may be said to have been initiated in
1878 by the differential lamp of F. von Hefner-Alteneck, and the
clutch lamp of C. F. Brush. The general principle of the former may be
explained as follows: There are two solenoids, placed one above the
other. The lower one, of thick wire, is in series with the two carbon
rods forming the arc, and is hence called the _series coil_. Above
this there is placed another solenoid of fine wire, which is called
the _shunt coil_. Suppose an iron rod to be placed so as to be partly
in one coil and partly in another; then when the coils are traversed
by currents, the iron core will be acted upon by forces tending to
pull it into these solenoids. If the iron core be attached to one end
of a lever, the other end of which carries the upper carbon, it will
be seen that if the carbons are in contact and the current is switched
on, the series coil alone will be traversed by the current, and its
magnetic action will draw down the iron core, and therefore pull the
carbons apart and strike the arc. The moment the carbons separate,
there will be a difference of potential between them, and the shunt
coil will then come into action, and will act on the core so as to
draw the carbons together. Hence the two solenoids act in opposition
to each other, one increasing and the other diminishing the length of
the arc, and maintaining the carbons in the proper position. In the
lamp of this type the upper carbon is in reality attached to a rod
having a side-rack gearing, with a train of wheels governed by a
pendulum. The action of the series coil on the mechanism is to first
lock or stop the train, and then lift it as a whole slightly. This
strikes the arc. When the arc is too long, the series coil lowers the
gear and finally releases the upper carbon, so that it can run down by
its own weight. The principle of a shunt and series coil operating on
an iron core in opposition is the basis of the mechanism of a number
of arc lamps. Thus the lamp invented by F. Krizik and L. Piette,
called from its place of origin the Pilsen lamp, comprises an iron
core made in the shape of a double cone or spindle (fig. 13), which is
so arranged in a brass tube that it can move into or out of a shunt
and series coil, wound the one with fine and the other with thick
insulated wire, and hence regulate the position of the carbon attached
to it. The movement of this core is made to feed the carbons directly
without the intervention of any clockwork, as in the case of the
Hefner-Alteneck lamp. In the clutch-lamp mechanism the lower carbon is
fixed, and the upper carbon rests upon it by its own weight and that
of its holder. The latter consists of a long rod passing through
guides, and is embraced somewhere by a ring capable of being tilted or
lifted by a finger attached to the armature of an electromagnet the
coils of which are in series with the arc. When the current passes
through the magnet it attracts the armature, and by tilting the ring
lifts the upper carbon-holder and hence strikes the arc. If the
current diminishes in value, the upper carbon drops a little by its
own weight, and the feed of the lamp is thus effected by a series of
small lifts and drops of the upper carbon (fig. 14). Another element
sometimes employed in arc-lamp mechanism is the brake-wheel regulator.
This is a feature of one form of the Brockie and of the
Crompton-Pochin lamps. In these the movement of the carbons is
effected by a cord or chain which passes over a wheel, or by a rack
geared with the brake wheel. When no current is passing through the
lamp, the wheel is free to move, and the carbons fall together; but
when the current is switched on, the chain or cord passing over the
brake wheel, or the brake wheel itself is gripped in some way, and at
the same time the brake wheel is lifted so that the arc is struck.
Although countless forms of self-regulating device have been invented
for arc lamps, nothing has survived the test of time so well as the
typical mechanisms which work with carbon rods in one line, one or both
rods being moved by a controlling apparatus as required. The early forms
of semi-incandescent arc lamp, such as those of R. Werdermann and
others, have dropped out of existence. These were not really true arc
lamps, the light being produced by the incandescence of the extremity of
a thin carbon rod pressed against a larger rod or block. The once famous
Jablochkoff candle, invented in 1876, consisted of two carbon rods about
4 mm. in diameter, placed parallel to each other and separated by a
partition of kaolin, steatite or other refractory non-conductor.
Alternating currents were employed, and the candle was set in operation
by a match or starter of high-resistance carbon paste which connected
the tips of the rods. When this burned off, a true arc was formed
between the parallel carbons, the separator volatilizing as the carbons
burned away. Although much ingenuity was expended on this system of
lighting between 1877 and 1881, it no longer exists. One cause of its
disappearance was its relative inefficiency in light-giving power
compared with other forms of carbon arc taking the same amount of power,
and a second equally important reason was the waste in carbons. If the
arc of the electric candle was accidentally blown out, no means of
relighting existed; hence the great waste in half-burnt candles. H.
Wilde, J. C. Jamin, J. Rapieff and others endeavoured to provide a
remedy, but without success.
It is impossible to give here detailed descriptions of a fraction of
the arc-lamp mechanisms devised, and it must suffice to indicate the
broad distinctions between various types. (1) Arc lamps may be either
_continuous-current_ or _alternating-current_ lamps. For outdoor
public illumination the former are greatly preferable, as owing to the
form of the illuminating power-curve they send the light down on the
road surface, provided the upper carbon is the positive one. For
indoor, public room or factory lighting, _inverted arc_ lamps are
sometimes employed. In this case the positive carbon is the lower one,
and the lamp is carried in an inverted metallic reflector shield, so
that the light is chiefly thrown up on the ceiling, whence it is
diffused all round. The alternating-current arc is not only less
efficient in mean spherical candle-power per watt of electric power
absorbed, but its distribution of light is disadvantageous for street
purposes. Hence when arc lamps have to be worked off an
alternating-current circuit for public lighting it is now usual to
make use of a _rectifier_, which rectifies the alternating current
into an unidirectional though pulsating current. (2.) Arc lamps may be
also classified, as above described, into _open_ or _enclosed arcs_.
