J. A. (John Ambrose) Fleming.

Electric lamps and electric lighting : a course of four lectures on electric illumination delivered at the Royal Institution of Great Britain online

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Online LibraryJ. A. (John Ambrose) FlemingElectric lamps and electric lighting : a course of four lectures on electric illumination delivered at the Royal Institution of Great Britain → online text (page 1 of 16)
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The Illustrations, Designs, and Text of these Lectures are entered at Stationers'
Hall, and are, therefore, copyright.




HE following pages are a trans-
cript, with some additions, of
a course of Four Lectures on
"Electric Illumination," deli-
vered by the Author to after-
noon audiences at the Royal
Institution, London. ' They are,
however, in no sense presented
as an adequate treatment of a
subject which has long since
outgrown the limits of a lecture,
or even of a course of lectures.

The original aim was merely to offer to a general
audience such non-technical explanations of the physical
effects and problems concerned in the modern applica-
tions of electricity for illuminating purposes as might
serve to further an intelligent interest in the subject,
and perhaps pave the way for a more serious study of
it. The growth of electric lighting is assisted by the
diffusion of all accurate (even if simple) information, on



the mode of production and employment of electric
currents for illuminating and other purposes.

Many of those who were present at the Lectures
expressed a desire to be again able to refer to the
descriptions and illustrations then given, and at their
request, and in the hope that they may be of use to
others, the lecture notes have been revised and pub-
lished. The aim throughout has been rather to deal
with principles than with details, and to give such
general and guiding explanations of terms and processes
as are essential for a grasp of the outlines of the sub-
ject. For this purpose simple drawings have been
given of as many of the experiments and apparatus
used as possible. For permission to reproduce a set
of photographs of views of the interior of rooms
artistically illuminated with incandescent lamps, and
which are to be found in the second Lecture, the author
is indebted to Mr. A. F. Davies, under whose guidance
these particular instances of electric lighting were
carried out.

J. A. F.

University College, London,
October, 1894.


(For Index to Contents, see page 225,)



ELECTBIC MEASUREMENTS ... ... ... ... 1 50

14 liberations.


ELECTRIC GLOW LAMPS ... ... ... ... 51 126

40 Illustrations.




11 Illustrations.


28 Illustrations.


... 171222


The reader is requested to make the follow in<j corrections:

Page 15, line 10 from top

instead of "movement of fixed plates,"

read "movement of suspended plates."

Page 69, line 4 from bottom

instead of " If a small piece of wire,"

read " If a small piece of iron wire."

Page 206, line 8 from bottom

instead of " 90lb. on the square inch,"
read " 60lb. on the square inch."

Page 207, bottom line

instead of "one of these larger machines,"
read "one of these smaller machines."


ELECTRIC LIGHTING in Great Britain. A Glance Backward over Fourteen
Years. Present Condition of Electric Lighting An Electric Current.
Its Chief Properties. Heating Power. Electrical Resistance.
Xanies of Electrical Units. Chemical Power of an Electric Current.
Hydraulic Analogies. Electric Pressure. Fall of Electric Pressure
down a Conductor. Ohm's Law. Joule's Law. Units of Work and
Power. The Watt as a Unit of Power. Incandescence of a Platinum
Wire. Spectroscopic Examination of a Heated Wire. Visible and
Invisible Radiation. Luminous Efficiency. Radiation from Bodies at
Various Temperatures. Efficiency of Various Sources of Light. The
Glow Lamp and Arc Lamp as Illuminants. Colours and Wave-
Lengths of Rays of Light. Similar and Dissimilar Sources of Light.
Colour-distinguishing Power. Causes of Colour. Comparison of
Brightness and Colour. Principles of Photometry. Limitations due
to the Eye.- Luminosity and Candle-power. Standards of Light.
Standards of Illumination. The Candle-foot. Comparison of Sun-
light and Moonlight. Comparison of Lights. Ritchie's Wedge.
Rumfordand Bunsen Photometers. Comparison of Lights of Different
Colours. Spectro-photometers. Results of Investigations.

