The well-known expression “per 1000 Cubic Feet” is not applicable to
electric light, and, instead, the Board of Trade Unit is employed.
By this term Unit is meant the quantity of energy contained in a
current of 1000 Ampères flowing under an Electro-motive Force of One
Volt during One Hour. In the early days of electric lighting the term
Volt-ampère was used, and has for convenience sake been shortened to
Watt; that is, the Volt or Unit of Electro-motive Force (or pressure)
is multiplied by the Ampère or Unit of current.
The Board of Trade Unit is, therefore, a Thousand Volt-ampères or Watts
per hour.
For example: 16 candle-power Swan lamps are assumed to take 60
Watts, which, if the electrical pressure is 100 Volts, would mean a
consumption of 0·6 Ampère; and, as an Electrical Horse-power equals 746
Watts, 12·4 lamps should theoretically be obtained per Horse-power,
which is, however, reduced in actual practice to 10 at the most, often
less.
The charge per Unit supplied by meter varies in England from 1_s._ to
7_d._
Price for electric Equivalent price for gas of equal
light. light.
_s._ _d._ _s._ _d._
1 0 } per Board { 6 10 per 1000 cubic feet.
0 9 } of { 5 1½ ” ”
0 7¼ } Trade { 4 2 ” ”
0 6 } Unit. { 3 5 ” ”
_Comparison of Cost of Gas and Electricity._
These prices, of course, include the manufacturer’s profit as well as
the loss in transmission through the mains and expenses of connecting
up to the consumer. The actual manufacturing cost of a station
maintaining 10,000 lights should not be more than 3_d._ per Unit, or
equivalent to gas at 1_s._ 8½_d._ per 1000 cubic feet.[2]
[2] See Table IV., page 87.
As petroleum lamps are used for the street lighting of many foreign
and colonial towns, the question arises, “Will it pay to substitute the
electric light?” Comparing the light given by a kerosene or petroleum
lamp with that from the incandescent electric lamp, the cost is greatly
in favour of oil; and, in fact, where the price of kerosene is under
1_s._ per gallon, electricity cannot compete if labour is cheap. On the
other hand, the trimming, lighting, and keeping in order of a number of
lamps scattered over a large area greatly augments the working cost, to
which must be added the breakages of chimneys, expense of wicks, also
the danger of fire. It is the safety of electricity which has caused
it to supplant oil both for public and private lighting in American
cities; even where the price of kerosene is not more than 6_d._ per
gallon there is a demand for the electric light, which is by far the
dearer illuminant, after making a liberal allowance for labour in
cleaning, filling, and lighting the oil lamp, also for depreciation of
the burners.
_Arc and Incandescent or Glow Lighting._
[Illustration: FIG. 3.]
Electric lighting can be obtained by means of arc or incandescent
lamps. The arc light is now well understood to be caused by the
extremely high temperature of the end of one or both the carbon
electrodes. The voltaic arc, Fig. 3, is formed by the minute particles
of carbon in a high state of combustion which the current appears
to break off and carry from one electrode to the other, the light,
however, being mainly due to the incandescence of the crater shown in
Fig. 3 on the upper carbon. In the incandescent or glow lamp light
is produced by the passage of a current of electricity through a
continuous fine thread or filament of carbon which becomes white-hot,
the destruction of the filament being prevented through its enclosure
in a glass bulb from which the air is exhausted. Figs. 4 and 5.
The first method is suitable for the lighting of streets where a
high-class illumination is required; also will be wanted for the
external lighting of shops, public-houses, and places of amusement, so
that arrangements must be made for arc lighting. The usual plan is to
charge at the same rate per Unit by meter as the incandescent lamps,
but to make an additional charge of 5_s._ to 7_s._ 6_d._ per lamp per
quarter for rent, and a further charge of 3_s._ per week for cleaning
and trimming.
The principal types are the Edison and the Swan, Fig. 4 and Fig. 5.
Incandescent lamps can be obtained to order from 1½ candle-power
upwards, but the 16 candle-power (nominal 20) or the 8 candle-power
(nominal 10) lamps are almost invariably employed. The latter give the
best effect, and can be worked to 10 candle-power without much risk,
they take about 30 watts as against 60 watts for the 16 candles; and
are not uneconomical, for nearly double the number can be worked with
the same energy. A new type of glow lamp, called the “Sunbeam,” has
been recently introduced, which contains a thick filament, and gives a
light of from 200 to 1500 candle-power, and can be employed instead of
an arc lamp with the same economy as the ordinary 16 candle-power type.
