V V²
log10L = 13.5 - -- - ------.
10 20,000
(See J. A. Fleming, "Characteristic Curves of Incandescent Lamps,"
_Phil. Mag._ May 1885).
Ageing of lamps.
All forms of incandescent or glow lamps are found to deteriorate in
light-giving power with use. In the case of carbon filaments this is due
to two causes. As already explained, carbon is scattered from the
filament and deposited upon the glass, and changes also take place in
the filament which cause it to become reduced in temperature, even when
subjected to the same terminal voltage. In many lamps it is found that
the first effect of running the lamp is slightly to increase its
candle-power, even although the voltage be kept constant; this is the
result of a small decrease in the resistance of the filament. The
heating to which it is subjected slightly increases the density of the
carbon at the outset; this has the effect of making the filament lower
in resistance, and therefore it takes more current at a constant
voltage. The greater part, however, of the subsequent decay in
candle-power is due to the deposit of carbon upon the bulb, as shown by
the fact that if the filament is taken out of the bulb and put into a
new clean bulb the candle-power in the majority of cases returns to its
original value. For every lamp there is a certain point in its career
which may be called the "smashing-point," when the candle-power falls
below a certain percentage of the original value, and when it is
advantageous to replace it by a new one. Variations of pressure in the
electric supply exercise a prejudicial effect upon the light-giving
qualities of incandescent lamps. If glow lamps, nominally of 100 volts,
are supplied from a public lighting-station, in the mains of which the
pressure varies between 90 and 110 volts, their life will be greatly
abbreviated, and they will become blackened much sooner than would be
the case if the pressure were perfectly constant. Since the candle-power
of the lamp varies very nearly as the fifth or sixth power of the
voltage, it follows that a variation of 10% in the electromotive force
creates a variation of nearly 50% in the candle-power. Thus a 16
candle-power glow lamp, marked for use at 100 volts, was found on test
to give the following candle-powers at voltages varying between 90 and
105: At 105 volts it gave 22.8 c.p.; at 100 volts, 16.7 c.p.; at 95
volts, 12.2 c.p.; and at 90 volts, 8.7 c.p. Thus a variation of 25% in
the candle-power was caused by a variation in voltage of only 5%. The
same kind of variation in working voltage exercises also a marked effect
upon the average duration of the lamp. The following figures show the
results of some tests on typical 3.1 watt lamps run at voltages above
the normal, taking the average life when worked at the marked volts
(namely, 100) as 1000 hours:
At 101 volts the life was 818 hours.
" 102 " " 681 "
" 103 " " 662 "
" 104 " " 452 "
" 105 " " 374 "
" 106 " " 310 "
Voltage regulators.
Self-acting regulators have been devised by which the voltage at the
points of consumption is kept constant, even although it varies at the
point of generation. If, however, such a device is to be effective, it
must operate very quickly, as even the momentary effect of increased
pressure is felt by the lamp. It is only therefore where the working
pressure can be kept exceedingly constant that high-efficiency lamps can
be advantageously employed, otherwise the cost of lamp renewals more
than counterbalances the economy in the cost of power. The slow changes
that occur in the resistance of the filament make themselves evident by
an increase in the watts per candle-power. The following table shows
some typical figures indicating the results of ageing in a 16
candle-power carbon-filament glow lamp:--
+----------+-------------+-------------+
|Hours run.|Candle-Power.| Watts per |
| | |Candle-Power.|
+----------+-------------+-------------+
| 0 | 16.0 | 3.16 |
| 100 | 15.8 | 3.26 |
| 200 | 15.86 | 3.13 |
| 300 | 15.68 | 3.37 |
| 400 | 15.41 | 3.53 |
| 500 | 15.17 | 3.51 |
| 600 | 14.96 | 3.54 |
| 700 | 14.74 | 3.74 |
+----------+-------------+-------------+
The gradual increase in watts per candle-power shown by this table does
not imply necessarily an increase in the total power taken by the lamp,
but is the consequence of the decay in candle-power produced by the
blackening of the lamp. Therefore, to estimate the value of an
incandescent lamp the user must take into account not merely the price
of the lamp and the initial watts per candle-power, but the rate of
decay of the lamp.
Edison effect.
