Electric lamps are of two varieties: (1) _Arc Lamps_ and (2)
_Incandescent_ or _Glow Lamps_. Under these headings we may briefly
consider the history, physical principles, and present practice of the
art of electric lighting.
1. _Arc Lamps._--If a voltaic battery of a large number of cells has its
terminal wires provided with rods of electrically-conducting carbon, and
these are brought in contact and then slightly separated, a form of
electric discharge takes place between them called the _electric arc_.
It is not quite certain who first observed this effect of the electric
current. The statement that Sir Humphry Davy, in 1801, first produced
and studied the phenomenon is probably correct. In 1808 Davy had
provided for him at the Royal Institution a battery of 2000 cells, with
which he exhibited the electric arc on a large scale.
The electric arc may be produced between any conducting materials
maintained at different potentials, provided that the source of electric
supply is able to furnish a sufficiently large current; but for
illuminating purposes pieces of hard graphitic carbon are most
convenient. If some source of continuous electric current is connected
to rods of such carbon, first brought into contact and then slightly
separated, the following facts may be noticed: With a low electromotive
force of about 50 or 60 volts no discharge takes place until the carbons
are in actual contact, unless the insulation of the air is broken down
by the passage of a small electric spark. When this occurs, the space
between the carbons is filled at once with a flame or luminous vapour,
and the carbons themselves become highly incandescent at their
extremities. If they are horizontal the flame takes the form of an arch
springing between their tips; hence the name _arc_. This varies somewhat
in appearance according to the nature of the current, whether continuous
or alternating, and according as it is formed in the open air or in an
enclosed space to which free access of oxygen is prevented. Electric
arcs between metal surfaces differ greatly in colour according to the
nature of the metal. When formed by an alternating current of high
electromotive force they resemble a lambent flame, flickering and
producing a somewhat shrill humming sound.
Electric arcs may be classified into continuous or alternating current
arcs, and open or enclosed arcs, carbon arcs with pure or chemically
impregnated carbons, or so-called flame arcs, and arcs formed with
metallic or oxide electrodes, such as magnetite. A continuous current
arc is formed with an electric current flowing always in the same
direction; an alternating current arc is formed with a periodically
reversed current. An open arc is one in which the carbons or other
material forming the arc are freely exposed to the air; an enclosed arc
is one in which they are included in a glass vessel. If carbons
impregnated with various salts are used to colour or increase the light,
the arc is called a chemical or flame arc. The carbons or electrodes may
be arranged in line one above the other, or they may be inclined so as
to project the light downwards or more in one direction. In a carbon arc
if the current is continuous the positive carbon becomes much hotter at
the end than the negative, and in the open air it is worn away, partly
by combustion, becoming hollowed out at the extremity into a _crater_.
At the same time the negative carbon gradually becomes pointed, and also
wears away, though much less quickly than the positive. In the
continuous-current open arc the greater part of the light proceeds from
the highly incandescent positive crater. When the arc is examined
through dark glasses, or by the optical projection of its image upon a
screen, a violet band or stream of vapour is seen to extend between the
two carbons, surrounded by a nebulous golden flame or aureole. If the
carbons are maintained at the right distance apart the arc remains
steady and silent, but if the carbons are impure, or the distance
between them too great, the true electric arc rapidly changes its place,
flickering about and frequently becoming extinguished; when this happens
it can only be restored by bringing the carbons once more into contact.
If the current is alternating, then the arc is symmetrical, and both
carbons possess nearly the same appearance. If it is enclosed in a
vessel nearly air-tight, the rate at which the carbons are burnt away is
greatly reduced, and if the current is continuous the positive carbon is
no longer cratered out and the negative no longer so much pointed as in
the case of the open arc.
Carbons.
Davy used for his first experiments rods of wood charcoal which had been
heated and plunged into mercury to make them better conductors. Not
until 1843 was it proposed by J. B. L. Foucault to employ pencils cut
from the hard graphitic carbon deposited in the interior of gas retorts.
In 1846 W. Greener and W. E. Staite patented a process for manufacturing
carbons for this purpose, but only after the invention of the Gramme
dynamo in 1870 any great demand arose for them. F. P. É. Carré in France
in 1876 began to manufacture arc lamp carbons of high quality from coke,
lampblack and syrup. Now they are made by taking some specially refined
form of finely divided carbon, such as the soot or lampblack formed by
cooling the smoke of burning paraffin or tar, or by the carbonization of
organic matter, and making it into a paste with gum or syrup. This
carbon paste is forced through dies by means of a hydraulic press, the
rods thus formed being subsequently baked with such precautions as to
preserve them perfectly straight. In some cases they are _cored_, that
is to say, have a longitudinal hole down them, filled in with a softer
carbon. Sometimes they are covered with a thin layer of copper by
electro-deposition. They are supplied for the market in sizes varying
from 4 or 5 to 30 or 40 millimetres in diameter, and from 8 to 16 in. in
length. The value of carbons for arc lighting greatly depends on their
purity and freedom from ash in burning, and on perfect uniformity of
structure. For ordinary purposes they are generally round in section,
but for certain special uses, such as lighthouse work, they are made
fluted or with a star-shaped section. The positive carbon is usually of
larger section than the negative. For continuous-current arcs a cored
carbon is generally used as a positive, and a smaller solid carbon as a
negative. For flame arc lamps the carbons are specially prepared by
impregnating them with salts of calcium, magnesium and sodium. The
calcium gives the best results. The rod is usually of a composite type.