The enclosed arc can be made to burn for 200 hours with one pair of
carbons, whereas open-arc lamps are usually only able to work, 8, 16
or 32 hours without recarboning, even when fitted with double carbons.
(3) Arc lamps are further divided into _focussing_ and _non-focussing_
lamps. In the former the lower carbon is made to move up as the upper
carbon moves down, and the arc is therefore maintained at the same
level. This is advisable for arcs included in a globe, and absolutely
necessary in the case of lighthouse lamps and lamps for optical
purposes. (4) Another subdivision is into _hand-regulated_ and
_self-regulating_ lamps. In the hand-regulated arcs the carbons are
moved by a screw attachment as required, as in some forms of
search-light lamp and lamps for optical lanterns. The carbons in large
search-light lamps are usually placed horizontally. The
self-regulating lamps may be classified into groups depending upon the
nature of the regulating appliances. In some cases the regulation is
controlled only by a _series coil_, and in others only by a _shunt
coil_. Examples of the former are the original Gülcher and Brush
clutch lamp, and some modern enclosed arc lamps; and of the latter,
the Siemens "band" lamp, and the Jackson-Mensing lamp. In series coil
lamps the variation of the current in the coil throws into or out of
action the carbon-moving mechanism; in shunt coil lamps the variation
in voltage between the carbons is caused to effect the same changes.
Other types of lamp involve the use both of shunt and series coils
acting against each other. A further classification of the
self-regulating lamps may be found in the nature of the carbon-moving
mechanism. This may be some modification of the Brush ring clutch,
hence called _clutch_ lamps; or some variety of _brake wheel_, as
employed in Brockie and Crompton lamps; or else some form of _electric
motor_ is thrown into or out of action and effects the necessary
changes. In many cases the arc-lamp mechanism is provided with a
_dash-pot_, or contrivance in which a piston moving nearly air-tight
in a cylinder prevents sudden jerks in the motion of the mechanism,
and thus does away with the "hunting" or rapid up-and-down movements
to which some varieties of clutch mechanism are liable. One very
efficient form is illustrated in the Thomson lamp and Brush-Vienna
lamp. In this mechanism a shunt and series coil are placed side by
side, and have iron cores suspended to the ends of a rocking arm held
partly within them. Hence, according as the magnetic action of the
shunt or series coil prevails, the rocking arm is tilted backwards or
forwards. When the series coil is not in action the _motion_ is free,
and the upper carbon-holder slides down, or the lower one slides up,
and starts the arc. The series coil comes into action to withdraw the
carbons, and at the same time locks the mechanism. The shunt coil then
operates against the series coil, and between them the carbon is fed
forwards as required. The control to be obtained is such that the arc
shall never become so long as to flicker and become extinguished, when
the carbons would come together again with a rush, but the feed should
be smooth and steady, the position of the carbons responding quickly
to each change in the current.
The introduction of enclosed arc lamps was a great improvement, in
consequence of the economy effected in the consumption of carbon and
in the cost of labour for trimming. A well-known and widely used form
of enclosed arc lamp is the Jandus lamp, which in large current form
can be made to burn for two hundred hours without recarboning, and in
small or midget form to burn for forty hours, taking a current of two
amperes at 100 volts. Such lamps in many cases conveniently replace
large sizes of incandescent lamps, especially for shop lighting, as
they give a whiter light. Great improvements have also been made in
inclined carbon arc lamps. One reason for the relatively low
efficiency of the usual vertical rod arrangement is that the crater
can only radiate laterally, since owing to the position of the
negative carbon no crater light is thrown directly downwards. If,
however, the carbons are placed in a downwards slanting position at a
small angle like the letter V and the arc formed at the bottom tips,
then the crater can emit downwards all the light it produces. It is
found, however, that the arc is unsteady unless a suitable magnetic
field is employed to keep the arc in position at the carbon tips. This
method has been adopted in the Carbone arc, which, by the employment
of inclined carbons, and a suitable electromagnet to keep the true arc
steady at the ends of the carbons, has achieved considerable success.
One feature of the Carbone arc is the use of a relatively high voltage
between the carbons, their potential difference being as much as 85
volts.
Arrangement.
Arc lamps may be arranged either (i.) in series, (ii.) in parallel or
(iii.) in series parallel. In the first case a number, say 20, may be
traversed by the same current, in that case supplied at a pressure of
1000 volts. Each must have a magnetic cut-out, so that if the carbons
stick together or remain apart the current to the other lamps is not
interrupted, the function of such a cut-out being to close the main
circuit immediately any one lamp ceases to pass current. Arc lamps
worked in series are generally supplied with a current from a constant
current dynamo, which maintains an invariable current of, say 10
amperes, independently of the number of lamps on the external circuit.