The scattering of carbon from the filament to the glass bulb produces
interesting physical effects, which have been studied by T. A. Edison,
W. H. Preece and J. A. Fleming. If into an ordinary carbon-filament glow
lamp a platinum plate is sealed, not connected to the filament but
attached to a third terminal, then it is found that when the lamp is
worked with continuous current a galvanometer connected in between the
middle plate and the positive terminal of the lamp indicates a current,
but not when connected in between the negative terminal of the lamp and
the middle plate. If the middle plate is placed between the legs of a
horse-shoe-shaped filament, it becomes blackened most quickly on the
side facing the negative leg. This effect, commonly called the _Edison
effect_, is connected with an electric discharge and convection of
carbon which takes place between the two extreme ends of the filament,
and, as experiment seems to show, consists in the conveyance of an
electric charge, either by carbon molecules or by bodies smaller than
molecules. There is, however, an electric discharge between the ends of
the filament, which rapidly increases with the temperature of the
filament and the terminal voltage; hence one of the difficulties of
manufacturing high-voltage glow lamps, that is to say, glow lamps for
use on circuits having an electromotive force of 200 volts and upwards,
is the discharge from one leg of the filament to the other.
Domestic use.
A brief allusion may be made to the mode of use of incandescent lamps
for interior and private lighting. At the present time hardly any other
method of distribution is adopted than that of an arrangement _in
parallel_; that is to say, each lamp on the circuit has one terminal
connected to a wire which finally terminates at one pole of the
generator, and its other terminal connected to a wire leading to the
other pole. The lamp filaments are thus arranged between the conductors
like the rungs of a ladder. In series with each lamp is placed a switch
and a fuse or cut-out. The lamps themselves are attached to some variety
of ornamental fitting, or in many cases suspended by a simple pendant,
consisting of an insulated double flexible wire attached at its upper
end to a ceiling rose, and carrying at the lower end a shade and socket
in which the lamp is placed. Lamps thus hung head downwards are
disadvantageously used because their _end-on candle-power_ is not
generally more than 60% of their maximum candle-power. In interior
lighting one of the great objects to be attained is uniformity of
illumination with avoidance of harsh shadows. This can only be achieved
by a proper distribution of the lamps. It is impossible to give any hard
and fast rules as to what number must be employed in the illumination of
any room, as a great deal depends upon the nature of the reflecting
surfaces, such as the walls, ceilings, &c. As a rough guide, it may be
stated that for every 100 sq. ft. of floor surface one 16 candle-power
lamp placed about 8 ft. above the floor will give a dull illumination,
two will give a good illumination and four will give a brilliant
illumination. We generally judge of the nature of the illumination in a
room by our ability to read comfortably in any position. That this may
be done, the horizontal illumination on the book should not be less than
one candle-foot. The following table shows approximately the
illuminations in candle-feet, in various situations, derived from actual
experiments:--
In a well-lighted room on the floor or tables 1.0 to 3.0 c.f.
On a theatre stage 3.0 to 4.0 c.f.
On a railway platform .05 to .5 c.f.
In a picture gallery .65 to 3.5 c.f.
The mean daylight in May in the interior
of a room 30.0 to 40.0 c.f.
In full sunlight 7000 to 10,000 c.f.
In full moonlight 1/60th to 1/100th c.f.
From an artistic point of view, one of the worst methods of lighting a
room is by pendant lamps, collected in single centres in large numbers.
The lights ought to be distributed in different portions of the room,
and so shaded that the light is received only by reflection from
surrounding objects. Ornamental effects are frequently produced by means
of candle lamps in which a small incandescent lamp, imitating the flame
of a candle, is placed upon a white porcelain tube as a holder, and
these small units are distributed and arranged in electroliers and
brackets. For details as to the various modes of placing conducting
wires in houses, and the various precautions for safe usage, the reader
is referred to the article ELECTRICITY SUPPLY. In the case of low
voltage metallic filament lamps when the supply is by alternating
current there is no difficulty in reducing the service voltage to any
lower value by means of a transformer. In the case of direct current the
only method available for working such low voltage lamps off higher
supply voltages is to arrange the lamps in series.
Additional information on the subjects treated above may be found in
the following books and original papers:--
Mrs Ayrton, _The Electric Arc_ (London, 1900); Houston and Kennelly,
_Electric Arc Lighting and Electric Incandescent Lighting_; S. P.
Thompson, _The Arc Light_, Cantor Lectures, Society of Arts (1895); H.
Nakano, "The Efficiency of the Arc Lamp," _Proc. American Inst. Elec.
Eng._ (1889); A. Blondel, "Public and Street Lighting by Arc Lamps,"
_Electrician_, vols. xxxv. and xxxvi. (1895); T. Heskett, "Notes on
the Electric Arc," _Electrician_, vol. xxxix. (1897); G. S. Ram, _The
Incandescent Lamp and its Manufacture_ (London, 1895); J. A. Fleming,
_Electric Lamps and Electric Lighting_ (London, 1899); J. A. Fleming,
"The Photometry of Electric Lamps," _Jour. Inst. Elec. Eng._ (1903),
32, p. 1 (in this paper a copious bibliography of the subject of
photometry is given); J. Dredge, _Electric Illumination_ (2 vols.,
London, 1882, 1885); A. P. Trotter, "The Distribution and Measurement
of Illumination," _Proc. Inst. C.E._ vol. cx. (1892); E. L. Nichols,
"The Efficiency of Methods of Artificial Illumination," _Trans.