The outer zone is pure carbon to give strength, the next zone contains
carbon mixed with the metallic salts, and the inner core is the same
but less compressed. In addition to the metallic salts a flux has to be
introduced to prevent the formation of a non-conducting ash, and this
renders it desirable to place the carbons in a downward pointing
direction to get rid of the slag so formed. Bremer first suggested in
1898 for this purpose the fluorides of calcium, strontium or barium.
When such carbons are used to form an electric arc the metallic salts
deflagrate and produce a flame round the arc which is strongly coloured,
the object being to produce a warm yellow glow, instead of the somewhat
violet and cold light of the pure carbon arc, as well as a greater
emission of light. As noxious vapours are however given off, flame arcs
can only be used out of doors. Countless researches have been made on
the subject of carbon manufacture, and the art has been brought to great
perfection.
Special manuals must be consulted for further information (see
especially a treatise on _Carbon making for all electrical purposes_,
by F. Jehl, London, 1906).
[Illustration: FIG. 4.]
[Illustration: FIG. 5.]
Physical phenomena.
The physical phenomena of the electric arc are best examined by forming
a carbon arc between two carbon rods of the above description, held in
line in a special apparatus, and arranged so as to be capable of being
moved to or from each other with a slow and easily regulated motion. An
arrangement of this kind is called a _hand-regulated arc lamp_ (fig. 4).
If such an arc lamp is connected to a source of electric supply having
an electromotive force preferably of 100 volts, and if some resistance
is included in the circuit, say about 5 ohms, a steady and continuous
arc is formed when the carbons are brought together and then slightly
separated. Its appearance may be most conveniently examined by
projecting its image upon a screen of white paper by means of an
achromatic lens. A very little examination of the distribution of light
from the arc shows that the illuminating or candle-power is not the same
in different directions. If the carbons are vertical and the positive
carbon is the upper of the two, the illuminating power is greatest in a
direction at an angle inclined about 40 or 50 degrees below the horizon,
and at other directions has different values, which may be represented
by the lengths of radial lines drawn from a centre, the extremities of
which define a curve called the _illuminating curve_ of the arc lamp
(fig. 5). Considerable differences exist between the forms of the
illuminating-power curves of the continuous and alternating current and
the open or enclosed arcs. The chief portion of the emitted light
proceeds from the incandescent crater; hence the form of the
illuminating-power curve, as shown by A. P. Trotter in 1892, is due to
the apparent area of the crater surface which is visible to an eye
regarding the arc in that direction. The form of the illuminating-power
curve varies with the length of the arc and relative size of the
carbons. Leaving out of account for the moment the properties of the arc
as an illuminating agent, the variable factors with which we are
concerned are (i.) the current through the arc; (ii.) the potential
difference of the carbons; (iii.) the length of the arc; and (iv.) the
size of the carbons. Taking in the first place the typical
direct-current arc between solid carbons, and forming arcs of different
lengths and with carbons of different sizes, it will be found that,
beginning at the lowest current capable of forming a true arc, the
potential difference of the carbons (the arc P.D.) decreases as the
current increases. Up to a certain current strength the arc is silent,
but at a particular critical value P.D. suddenly drops about 10 volts,
the current at the same time rising 2 or 3 amperes. At that moment the
arc begins to _hiss_, and in this hissing condition, if the current is
still further increased, P.D. remains constant over wide limits. This
drop in voltage on hissing was first noticed by A. Niaudet (_La Lumière
électrique_, 1881, 3, p. 287). It has been shown by Mrs Ayrton (_Journ.
Inst. Elec. Eng._ 28, 1899, p. 400) that the hissing is mainly due to
the oxygen which gains access from the air to the crater, when the
latter becomes so large by reason of the increase of the current as to
overspread the end of the positive carbon. According to A. E. Blondel
and Hans Luggin, hissing takes place whenever the current density
becomes greater than about 0.3 or 0.5 ampere per square millimetre of
crater area.
The relation between the current, the carbon P.D., and the length of
arc in the case of the direct-current arc has been investigated by
many observers with the object of giving it mathematical expression.
Let V stand for the potential difference of the carbons in volts, A
for the current through the arc in amperes, L for the length of the
arc in millimetres, R for the resistance of the arc; and let a, b, c,
d, &c., be constants. Erik Edlund in 1867, and other workers after
him, considered that their experiments showed that the relation
between V and L could be expressed by a simple linear equation,