If the lamps, however, are worked in series off a constant potential
circuit, such as one supplying at the same time incandescent lamps,
provision must be made by which a resistance coil can be substituted for
any one lamp removed or short-circuited. When lamps are worked in
parallel, each lamp is independent, but it is then necessary to add a
resistance in series with the lamp. By special devices three lamps can
be worked in series of 100 volt circuits. Alternating-current arc lamps
can be worked off a high-tension circuit in parallel by providing each
lamp with a small transformer. In some cases the alternating
high-tension current is _rectified_ and supplied as a unidirectional
current to lamps in series. If single alternating-current lamps have to
be worked off a 100 volt alternating-circuit, each lamp must have in
series with it a choking coil or economy coil, to reduce the circuit
pressure to that required for one lamp. Alternating-current lamps take a
larger _effective_ current, and work with a less effective or virtual
carbon P.D., than continuous current arcs of the same wattage.
Cost.
The cost of working public arc lamps is made up of several items. There
is first the cost of supplying the necessary electric energy, then the
cost of carbons and the labour of recarboning, and, lastly, an item due
to depreciation and repairs of the lamps. An ordinary type of open 10
ampere arc lamp, burning carbons 15 and 9 mm. in diameter for the
positive and negative, and working every night of the year from dusk to
dawn, uses about 600 ft. of carbons per annum. If the positive carbon is
18 mm. and the negative 12 mm., the consumption of each size of carbon
is about 70 ft. per 1000 hours of burning. It may be roughly stated that
at the present prices of plain open arc-lamp carbons the cost is about
15s. per 1000 hours of burning; hence if such a lamp is burnt every
night from dusk to midnight the annual cost in that respect is about £1,
10s. The annual cost of labour per lamp for trimming is in Great Britain
from £2 to £3; hence, approximately speaking, the cost per annum of
maintenance of a public arc lamp burning every night from dusk to
midnight is about £4 to £5, or perhaps £6, per annum, depreciation and
repairs included. Since such a 10 ampere lamp uses half a Board of Trade
unit of electric energy every hour, it will take 1000 Board of Trade
units per annum, burning every night from dusk to midnight; and if this
energy is supplied, say at 1½d. per unit, the annual cost of energy will
be about £6, and the upkeep of the lamp, including carbons, labour for
trimming and repairs, will be about £10 to £11 per annum. The cost for
labour and carbons is considerably reduced by the employment of the
enclosed arc lamp, but owing to the absorption of light produced by the
inner enclosing globe, and the necessity for generally employing a
second outer globe, there is a lower resultant candle-power per watt
expended in the arc. Enclosed arc lamps are made to burn without
attention for 200 hours, singly on 100 volt circuits, or two in series
on 200 volt circuits, and in addition to the cost of carbons per hour
being only about one-twentieth of that of the open arc, they have
another advantage in the fact that there is a more uniform distribution
of light on the road surface, because a greater proportion of light is
thrown out horizontally.
It has been found by experience that the ordinary type of open arc lamp
with vertical carbons included in an opalescent globe cannot compete in
point of cost with modern improvements in gas lighting as a means of
street illumination. The violet colour of the light and the sharp
shadows, and particularly the non-illuminated area just beneath the
lamp, are grave disadvantages. The high-pressure flame arc lamp with
inclined chemically treated carbons has, however, put a different
complexion on matters. Although the treated carbons cost more than the
plain carbons, yet there is a great increase of emitted light, and a
9-ampere flame arc lamp supplied with electric energy at 1½d. per unit
can be used for 1000 hours at an inclusive cost of about £s to £6, the
mean emitted illumination being at the rate of 4 c.p. per watt absorbed.
In the Carbone arc lamp, the carbons are worked at an angle of 15° or
20° to each other and the arc is formed at the lower ends. If the
potential difference of the carbons is low, say only 50-60 volts, the
crater forms between the tips of the carbons and is therefore more or
less hidden. If, however, the voltage is increased to 90-100 then the
true flame of the arc is longer and is curved, and the crater forms at
the exteme tip of the carbons and throws all its light downwards. Hence
results a far greater mean hemispherical candle power (M.H.S.C.P.), so
that whereas a 10-ampere 60 volt open arc gives at most 1200 M.H.S.C.P.,
a Carbone 10-ampere 85 volt arc will give 2700 M.H.S.C.P. Better results
still can be obtained with impregnated carbons. But the flame arcs with
impregnated carbons cannot be enclosed, so the consumption of carbon is
greater, and the carbons themselves are more costly, and leave a greater
ash on burning; hence more trimming is required. They give a more
pleasing effect for street lighting, and their golden yellow globe of
light is more useful than an equally costly plain arc of the open type.
This improvement in efficiency is, however, accompanied by some
disadvantages. The flame arc is very sensitive to currents of air and
therefore has to be shielded from draughts by putting it under an
"economizer" or chamber of highly refractory material which surrounds
the upper carbon, or both carbon tips, if the arc is formed with
inclined carbons. (For additional information on flame arc lamps see a
paper by L. B. Marks and H. E. Clifford, _Electrician_, 1906, 57, p.
975.)
2. _Incandescent Lamps._--Incandescent electric lighting, although not
the first, is yet in one sense the most obvious method of utilizing
electric energy for illumination. It was evolved from the early observed
fact that a conductor is heated when traversed by an electric current,
and that if it has a high resistance and a high melting-point it may be
rendered incandescent, and therefore become a source of light. Naturally
every inventor turned his attention to the employment of wires of
refractory metals, such as platinum or alloys of platinum-iridium, &c.,
for the purpose of making an incandescent lamp. F. de Moleyns
experimented in 1841, E. A. King and J. W. Starr in 1845, J. J. W.