American Inst. Elec. Eng._ vol. vi. (1889); Sir W. de W. Abney,
_Photometry_, Cantor Lectures, Society of Arts (1894); A. Blondel,
"Photometric Magnitudes and Units," _Electrician_ (1894); J. E.
Petavel, "An Experimental Research on some Standards of Light," _Proc.
Roy. Soc._ lxv. 469 (1899); F. Jehl, _Carbon-Making for all Electrical
Purposes_ (London, 1906); G. B. Dyke, "On the Practical Determination
of the Mean Spherical Candle Power of Incandescent and Arc Lamps,"
_Phil. Mag._ (1905); the _Preliminary Report of the Sub-Committee of
the American Institute of Electrical Engineers_ on "Standards of
Light"; Clifford C. Paterson, "Investigations on Light Standards and
the Present Condition of the High Voltage Glow Lamp," _Jour. Inst.
Elec. Eng._ (January 24, 1907); J. Swinburne, "New Incandescent
Lamps," _Jour. Inst. Elec. Eng._ (1907); L. Andrews, "Long Flame Arc
Lamps," Jour. Inst. Elec. Eng. (1906); W. von Bolton and O. Feuerlein,
"The Tantalum Lamp," _The Electrician_ (Jan. 27, 1905). Also the
current issues of _The Illuminating Engineer_. (J. A. F.)
Methods of charging.
_Commercial Aspects._--The cost of supplying electricity depends more
upon the rate of supply than upon the quantity supplied; or, as John
Hopkinson put it, "the cost of supplying electricity for 1000 lamps for
ten hours is very much less than ten times the cost of supplying the
same number of lamps for one hour." Efforts have therefore been made to
devise a system of charge which shall in each case bear some relation to
the cost of the service. Consumers vary largely both in respect to the
quantity and to the period of their demands, but the cost of supplying
any one of them with a given amount of electricity is chiefly governed
by the amount of his maximum demand at any one time. The reason for this
is that it is not generally found expedient to store electricity in
large quantities. Electricity supply works generate the electricity for
the most part at the moment it is used by the consumer. Electric lamps
are normally in use on an average for only about four hours per day, and
therefore the plant and organization, if employed for a lighting load
only, are idle and unremunerative for about 20 hours out of the 24. It
is necessary to have in readiness machinery capable of supplying the
maximum possible requirements of all the consumers at any hour, and this
accounts for a very large proportion of the total cost. The cost of raw
material, viz. coal, water and stores consumed in the generation of
electricity sold, forms relatively only a small part of the total cost,
the major part of which is made up of the fixed charges attributable to
the time during which the works are unproductive. This makes it very
desirable to secure demands possessing high "load" and "diversity"
factors. The correct way to charge for electricity is to give liberal
rebates to those consumers who make prolonged and regular use of the
plant, that is to say, the lower the "peak" demand and the more
continuous the consumption, the better should be the discount. The
consumer must be discouraged from making sudden large demands on the
plant, and must be encouraged, while not reducing his total consumption,
to spread his use of the plant over a large number of hours during the
year. Mr Arthur Wright has devised a tariff which gives effect to this
principle. The system necessitates the use of a special indicator--not
to measure the quantity of electricity consumed, which is done by the
ordinary meter--but to show the maximum amount of current taken by the
consumer at any one time during the period for which he is to be
charged. In effect it shows the proportion of plant which has had to be
kept on hand for his use. If the indicator shows that say twenty lamps
is the greatest number which the consumer has turned on simultaneously,
then he gets a large discount on all the current which his ordinary
meter shows that he has taken beyond the equivalent of one hour's daily
use of those twenty lamps. Generally the rate charged under this system
is 7d. per unit for the equivalent of one hour's daily use of the
maximum demand and 1d. per unit for all surplus. It is on this principle
that it pays to supply current for tramway and other purposes at a price
which primâ facie is below the cost of production; it is only apparently
so in comparison with the cost of producing electricity for lighting
purposes. In the case of tramways the electricity is required for 15 or
16 hours per day. Electricity for a single lamp would cost on the basis
of this "maximum-demand-indicator" system for 15 hours per day only
1.86d. per unit. In some cases a system of further discounts to very
large consumers is combined with the Wright system. Some undertakers
have abandoned the Wright system in favour of average flat rates, but
this does not imply any failure of the Wright system; on the contrary,
the system, having served to establish the most economical consumption
of electricity, has demonstrated the average rate at which the
undertakers are able to give the supply at a fair profit, and the
proportion of possible new customers being small the undertakers find it
a simplification to dispense with the maximum demand indicator. But in
some cases a mistake has been made by offering the unprofitable
early-closing consumers the option of obtaining electricity at a flat
rate much lower than their load-factor would warrant and below cost
price. The effect of this is to nullify the Wright system of charging,
for a consumer will not elect to pay for his electricity on the Wright
system if he can obtain a lower rate by means of a flat rate system.