[Illustration: FIG. 4. FIG. 5.]
_Life of Incandescent Lamps._
In estimating the annual cost of lighting, the renewals of lamps must
be taken into account; and although some lamps have worked 3000 or even
4000 hours, a life of 1000 working hours is the highest average it is
safe to assume in practical work under even the best conditions, that
is, using secondary batteries and never over running. The average
life of 130 lamps on H.M.S. troopship _Malabar_ was 3799 hours each,
the shortest life being 638½ hours for 18 yard-arm lamps of 32
candle-power. If the current is allowed to fluctuate, the average life
would be very much less; it is an unsettled question whether long-lived
lamps are really economical, by reason of the blackening of the globes,
which takes place after the lamp has been worked some time, and is
probably due to small particles of carbon thrown off from the filament
being deposited on the glass. It has been suggested that attrition of
the filament is going on all the time the lamp is at work, and that
the heated atoms striking against the filament may account for the
blackening, in that the mean free path of the atoms would be greater
in a perfect vacuum than in the air, consequently they would abrade
the filament with considerable force. If lamps were sold at 1_s._ each
instead of 3_s._ 6_d._, which is now the price for not less than a
thousand, it would be more economical to change them at the first signs
of blackening, even if the life did not exceed 500 hours.
The diagram, Fig. 6, has been so arranged that the amount of light
required in a given district can be ascertained for any period of the
day or night; it has been calculated from the observations taken daily
at one of the Berlin central-stations by the engineer to the company.
Six hundred and forty watts are assumed, for the purposes of the
diagram, to be the equivalent of a horse-power, instead of 736, as the
German electrical horse-power is 736 watts instead of 746 watts.
[Illustration: FIG. 6.]
The table, Fig. 6, has two vertical scales, A and B, each giving the
kilowatts[3] and corresponding horse-power. A is drawn to a scale ten
times greater than B, with the object of noting the smaller amount of
lights required for street illumination. The horizontal line is divided
into hours, and represents a day’s lighting in the middle of December
and the end of July, so as to show the maximum and minimum amount of
current that will be required. In the lighting of a town there are
two classes of illumination, the amount taken by the public, which is
uncertain, and that employed for street lighting, which is a known
quantity.
[3] 1000 watts.
The curves, II and II A, represent the private lighting of houses,
hotels, theatres, and shops of different kinds in December and in
July, the curve, II A, being in dotted lines clearly shows what a vast
difference there is in the amount of light, and consequently the amount
of energy required in the generating station, as compared with curve
II, which is taken when the days are longest.
The rectangles, I and I A, show the street illumination, and are drawn
to suit scale A; half an hour after sunset all the lamps are turned on,
and the work reaches its maximum suddenly, and continues the same until
12 o’clock, when, according to the municipal decrees, it either falls
one or two gradations until half an hour before sunrise, when all the
lamps are extinguished. The calculations are based on the assumption of
640 watts to the horse-power, instead of 736, which is the theoretical
efficiency of a German horse-power.
If a number of diagrams are taken on this method for different periods
of the year, the constant work can be ascertained. This knowledge is
most valuable when calculating the most economical area for the mains,
which is then easily accomplished by means of Forbes’ tables, which are
based on Sir William Thomson’s well-known rule.
The lines, 2 and 2 A show the constant work at the same two
periods of the year from which the diagrams are taken. The constant
work at the end of December will be found to amount to 20 per cent.
of the total work, and that at the middle of June to 15 per cent. By
summing up the average work for all the days in the year we obtain the
cost per annum, and adding to this the expense of management, interest,
&c., and knowing the local conditions, we can fix what proportion of
the day’s work is admissible as loss. With the Edison system at Berlin,
5 per cent. is taken as average loss; thus, at the end of December,
it amounts, with the maximum number of lights, to 18·8 per cent., and
with the minimum to 1·1 per cent.; in the middle of July the maximum
is 15·8 per cent., and the minimum 0·5 per cent. The dynamos must, of
course, be of sufficient power to be able to overcome this loss, which
only shows itself periodically; therefore the generating plant may
be constructed to give, nominally, 20 to 30 per cent. less than the
maximum work, and be capable of being pushed to the full amount for a
short time only.