Watson in 1853, and W. E. Staite in 1848, but these inventors achieved
no satisfactory result. Part of their want of success is attributable to
the fact that the problem of the economical production of electric
current by the dynamo machine had not then been solved. In 1878 T. A.
Edison devised lamps in which a platinum wire was employed as the
light-giving agent, carbon being made to adhere round it by pressure.
Abandoning this, he next directed his attention to the construction of
an "electric candle," consisting of a thin cylinder or rod formed of
finely-divided metals, platinum, iridium, &c., mixed with refractory
oxides, such as magnesia, or zirconia, lime, &c. This refractory body
was placed in a closed vessel and heated by being traversed by an
electric current. In a further improvement he proposed to use a block of
refractory oxide, round which a bobbin of fine platinum or
platinum-iridium wire was coiled. Every other inventor who worked at the
problem of incandescent lighting seems to have followed nearly the same
path of invention. Long before this date, however, the notion of
employing carbon as a substance to be heated by the current had entered
the minds of inventors; even in 1845 King had employed a small rod of
plumbago as the substance to be heated. It was obvious, however, that
carbon could only be so heated when in a space destitute of oxygen, and
accordingly King placed his plumbago rod in a barometric vacuum. S. W.
Konn in 1872, and S. A. Kosloff in 1875, followed in the same direction.
Carbon filament lamp.
No real success attended the efforts of inventors until it was finally
recognized, as the outcome of the work by J. W. Swan, T. A. Edison, and,
in a lesser degree, St. G. Lane Fox and W. E. Sawyer and A. Man, that
the conditions of success were as follow: First, the substance to be
heated must be carbon in the form of a thin wire rod or thread,
technically termed a _filament_; second, this must be supported and
enclosed in a vessel formed entirely of glass; third, the vessel must be
exhausted as perfectly as possible; and fourth, the current must be
conveyed into and out of the carbon filament by means of platinum wires
hermetically sealed through the glass.
One great difficulty was the production of the carbon filament. King,
Sawyer, Man and others had attempted to cut out a suitably shaped
piece of carbon from a solid block; but Edison and Swan were the first
to show that the proper solution of the difficulty was to carbonize an
organic substance to which the necessary form had been previously
given. For this purpose cardboard, paper and ordinary thread were
originally employed, and even, according to Edison, a mixture of
lampblack and tar rolled out into a fine wire and bent into a spiral.
At one time Edison employed a filament of bamboo, carbonized after
being bent into a horse-shoe shape. Swan used a material formed by
treating ordinary crochet cotton-thread with dilute sulphuric acid,
the "parchmentized thread" thus produced being afterwards carbonized.
In the modern incandescent lamp the filament is generally constructed
by preparing first of all a form of soluble cellulose. Carefully
purified cotton-wool is dissolved in some solvent, such as a solution
of zinc chloride, and the viscous material so formed is forced by
hydraulic pressure through a die. The long thread thus obtained, when
hardened, is a semi-transparent substance resembling cat-gut, and when
carefully carbonized at a high temperature gives a very dense and
elastic form of carbon filament. It is cut into appropriate lengths,
which after being bent into horse-shoes, double-loops, or any other
shape desired, are tied or folded round carbon formers and immersed in
plumbago crucibles, packed in with finely divided plumbago. The
crucibles are then heated to a high temperature in an ordinary
combustion or electric furnace, whereby the organic matter is
destroyed, and a skeleton of carbon remains. The higher the
temperature at which this carbonization is conducted, the denser is
the resulting product. The filaments so prepared are sorted and
measured, and short leading-in wires of platinum are attached to their
ends by a carbon cement or by a carbon depositing process, carried out
by heating electrically the junction of the carbon and platinum under
the surface of a hydrocarbon liquid. They are then mounted in bulbs
of lead glass having the same coefficient of expansion as platinum,
through the walls of which, therefore, the platinum wires can be
hermetically sealed. The bulbs pass into the exhausting-room, where
they are exhausted by some form of mechanical or mercury pump. During
this process an electric current is sent through the filament to heat
it, in order to disengage the gases occluded in the carbon, and
exhaustion must be so perfect that no luminous glow appears within the
bulb when held in the hand and touched against one terminal of an
induction coil in operation.
In the course of manufacture a process is generally applied to the
carbon which is technically termed "treating." The carbon filament is
placed in a vessel surrounded by an atmosphere of hydrocarbon, such as
coal gas or vapour of benzol. If current is then passed through the
filament the hydrocarbon vapour is decomposed, and carbon is thrown
down upon the filament in the form of a lustrous and dense deposit
having an appearance like steel when seen under the microscope. This
deposited carbon is not only much more dense than ordinary carbonized
organic material, but it has a much lower specific electric
resistance. An untreated carbon filament is generally termed the
primary carbon, and a deposited carbon the secondary carbon. In the
process of treating, the greatest amount of deposit is at any places
of high resistance in the primary carbon, and hence it tends to cover
up or remedy the defects which may exist. The bright steely surface of
a well-treated filament is a worse radiator than the rougher black
surface of an untreated one; hence it does not require the expenditure
of so much electric power to bring it to the same temperature, and
probably on account of its greater density it ages much less rapidly.
[Illustration: FIG. 15.]
[Illustration: FIG. 16.--Incandescent Lamp Sockets.]