Thus the long-hour profitable consumer is made to pay a much higher
price than he need be charged, in order that the unprofitable short-hour
consumer may be retained and be made actually still more unprofitable.
It is not improbable that ultimately the supply will be charged for on
the basis of a rate determined by the size and character of the
consumer's premises, or the number and dimensions of the electrical
points, much in the same way as water is charged for by a water rate
determined by the rent of the consumer's house and the number of water
taps.
Wiring of houses.
Most new houses within an electricity supply area are wired for
electricity during construction, but in several towns means have to be
taken to encourage small shopkeepers and tenants of small houses to use
electricity by removing the obstacle of the first outlay on wiring. The
cost of wiring may be taken at 15s. to £2 per lamp installed including
all necessary wire, switches, fuses, lamps, holders, casing, but not
electroliers or shades. Many undertakers carry out wiring on the easy
payment or hire-purchase system. Parliament has sanctioned the adoption
of these systems by some local authorities and even authorized them to
do the work by direct employment of labour. The usual arrangement is to
make an additional charge of ½d. per unit on all current used, with a
minimum payment of 1s. per 8 c.p. lamp, consumers having the option of
purchasing the installation at any time on specified conditions. The
consumer has to enter into an agreement, and if he is only a tenant the
landlord has to sign a memorandum to the effect that the wiring and
fittings belong to the supply undertakers. Several undertakers have
adopted a system of maintenance and renewal of lamps, and at least one
local authority undertakes to supply consumers with lamps free of
charge.
Consumption.
There is still considerable scope for increasing the business of
electricity supply by judicious advertising and other methods.
Comparisons of the kilowatt hour consumption per capita in various towns
show that where an energetic policy has been pursued the profits have
improved by reason of additional output combined with increased load
factor. The average number of equivalent 8 c.p. lamps connected per
capita in the average of English towns is about 1.2. The average number
of units consumed per capita per annum is about 23, and the average
income per capita per annum is about 5s. In a number of American cities
20s. per capita per annum is obtained. In the United States a
co-operative electrical development association canvasses both the
general public and the electricity supply undertakers. Funds are
provided by the manufacturing companies acting in concert with the
supply authorities and contractors, and the spirit underlying the work
is to advertise the merits of electricity--not any particular company or
interest. Their efforts are directed to securing new consumers and
stimulating the increased and more varied use of electricity among
actual consumers.
All supply undertakers are anxious to develop the consumption of
electricity for power purposes even more than for lighting, but the
first cost of installing electric motors is a deterrent to the adoption
of electricity in small factories and shops, and most undertakers are
therefore prepared to let out motors, &c., on hire or purchase on
varying terms according to circumstances.
A board of trade unit will supply one 8 c.p. carbon lamp of 30 hours or
30 such lamps for one hour. In average use an incandescent lamp will
last about 800 hours, which is equal to about 12 months normal use; a
good lamp will frequently last more than double this time before it
breaks down.
A large number of towns have adopted electricity for street lighting.
Frank Bailey has furnished particulars of photometric tests which he has
made on new and old street lamps in the city of London. From these tests
the following comparative figures are deduced:--
Average total Cost
Gas-- per c.p. per annum.
Double burner ordinary low pressure incandescent
(mean of six tests) 11.1d.
Single burner high-pressure gas 9.0
Double burner high-pressure gas 11.7
Arc lamp--
Old type of lantern 8
Flame arc 5
From these tests of candle-power the illumination at a distance of 100
ft. from the source is estimated as follows:--
Candle Ft. Ratio.