HOEVER undertakes to describe the re-
markable progress during the last two
decades of the art of electric illumination
must certainly direct attention to three
important dates which, in England at any
rate, formed turning points in the history
of this application of scientific knowledge.
^ In the year 1879 the Government of our

country had its attention directed for the first time to electric
lighting as a possible subject of legislation, and referred the
whole matter to a Select Committee of the House of Commons,
of which Lord Playfair was appointed Chairman. This Com-
mittee met, and took voluminous evidence from numerous and


various experts, but the broad conclusion reached in the report
which it finally presented was generally the expression of
opinion that there was no reasonable scientific ground at that
date for supposing that domestic electric lighting had obtained
a sufficient footing to entitle it to be described as a practical
success, and that, therefore, there seemed no useful result to be
attained by interfering at once, by special legislation, with
electric lighting. Passing over a gap of three years, we find
in the year 1882 the whole aspect of affairs entirely changed
by the completed invention of the electric glow lamp. At
the beginning of that year the first Crystal Palace Electrical
Exhibition enabled the public to properly appreciate the
extent to which the then perfected electric incandescence
lamp had revolutionised artificial lighting, and also the charm
and beauty of that illuminant. Schemes were rapidly put
forward for the supply of electric current from public
generating stations for the purpose of electric lighting,
and the enthusiasm which was thus awakened in the public
mind led inventors and promoters to prognosticate a very
speedy and entire revolution in the art of illumination. As a
result, immediate legislative action was taken to control
the public supply of electric current. In 1882 a Bill was
introduced into the House of Commons entitled "An Act
for Facilitating Electric Lighting." We need not pause to
enumerate the various causes which interfered with the
immediate fulfilment of the sanguine expectations which were
formed in 1882 as to the immediate future of electric lighting.
Practical difficulties presented themselves the moment that
many of the immature schemes which were then launched
were attempted to be put into practice. The dynamo machine
in some of its forms had hardly emerged from the condition
of being a large laboratory or workshop instrument, and
mechanical engineers bad not yet brought to bear upon it
that knowledge which subsequently enabled them to convert it
into a most efficient and trustworthy machine for generating


electric current for public electric supply. Countless details
remained to be perfected in the many arrangements for dis-
tributing, using, and measuring electric current so produced.
From the commercial point of view much information had
to be slowly accumulated before even approximately correct
opinions could be formed as to the revenue to be gained from
the sale of electric energy for domestic purposes, and there
was at that date but little information available for enabling an
accurate forecast to be made concerning the probable average
annual consumption of electrical energy by incandescent lamps
when used instead of gas jets in different classes of buildings for
illuminating purposes. There is no doubt, however, that the
Act of 1882, though much abused at the time, performed the
important function of preventing the survival of the unfit.

Between 1882 and 1888 exceedingly important improvements
(some of which it will be necessary later on to examine) were
made, and in 1888 the time seemed ripe for a fresh forward
movement. This was effected by the pressure brought upon
the Legislature to repeal one of the clauses in the Act of 1882,
by which revision much more favourable conditions were
created for inviting the support of capital ; and as soon as the
Electric Lighting Amendment Act of 1888 was an accomplished
fact, a very important inquiry was held by the Board of Trade
in May, 1889, in the Westminster Town Hall, London, under
the chairmanship of Major Marindin. At this inquiry, which
lasted for eighteen days, the whole subject of electric
lighting by public supply, especially with reference to the
needs of London, was carefully debated by many of the
leading scientific and legal experts, and, as a result, the
Metropolis was divided up into certain areas of electric
supply, and conditions were laid down under which the
distribution of current might be undertaken either by Public
Companies or by the Local Authorities. Up to the date
of that inquiry the total amount of electric lighting in London

B 2


or in the Provinces had been, comparatively speaking, very
small. From and after that date it has advanced by leaps and
bounds. The progress made in the use of the incandescent
lamp as a means of artificial illumination is shown by the
following figures, and graphically indicated in Fig. 1.