Finally, the lamp is provided with a collar having two sole plates on
it, to which the terminal wires are attached, or else the terminal
wires are simply bent into two loops; in a third form, the Edison
screw terminal, it is provided with a central metal plate, to which
one end of the filament is connected, the other end being joined to a
screw collar. The collars and screws are formed of thin brass embedded
in plaster of Paris, or in some material like vitrite or black glass
(fig. 15). To put the lamp into connexion with the circuit supplying
the current, it has to be fitted into a socket or holder. Three of the
principal types of holder in use are the bottom contact (B.C.) or
Dornfeld socket, the Edison screw-collar socket and the Swan or loop
socket. In the socket of C. Dornfeld (fig. 16, a and a´) two spring
pistons, in contact with the two sides of the circuit, are fitted into
the bottom of a short metallic tube having bayonet joint slots cut in
the top. The brass collar on the lamp has two pins, by means of which
a bayonet connexion is made between it and the socket; and when this
is done, the spring pins are pressed against the sole plates on the
lamp. In the Edison socket (fig. 16, b) a short metal tube with an
insulating lining has on its interior a screw sleeve, which is in
connexion with one wire of the circuit; at the bottom of the tube, and
insulated from the screw sleeve, is a central metal button, which is
in connexion with the other side of the circuit. On screwing the lamp
into the socket, the screw collar of the lamp and the boss or plate at
the base of the lamp make contact with the corresponding parts of the
socket, and complete the connexion. In some cases a form of switch is
included in the socket, which is then termed the key-holder. For loop
lamps the socket consists of an insulated block, having on it two
little hooks, which engage with the eyes of the lamp. This insulating
block also carries some form of spiral spring or pair of spring loops,
by means of which the lamp is pressed away from the socket, and the
eyes kept tight by the hooks. This spring or Swan socket (fig. 16, c)
is found useful in places where the lamps are subject to vibration,
for in such cases the Edison screw collar cannot well be used, because
the vibration loosens the contact of the lamp in the socket. The
sockets may be fitted with appliances for holding ornamental shades or
conical reflectors.
The incandescent filament being a very brilliant line of light,
various devices are adopted for moderating its brilliancy and
distributing the light. A simple method is to sand-blast the exterior
of the bulb, whereby it acquires an appearance similar to that of
ground glass, or the bare lamp may be enclosed in a suitable glass
shade. Such shades, however, if made of opalescent or semi-opaque
glass, absorb 40 to 60% of the light; hence various forms of dioptric
shade have been invented, consisting of clear glass ruled with
prismatic grooves in such a manner as to diffuse the light without any
very great absorption. Invention has been fertile in devising etched,
coloured, opalescent, frosted and ornamental shades for decorative
purposes, and in constructing special forms for use in situations,
such as mines and factories for explosives, where the globe containing
the lamp must be air-tight. High candle-power lamps, 500, 1000 and
upwards, are made by placing in one large glass bulb a number of
carbon filaments arranged in parallel between two rings, which are
connected with the main leading-in wires. When incandescent lamps are
used for optical purposes it is necessary to compress the filament
into a small space, so as to bring it into the focus of a lens or
mirror. The filament is then coiled or crumpled up into a spiral or
zigzag form. Such lamps are called _focus lamps_.
Classification of lamps.
Incandescent lamps are technically divided into high and low voltage
lamps, high and low efficiency lamps, standard and fancy lamps. The
difference between high and low efficiency lamps is based upon the
relation of the power absorbed by the lamp to the candle-power emitted.
Every lamp when manufactured is marked with a certain figure, called the
_marked volts_. This is understood to be the electromotive force in
volts which must be applied to the lamp terminals to produce through the
filament a current of such magnitude that the lamp will have a
practically satisfactory life, and give in a horizontal direction a
certain candle-power, which is also marked upon the glass. The numerical
product of the current in amperes passing through the lamp, and the
difference in potential of the terminals measured in volts, gives the
total power taken up by the lamp in watts; and this number divided by
the candle-power of the lamp (taking generally a horizontal direction)
gives the _watts per candle-power_. This is an important figure, because
it is determined by the temperature; it therefore determines the quality
of the light emitted by the lamp, and also fixes the average duration of
the filament when rendered incandescent by a current. Even in a good
vacuum the filament is not permanent. Apart altogether from accidental
defects, the carbon is slowly volatilized, and carbon molecules are also
projected in straight lines from different portions of the filament.
This process not only causes a change in the nature of the surface of
the filament, but also a deposit of carbon on the interior of the bulb,
whereby the glass is blackened and the candle-power of the lamp reduced.
The volatilization increases very rapidly as the temperature rises.
Hence at points of high resistance in the filament, more heat being
generated, a higher temperature is attained, and the scattering of the
carbon becomes very rapid; in such cases the filament is sooner or later
cut through at the point of high resistance. In order that incandescent
lighting may be practically possible, it is essential that the lamps
shall have a certain _average life_, that is, duration; and this useful
duration is fixed not merely by the possibility of passing a current
through the lamp at all, but by the rate at which the candle-power
diminishes. The decay of candle-power is called the _ageing_ of the
lamp, and the useful life of the lamp may be said to be that period of
its existence before it has deteriorated to a point when it gives only
75% of its original candle-power. It is found that in practice carbon
filament lamps, as at present made, if worked at a higher efficiency
than 2½ watts per candle-power, exhibit a rapid deterioration in
candle-power and an abbreviated life. Hence lamp manufacturers classify
lamps into various classes, marked for use say at 2½, 3, 3½ and 4 watts
per candle. A 2½ watt per candle lamp would be called a _high-efficiency
lamp_, and a 4 watt per candle lamp would be called a _low-efficiency_
lamp. In ordinary circumstances the low-efficiency lamp would probably
have a longer life, but its light would be less suitable for many
purposes of illumination in which colour discrimination is required.