Double ordinary incandescent gas lamp
illumination 0.013 = 1.0
Single high pressure ordinary incandescent
gas lamp illumination 0.016 = 1.24
Double high pressure ordinary incandescent
gas lamp illumination 0.027 = 2.10
Ordinary arc lamp 0.060 = 4.50
Flame arc lamp 0.120 = 9.00
The cost of electricity, light for light, is very much less than that of
gas. The following comparative figures relating to street lighting at
Croydon have been issued by the lighting committee of that
corporation:--
+----------------------+---------+------------+--------+------------+-------------+
| Type of Lamp. | Number | Distance | Total |Average c.p.|Cost per c.p.|
| |of Lamps.|apart (yds.)| Cost. | per Mile. | per annum. |
+----------------------+---------+------------+--------+------------+-------------+
| Incandescent gas | 2,137 | 80 | £7,062 | 839 | 15.86d. |
| Incandescent electric| 90 | 66 | 288 | 1,373 | 13.71 |
| Electric arcs | 428 | 65 | 7,212 | 10,537 | 11.32 |
+----------------------+---------+------------+--------+------------+-------------+
Apart from cheaper methods of generation there are two main sources of
economy in electric lighting. One is the improved arrangement and use of
electrical installations, and the other is the employment of lamps of
higher efficiency. As regards the first, increased attention has been
given to the position, candle-power and shading of electric lamps so as
to give the most effective illumination in varying circumstances and to
avoid excess of light. The ease with which electric lamps may be
switched on and off from a distance has lent itself to arrangements
whereby current may be saved by switching off lights not in use and by
controlling the number of lamps required to be alight at one time on an
electrolier. Appreciable economies are brought about by the scientific
disposition of lights and the avoidance of waste in use. As regards the
other source of economy, the Nernst, the tantalum, the osram, and the
metallized carbon filament lamp, although costing more in the first
instance than carbon lamps, have become popular owing to their economy
in current consumption. Where adopted largely they have had a distinct
effect in reducing the rate of increase of output from supply
undertakings, but their use has been generally encouraged as tending
towards the greater popularity of electric light and an ultimately wider
demand. Mercury vapour lamps for indoor and outdoor lighting have also
proved their high efficiency, and the use of flame arc lamps has greatly
increased the cheapness of outdoor electric lighting.
The existence of a "daylight load" tends to reduce the all-round cost of
generating and distributing electricity. This daylight load is partly
supplied by power for industrial purposes and partly by the demand for
electricity in many domestic operations. The use of electric heating and
cooking apparatus (including radiators, ovens, grills, chafing dishes,
hot plates, kettles, flat-irons, curling irons, &c.) has greatly
developed, and provides a load which extends intermittently throughout
the greater part of the twenty-four hours. Electric fans for home
ventilation are also used, and in the domestic operations where a small
amount of power is required (as in driving sewing machines, boot
cleaners, washing machines, mangles, knife cleaners, "vacuum" cleaners,
&c.) the electric motor is being largely adopted. The trend of affairs
points to a time when the total demand from such domestic sources will
greatly exceed the demand for lighting only. The usual charges for
current to be used in domestic heating or power operations vary from 1d.
to 2d. per unit. As the demand increases the charges will undergo
reduction, and there will also be a reflex action in bringing down the
cost of electricity for lighting owing to the improved load factor
resulting from an increase in the day demand. In the cooking and heating
and motor departments also there has been improvement in the efficiency
of the apparatus, and its economy is enhanced by the fact that current
may be switched on and off as required.
Testing meters.
The Board of Trade are now prepared to receive electric measuring
instruments for examination or testing at their electrical standardizing
laboratory, where they have a battery power admitting of a maximum
current of 7000 amperes to be dealt with. The London county council and
some other corporations are prepared upon requisition to appoint
inspectors to test meters on consumers' premises.
Wiring rules.
All supply undertakers now issue rules and regulations for the
efficient wiring of electric installations. The rules and regulations
issued by the institution of electrical engineers have been accepted
by many local authorities and companies, and also by many of the fire
insurance companies. The Phoenix fire office rules were the first to
be drawn up, and are adopted by many of the fire offices, but some
other leading insurance offices have their own rules under which risks
are accepted without extra premium. In the opinion of the insurance
companies "the electric light is the safest of all illuminants and is
preferable to any others when the installation has been thoroughly
well put up." Regulations have also been issued by the London county
council in regard to theatres, &c., by the national board of fire
underwriters of America (known as the "National Electrical Code"), by
the fire underwriters association of Victoria (Commonwealth of
Australia), by the Calcutta fire insurance agents association and
under the Canadian Electric Light Inspection Act. In Germany rules
have been issued by the Verband Deutscher Elektrotechniker and by the
union of private fire insurance companies of Germany, in Switzerland
by the Association Suisse des électriciens, in Austria by the
Elektrotechnischer Verein of Vienna, in France by ministerial decree
and by the syndicat professionel des industries électriques. (For
reprints of these regulations see _Electrical Trades Directory_.)
(E. Ga.)