At the end of 1890 there were probably in use about
180,000 incandescent lamps in London, but at the end of 1892
some 500,000 were being employed, and at the end of 1893

1.000,000 8-c.p. Lamps Connected





400,000- /

300,000 _ri<V1






100,000 ^f" x *"^




1880 1890 1891 1892 1893

FIG. 1. Diagram showing the Progress of Electric Lighting in London
and the Provinces in Five Years. The altitude of the vertical black lines
represents to scale the growth, from year to year, in the total number of
lamps supplied.

700,000. In the provinces the grand totals at the end
of 1892 and 1893 were probably 147,000 and 425,000.
Hence, while in 1888 the total number of incandescent lamps
in use in the United Kingdom probably hardly exceeded
100,000, even if so many, we find that at the end of 1893
there was a grand total of rather more than 1,125 000 electric
glow lamps in use in the United Kingdom. There are at the


present time (1894) some thirteen or fourteen Companies and
Corporations supplying electric current for lighting purposes
in London, and these have a total length of more than
250 miles of copper mains laid down under the streets. In the
Provinces there are between 60 and 70 towns in which a
similar public electric supply is given, either by the Local
Authority or by a Public Company, and these "undertakers,"
as they are called in the Act, have a total length of probably
more than 400 miles of mains in use at present. Some idea
may be formed of the extent to which electric lighting has
proceeded in the United Kingdom in the five years which
have elapsed since the Board of Trade inquiry was held at
the Westminster Town Hall in 1889, by picturing to ourselves
an underground electric main or pair of conductors of copper
stretching across Great Britain from Penzance to Perth, with
an 8-candle-power incandescent lamp placed every yard along
this distance. This would roughly represent the total usage
of electric lamps and length of electric mains at the present
date (1894).

The 700,000 incandescent electric lamps in London have
probably already begun to make their presence felt by the Gas
Companies. If these 700,000 glow lamps were replaced by gas
jets of equal illuminating power, their annual consumption
of gas might at least amount to about 1,000 million cubic feet.
The average rate of increase of the metropolitan gas consump-
tion between 1886 and 1891 was 947 million cubic feet, the
increase for 1891 alone being 1,313 millions. In 1892, how-
ever, the year when the London Electric Lighting Companies
got fairly to work, this increase fell to 35| millions ; and in
1893 the consumption was less by 1,007 million cubic feet.
The year 1893 was unusually bright and sunny in the summer
and free from fog in winter in London. Hence this cause
amongst others may have operated to arrest the growth of gas,
but it is clear that a stage has now been reached in which the


older illuminant will begin to feel the competition of the
younger. An industry which has progressed with such rapid
strides, and which in the short space of half a decade has made
electric lighting in large towns one of the necessary luxuries
of life, is not likely to be arrested in its present stage ; and as
it has, in its various aspects, not only much scientific and
artistic, but also considerable practical interest for every
householder, your attention in this and the three succeeding
lectures will be directed to the elucidation of facts which
ought to be known by every user of the electric light, and the
knowledge of which will enable him to understand something
of the principles which underlie the art of electric illumination,
and to comprehend as well the aid which it can render, when
properly applied, to beautify and please.

It will be necessary to open the whole of our discussion by
some simple illustrations of the meaning of fundamental terms.
Every science as well as every art has its necessary technical
terms, and even if these words at first sound strangely, they are
not therefore necessarily difficult to understand. We are all
familiar with the fact that electric illumination depends upon
the utilisation of something which we call the electric current.
Little by little scientific research may open up a pathway
towards a fuller understanding of the true nature of an electric
current, but at the present moment all that we are able to
say is that we know of what it can do, how it is produced,
and the manner in which it can properly be measured. Two
principal facts connected with it are, that when a conductor,
such as a metallic wire or a carbon filament, or any other
material which is capable of being employed as a conductor, is
traversed by an electric current, heat is generated in the con-
ductor, and the space round the conductor becomes capable
of influencing a magnetic needle. These facts can be simply
illustrated by passing an electric current through an iron wire
thus (Fig. 2). You will notice that as the current is gradually


increased the iron wire is brought up from a condition in
which it is only slightly warm to one in which it becomes
visibly red hot in the dark, and finally brilliantly incandescent.
At the same time if I explore the region round about the wire
with a suspended compass needle, we find that at every point
in the neighbourhood of the wire the magnetic needle places
itself, or tries to place itself, in a position perpendicular to the
wire. This fact, of capital importance, was discovered by