The possibility of employing high-efficiency lamps depends greatly on
the uniformity of the electric pressure of the supply. If the voltage is
exceedingly uniform, then high-efficiency lamps can be satisfactorily
employed; but they are not adapted for standing the variations in
pressure which are liable to occur with public supply-stations, since,
other things being equal, their filaments are less substantial. The
classification into high and low voltage lamps is based upon the watts
per candle-power corresponding to the marked volts. When incandescent
lamps were first introduced, the ordinary working voltage was 50 or 100,
but now a large number of public supply-stations furnish current to
consumers at a pressure of 200 or 250 volts. This increase was
necessitated by the enlarging area of supply in towns, and therefore the
necessity for conveying through the same subterranean copper cables a
large supply of electric energy without increasing the maximum current
value and the size of the cables. This can only be done by employing a
higher working electromotive force; hence arose a demand for
incandescent lamps having marked volts of 200 and upwards, technically
termed high-voltage lamps. The employment of higher pressures in public
supply-stations has necessitated greater care in the selection of the
lamp fittings, and in the manner of carrying out the wiring work. The
advantages, however, of higher supply pressures, from the point of view
of supply-stations, are undoubted. At the same time the consumer desired
a lamp of a higher efficiency than the ordinary carbon filament lamp.
The demand for this stimulated efforts to produce improved carbon lamps,
and it was found that if the filament were exposed to a very high
temperature, 3000° C. in an electric furnace, it became more refractory
and was capable of burning in a lamp at an efficiency of 2½ watts per
c.p. Inventors also turned their attention to substances other than
carbon which can be rendered incandescent by the electric current.
Oxide filaments.
The luminous efficiency of any source of light, that is to say, the
percentage of rays emitted which affect the eye as light compared with
the total radiation, is dependent upon its temperature. In an ordinary
oil lamp the luminous rays do not form much more than 3% of the total
radiation. In the carbon-filament incandescent lamp, when worked at
about 3 watts per candle, the luminous efficiency is about 5%; and in
the arc lamp the radiation from the crater contains about 10 to 15% of
eye-affecting radiation. The temperature of a carbon filament working at
about 3 watts per candle is not far from the melting-point of platinum,
that is to say, is nearly 1775° C. If it is worked at a higher
efficiency, say 2.5 watts per candle-power, the temperature rises
rapidly, and at the same time the volatilization and molecular
scattering of the carbon is rapidly increased, so that the average
duration of the lamp is very much shortened. An improvement, therefore,
in the efficiency of the incandescent lamp can only be obtained by
finding some substance which will endure heating to a higher temperature
than the carbon filament. Inventors turned their attention many years
ago, with this aim, to the refractory oxides and similar substances.
Paul Jablochkoff in 1877 described and made a lamp consisting of a piece
of kaolin, which was brought to a state of incandescence first by
passing over it an electric spark, and afterwards maintained in a state
of incandescence by a current of lower electromotive force. Lane Fox and
Edison, in 1878, proposed to employ platinum wires covered with films of
lime, magnesia, steatite, or with the rarer oxides, zirconia, thoria,
&c.; and Lane Fox, in 1879, suggested as an incandescent substance a
mixture of particles of carbon with the earthy oxides. These earthy
oxides--magnesia, lime and the oxides of the rare earths, such as
thoria, zirconia, erbia, yttria, &c.--possess the peculiarity that at
ordinary temperatures they are practically non-conductors, but at very
high temperatures their resistance at a certain point rapidly falls, and
they become fairly good conductors. Hence if they can once be brought
into a state of incandescence a current can pass through them and
maintain them in that state. But at this temperature they give up oxygen
to carbon; hence no mixtures of earthy oxides with carbon are permanent
when heated, and failure has attended all attempts to use a carbon
filament covered with such substances as thoria, zirconia or other of
the rare oxides.
Nernst lamp.
H. W. Nernst in 1897, however, patented an incandescent lamp in which
the incandescent body consists entirely of a slender rod or filament of
magnesia. If such a rod is heated by the oxy-hydrogen blowpipe to a high
temperature it becomes conductive, and can then be maintained in an
intensely luminous condition by passing a current through it after the
flame is withdrawn. Nernst found that by mixing together, in suitable
proportions, oxides of the rare earths, he was able to prepare a
material which can be formed into slender rods and threads, and which is
rendered sufficiently conductive to pass a current with an electromotive
force as low as 100 volts, merely by being heated for a few moments with
a spirit lamp, or even by the radiation from a neighbouring platinum
spiral brought to a state of incandescence.
[Illustration: FIG. 17.--Nernst Lamp A Type.]
[Illustration: FIG. 18.--Nernst Lamp, Burners for B Type. a, low
voltage; b, high voltage.]