FIG. 2. An iron wire W is rendered incandescent by an electric current
sent through it from a battery B. The magnetic needle N S held near it
sets itself across the wire.

H. C. Oersted in 1820, and in the Latin memoir in which he
describes this epoch-making discovery he employs the following
striking phrase to express the behaviour of a magnetic needle
to the wire conveying the current. He says, " The electric
conflict performs circles round the wire ; " and that which he
called the electric conflict round the wire, we now in more
modern language call the magnetic field embracing the con-
ductor. We shall return in a later lecture to this last fact.


Meanwhile I wish at present to fasten your attention on the
heating qualities of an electric current, and the laws of that
heat production and radiation. The same electric current
produces heat at very different rates in different conductors,
and the quality of a body in virtue of which the electric current
produces heat in passing through it is called its electric
resistance. If the same electric current is passed through con-
ducting wires of similar dimensions, but of different materials,
it produces in them different quantities of heat in the same
time. Before you (Fig. 3) is a chain composed of spirals
of iron and copper wire. These wires are each of the same
length and of the same diameter. Sending through this com-

FIG. 3. A chain W composed of alternate spirals of copper C and iron I
is traversed by an electric current sent through it by a battery B. The
iron links become red hot, the copper links only slightly warm.

pound chain an electric current, we notice that the iron wire
links are very soon brought up to a bright red heat, whilst the
copper links, though slightly warm, are not visibly hot. We
have, therefore, before us an illustration of the fact that a
current heats the conductor, but that each conductor has a
specific quality called its electrical resistance, in virtue of which
the same strength of current produces heat in it at a rate
depending on the nature of the material. Other things being
equal, the bodies which are most heated are said to have
the highest resistance.


It is now necessary to notice the units in which these two
quantities, namely, electric current and electric resistance, are
measured. For the sake of distinction, units of electric
quantities are named after distinguished men. We follow a
similar custom in some respects in common life, as when we
speak of a " Gladstone " bag or a " Hansom " cab, and
abbreviate these terms into a gladstone and a hansom.
Primarily the distinctive words here used are the names of
persons, but by application and abbreviation they become the
names of things. An electric current is measured in terms of
a unit current which is called an ampere, and electrical
resistance is measured in terms of a unit which is called an
ohm, these being respectively named after two great investi-

FIG. 4. Glass lantern trough containing two lead plates n p, and a
solution of sugar of lead. When a current from a battery B is sent through
the cell it deposits the lead in tufts on the negative plate n.

gators, Andre Marie Ampere and Georg Simon Ohm. In
order to understand the mode in which an electric current
can thus be denned, we must direct attention to another
property of electric currents, namely, their power of decom-
posing solutions of metallic salts. You are all familiar with
the substance which is called sugar of lead, or, in chemical
language, acetate of lead. Placing in a small glass trough
a solution of acetate of lead and two lead plates (see Fig. 4), I
place the cell in the electric lantern and project the image
upon the screen. If an electric current is passed through
the solution from one lead plate to the other it decomposes


the solution of acetate of lead, extricating from the solution
molecules of lead and depositing them on one of the lead
plates, and you see the tufted crystals of lead being built up
in frond-like form on the negative pole in the cell. We might
employ, in preference to a solution of acetate of lead, a solution
of nitrate of silver, which is the basis of most marking inks,
and the same effect would be seen. It was definitely proved
by Faraday that we might define the strength of an electric
current by the amount of metal which it extricates from the
solution of a metallic salt in one second, minute, or hour. The
Board of Trade Committee on Electrical Standards have now
given a definition of what is to be understood by an electric
current of one ampere in the following terms : An electric
current of one ampere is a current which will in one hour
extricate from a solution of nitrate of silver 4-025 grammes
of silver.* Otherwise we might put it in this manner : A
current of electricity is said to have a strength of one ampere
if, when passed through a solution of nitrate of silver, it
decomposes it and deposits on the negative plate one ounce of
silver in very nearly seven hours. We are acquainted in the
laboratory with currents of electricity so small that they
would take 100,000 years of continuous action to deposit
one ounce of silver, and we are familiar in electric lighting
practice with currents great enough to deposit one hundred-
weight of silver in thirty minutes. The simple experiment
just shown is the basis of the whole art of electro-plating.
Hence, when we speak later of a current of one ampere, or ten
amperes you will be able to realise in thought precisely what
such a current is able to achieve in chemical decomposition.
It may be convenient at this stage to bring to your notice the
fact that an 8 candle-power incandescent lamp working at 100
volts usually takes a current of about one-third of an ampere,
a current which would deposit by electro -plating action one
ounce of silver in about twenty- one hours.

* 28'3495 grammes 1 ounce avoirdupois.



We pass next to consider another important matter, viz.,
that of electric pressure or potential ; and we shall be helped
in grasping this idea by considering the corresponding concep-
tion in the case of the flow of fluids. When a fluid such as
water flows along a pipe it does so in virtue of the fact that
there is a difference of pressure between different points in
the pipe, and the water flows in the pipe from the place where
the pressure is greatest to the place where the pressure is
least. On the table before you is a horizontal pipe (Fig. 5)
which is connected with a cistern of water, and which delivers

FIG. 5. Horizontal pipe P, having six vertical gauge tubes attached to
it. The pipe P is in connection with a water cistern C, and when the tap
T is shut the water stands up at the same level, A B, in all the tubes.

water to another receptacle at a lower level. In that pipe are
placed a number of vertical glass tubes to enable us to measure
the pressure in the pipe at any instant. The pressure at the
foot of each gauge glass is exactly measured by the head or
elevation of the water in the vertical gauge glass, and at the
present moment, when the outlet from the horizontal pipe is
closed, you will notice that the water in all the gauge glasses
stands up to the same height as the water in the cistern. In
other words, the pressure in the pipe is everywhere the same.


Opening the outlet tap we allow the water to flow out from
the pipe, and you will then observe that the water sinks (see
Fig. 6) in each gauge glass, and, so far from being now uniform
in height, there is seen to be a regular fall in pressure
along the pipe, the gauge glass nearest the cistern showing
the greatest pressure, the next one less, the next one less
still, and so on, the pressure in the horizontal pipe gradually
diminishing as we proceed along towards the tap by which
the water is flowing out. This fall in pressure along the pipe
takes place in every gas and water pipe, and is called the

FIG. 6. Horizontal pipe P, through which water is flowing from a
cistern C to a reservoir R. When the tap T is open the water stands at
gradually decreasing heights in the pressure tubes. The dotted line A B
shows the hydraulic gradient.

hydraulic gradient in the pipe. The flow of water takes
place in virtue of this gradient of pressure. It will be next
necessary to explain to you that there is an exactly similar
phenomenon in the case of an electric current in a wire, and
that there is a quantity which we may call the electric pres-
sure, which diminishes in amount as we proceed along the
wire when the current is flowing in it. In order to under-
stand the manner in which this electric pressure can be



measured, a few preliminary experiments will be essential.
Every body which is charged with electricity has, in virtue of
that charge, a certain electrical potential, or pressure, as it is

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Online LibraryJ. A. (John Ambrose) FlemingElectric lamps and electric lighting : a course of four lectures on electric illumination delivered at the Royal Institution of Great Britain → online text (page 1 of 16)