The Nernst lamp, therefore (fig. 17), consists of a slender rod of the
mixed oxides attached to platinum wires by an oxide paste. Oxide
filaments of this description are not enclosed in an exhausted glass
vessel, and they can be brought, without risk of destruction, to a
temperature considerably higher than a carbon filament; hence the lamp
has a higher luminous efficiency. The material now used for the oxide
rod or "glower" of Nernst lamps is a mixture of zirconia and yttria,
made into a paste and squirted or pressed into slender rods. This
material is non-conductive when cold, but when slightly heated it
becomes conductive and then falls considerably in resistance. The
glower, which is straight in some types of the lamp but curved in
others, is generally about 3 or 4 cm. long and 1 or 2 mm. in diameter.
It is held in suitable terminals, and close to it, or round it, but
not touching it, is a loose coil of platinum wire, also covered with
oxide and called the "heater" (fig. 18). In series with it is a spiral
of iron wire, enclosed in a bulb full of hydrogen, which is called the
"ballast resistance." The socket also contains a switch controlled by
an electromagnet. When the current is first switched on it passes
through the heater coil which, becoming incandescent, by radiation
heats the glower until it becomes conductive. The glower then takes
current, becoming itself brilliantly incandescent, and the
electromagnet becoming energized switches the heater coil out of
circuit. The iron ballast wire increases in resistance with increase
of current, and so operates to keep the total current through the
glower constant in spite of small variations of circuit voltage. The
disadvantages of the lamp are (1) that it does not light immediately
after the current is switched on and is therefore not convenient for
domestic use; (2) that it cannot be made in small light units such as
5 c.p.; (3) that the socket and fixture are large and more complicated
than for the carbon filament lamp. But owing to the higher
temperature, the light is whiter than that of the carbon glow lamp,
and the efficiency or candle power per watt is greater. Since,
however, the lamp must be included in an opal globe, some considerable
part of this last advantage is lost. On the whole the lamp has found
its field of operation rather in external than in domestic lighting.
Metallic filament lamps.
Great efforts were made in the latter part of the 19th century and the
first decade of the 20th to find a material for the filament of an
incandescent lamp which could replace carbon and yet not require a
preliminary heating like the oxide glowers. This resulted in the
production of refractory metallic filament lamps made of osmium,
tantalum, tungsten and other rare metals. Auer von Welsbach suggested
the use of osmium. This metal cannot be drawn into wire on account of
its brittleness, but it can be made into a filament by mixing the finely
divided metal with an organic binding material which is carbonized in
the usual way at a high temperature, the osmium particles then cohering.
The difficulty has hitherto been to construct in this way metallic
filament lamps of low candle power (16 c.p.) for 220 volt circuits, but
this is being overcome. When used on modern supply circuits of 220 volts
a number of lamps may be run in series, or a step-down transformer
employed.
[Illustration: FIG. 19.--Tantalum Lamp.]
The next great improvement came when W. von Bolton produced the tantalum
lamp in 1904. There are certain metals known to have a melting point
about 2000° C. or upwards, and of these tantalum is one. It can be
produced from the potassium tantalo-fluoride in a pulverulent form. By
carefully melting it _in vacuo_ it can then be converted into the
reguline form and drawn into wire. In this condition it has a density of
16.6 (water = 1), is harder than platinum and has greater tensile
strength than steel, viz. 95 kilograms per sq. mm., the value for good
steel being 70 to 80 kilograms per sq. mm. The electrical resistance at
15° C. is 0.146 ohms per metre with section of 1 sq. mm. after annealing
at 1900° C. _in vacuo_ and therefore about 6 times that of mercury; the
temperature coefficient is 0.3 per degree C. At the temperature assumed
in an incandescent lamp when working at 1.5 watts per c.p. the
resistance is 0.830 ohms per metre with a section of 1 sq. mm. The
specific heat is 0.0365. Bolton invented methods of producing tantalum
in the form of a long fine wire 0.05 mm. in diameter. To make a 25 c.p.
lamp 650 mm., or about 2 ft., of this wire are wound backwards and
forwards zigzag on metallic supports carried on a glass frame, which is
sealed into an exhausted glass bulb. The tantalum lamp so made (fig.
19), working on a 110 volt circuit takes 0.36 amperes or 39 watts, and
hence has an efficiency of about 1.6 watts per c.p. The useful life,
that is the time in which it loses 20% of its initial candle power, is
about 400-500 hours, but in general a life of 800-1000 hours can be
obtained. The bulb blackens little in use, but the life is said to be
shorter with alternating than with direct current. When worked on
alternating current circuits the filament after a time breaks up into
sections which become curiously sheared with respect to each other but
still maintain electrical contact. The resistance of tantalum increases
with the temperature; hence the temperature coefficient is positive, and
sudden rises in working voltage do not cause such variations in
candle-power as in the case of the carbon lamp.
Patents have also been taken out for lamps made with filaments of such
infusible metals as tungsten and molybdenum, and Siemens and Halske,
Sanders and others, have protected methods for employing zirconium and
other rare metals. According to the patents of Sanders (German patents
Nos. 133701, 137568, 137569) zirconium filaments are manufactured from
the hydrogen or nitrogen compounds of the rare earths by the aid of some
organic binding material. H. Kuzel of Vienna (British Patent No. 28154
of 1904) described methods of making metallic filaments from any metal.
He employs the metals in a colloidal condition, either as hydrosol,
organosol, gel, or colloidal suspension. The metals are thus obtained in
a gelatinous form, and can be squirted into filaments which are dried
and reduced to the metallic form by passing an electric current through
them (_Electrician_, 57, 894). This process has a wide field of
application, and enables the most refractory and infusible metals to be
obtained in a metallic wire form. The zirconium and tungsten wire lamps
are equal to or surpass the tantalum lamp in efficiency and are capable
of giving light, with a useful commercial life, at an efficiency of
about one watt per candle. Lamps called osram lamps, with filaments
composed of an alloy of osmium and tungsten (wolfram), can be used with
a life of 1000 hours when run at an efficiency of about 1.5 watts per
candle.
Tungsten lamps are made by the processes of Just and Hanaman (German
patent No. 154262 of 1903) and of Kuzel, and at a useful life of 1000
hours, with a falling off in light-giving power of only 10-15%, they
have been found to work at an efficiency of one to 1.25 watts per c.p.
Further collected information on modern metallic wire lamps and the
patent literature thereof will be found in an article in the _Engineer_
for December 7, 1906.
Mention should also be made of the Helion filament glow lamp in which
the glower is composed largely of silicon, a carbon filament being used
as a base. This filament is said to have a number of interesting
qualities and an efficiency of about 1 watt per candle (see the
_Electrician_, 1907, 58, p. 567).
Mercury vapour lamps.
The mercury vapour lamps of P. Cooper-Hewitt, C. O. Bastian and others
have a certain field of usefulness. If a glass tube, highly exhausted,
contains mercury vapour and a mercury cathode and iron anode, a current
can be passed through it under high electromotive force and will then be
maintained when the voltage is reduced. The mercury vapour is rendered
incandescent and glows with a brilliant greenish light which is highly
actinic, but practically monochromatic, and is therefore not suitable
for general illumination because it does not reveal objects in their
daylight colours. It is, however, an exceedingly economical source of
light. A 3-ampere Cooper-Hewitt mercury lamp has an efficiency of 0.15
to 0.33 watts per candle, or practically the same as an arc lamp, and
will burn for several thousand hours. A similar lamp with mercury vapour
included in a tube of _uviol_ glass specially transparent to
ultra-violet light (prepared by Schott & Co. of Jena) seems likely to
replace the Finsen arc lamp in the treatment of lupus. Many attempts
have been made to render the mercury vapour lamp polychromatic by the
use of amalgams of zinc, sodium and bismuth in place of pure mercury for
the negative electrode.
Photometry of glow lamps.
An important matter in connexion with glow lamps is their photometry.
The arrangement most suitable for the photometry and testing of
incandescent lamps is a gallery or room large enough to be occupied by
several workers, the walls being painted dead black. The photometer,
preferably one of the Lummer-Brodhun form, is set up on a gallery or
bench. On one side of it must be fixed a working standard, which as
first suggested by Fleming is preferably a large bulb incandescent lamp
with a specially "aged" filament. Its candle-power can be compared, at
regular intervals and known voltages, with that of some accepted flame
standard, such as the 10 candle pentane lamp of Vernon Harcourt. In a
lamp factory or electrical laboratory it is convenient to have a number
of such large bulb standard lamps. This working standard should be
maintained at a fixed distance on one side of the photometer, such that
when worked at a standard voltage it creates an illumination of one
candle-foot on one side of the photometer disk. The incandescent lamp to
be examined is then placed on the other side of the photometer disk on a
travelling carriage, so that it can be moved to and fro. Arrangements
must be made to measure the current and the voltage of this lamp under
test, and this is most accurately accomplished by employing a
potentiometer (q.v.). The holder which carries the lamp should allow the
lamp to be held with its axis in any required position; in making normal
measurements the lamp should have its axis vertical, the filament being
so situated that none of the turns or loops overlies another as seen
from the photometer disk. Observations can then be made of the
candle-power corresponding to different currents and voltages.
The candle-power of the lamp varies with the other variables in
accordance with exponential laws of the following kind:--
If A is the current in amperes through the lamp, V the voltage or
terminal potential difference, W the power absorbed in watts, _c.p._
the maximum candle-power, and a, b, c, &c., constants, it has been
found that A and _c.p._ are connected by an exponential law such that
c.p. = aA^x
For carbon filament lamps x is a number lying between 5 and 6,
generally equal to 5.5 or 5.6. Also it has been found that c.p. = bW³
very nearly, and that
c.p. = cV^y nearly
where c is some other constant, and for carbon filaments y is a number
nearly equal to 6. It is obvious that if the candle-power of the lamp
varies very nearly as the 6th power of the current and of the voltage,
the candle-power must vary as the cube of the wattage.
Sir W. de W. Abney and E. R. Festing have also given a formula
connecting candle-power and watts equivalent to c.p. = (W - d)² where
d is a constant.
In the case of the tantalum lamp the exponent x has a value near to 6,
but the exponent y is a number near to 4, and the same for the osmium
filament. Hence for these metallic glowers a certain percentage
variation of voltage does not create so great a variation in
candle-power as in the case of the carbon lamp.
Curves delineating the relation of these variables for any
incandescent lamp are called its _characteristic-curves_. The life or
average duration is a function of W/c.p., or of the _watts per
candle-power_, and therefore of the voltage at which the lamp is
worked. It follows from the above relation that the watts per
candle-power vary inversely as the fourth power of the voltage.
From limited observations it seems that the average life of a
carbon-filament lamp varies as the fifth or sixth power of the watts
per candle-power. If V is the voltage at which the lamp is worked and
L is its average life, then L varies roughly as the twenty-fifth power
of the reciprocal of the voltage, or