MEASURING ELECTRICITY
There are many people whose acquaintance with electricity consists
mainly in paying the electric light bill. To such the instruments
whereby electricity is measured will make a specially interesting
appeal.
Current is sold in Great Britain at so much per Board of Trade Unit. To
state what that is needs a preliminary explanation of the other units
employed in connection with electric currents.
The public electricity supply in any district is announced to be so many
volts, it may be 100, 200 or perhaps 230, but whatever it be, it is
always so many "volts." Then the electrician speaks lightly of numbers
of "amperes," he may even talk of the number of "watts" used by the
lamps, while occasionally the word "ohm" will leak out. Among these
terms the general reader is apt to become completely fog-bound. But
really they are quite simple if once understood, and, as we shall see in
a moment, there are some very remarkable instruments for measuring them,
some of which exhibit a delicacy truly astonishing.
It is well at the outset to try and divest ourselves of the idea that
there is anything mysterious or occult about electricity. It is quite
true that there are many things about it very little understood even by
the most learned, but for ordinary practical purposes it may be regarded
as a fluid, which flows along a wire just as water flows along a pipe.
The wire is, electrically speaking, a "hole" through the air or other
non-conducting substance with which it is surrounded. A water-pipe being
a hole through a bar of iron, so the copper core of an electrical wire
is, so far as the current is concerned, but a hole through the centre of
a tube of silk, cotton, rubber or whatever it be. Electricity can flow
through certain solids just as water can flow through empty space.
Water will not flow through a pipe unless a pressure be applied to it
somewhere. In a pipe the ends of which are at the same level water will
lie inert and motionless. Lower one end, however, and the pressure
produced by gravity--in other words, the weight of the water--will cause
it to move. In like manner pressure produced by the action of a pump
will make water flow. On the other hand, when it moves it encounters
resistance, through the water rubbing against the walls of the pipe.
Similarly, an electrical pressure is necessary before a current of
electricity will flow. And every conductor offers more or less
resistance to the flow of current, thus opposing the action of the
pressure. Before current will flow through your domestic glow-lamps and
cause them to give light there must be a pressure at work, and that
pressure is described as so many volts.
A battery is really a little automatic electrical pump for producing an
electrical pressure. And the volt, which is a legal measure, just as
much as a pound or a yard, is a certain fraction of the pressure
produced by a certain battery known as Clark's Cell. It is not necessary
here to say exactly what that fraction is, but it will give a general
idea to state that the ordinary Leclanche or dry cell, such as is used
for electric bells, produces a pressure of about one and a half volts.
Thus we see the volt is the electrical counterpart of the term "pound
per square inch" which is used in the case of water pressure.
A flow of water is measured in gallons per minute. An electrical current
is measured in coulombs per second. Thus the coulomb is the electrical
counterpart of the gallon. But in this particular we differ slightly in
our methods of talking of water and electricity. Gallons per minute or
per hour is the invariable term in the former case, but in the latter
we do not speak of coulombs per second, although that is what we mean,
for we have a special name for one coulomb per second, and that same is
ampere. One ampere is one coulomb per second, two amperes are two
coulombs per second, and so on.
There is no recognised term to denote the resistance which a water-pipe
offers to the passage of water through it, but in the similar case with
electricity there is a term specially invented for the purpose, the ohm.
Legally it is the resistance of a column of mercury of a certain size
and weight. A rough idea of it is given by the fact that a copper wire a
sixteenth of an inch thick and 400 feet long has a resistance of about
one ohm.
The three units--the volt, ampere and ohm--are so related that a
pressure of one volt acting upon a circuit with a resistance of one ohm
will produce a current of one ampere.
A current can do work; when it lights or heats your room or drives a
tramcar it is doing work; and the rate at which a current does work is
found by multiplying together the number of volts and the number of
amperes. The result is in still another unit, the watt. And 1000 watts
is a kilowatt. Finally, to crown the whole story, a kilowatt for one
hour is a Board of Trade unit.
So for every unit which you pay for in the quarterly bill you have had a
current equal to 1000 watts for an hour. To give a concrete example, if
the pressure of your supply is 200 volts, and you take a current of five
amperes for an hour, you will have consumed one B.T.U.
Perhaps it will give added clearness to this explanation to tabulate the
terms as follow:--
_Volt_ = The unit of pressure, analogous to "pounds per square
inch" in the case of water.
_Coulomb_ = The measure of quantity, analogous to the gallon.
_Ampere_ = The measure of the "strength" of a current, meaning one
coulomb per second.
_Watt_ = The unit denoting the power for work of any current. It is
the result of multiplying together volts and amperes.
_Kilowatt_ = 1000 watts.
_Board of Trade Unit_ = A current of one kilowatt flowing for one
hour.
[Illustration: _By permission of Dupont Powder Co._
A FINE CROP
Celery grown on soil tilled by dynamite.--_See_ p. 24]
In practice the measurements are generally made by means of the
connection between electricity and magnetism. A current of electricity
is a magnet. Whenever a current is flowing it is surrounded by a region
in which magnetism can be felt. This region is called the magnetic
field, and the strength of the field varies with the strength that is
the number of amperes in the current. If a wire carrying a current be
wound up into a coil it is evident that the magnetic field will be more
intense than if the wire be straight, for it will be concentrated into a
smaller area. Iron, with its peculiar magnetic properties, if placed in
a magnetic field seems to draw the magnetic forces towards itself, and
consequently, if the wire be wound round a core of iron, the magnetism
due to the current will be largely concentrated at the ends of the core.
But the main principle remains--in any given magnet the magnetic power
exhibited will be in proportion to the current flowing.
The switchboard at a generating station is always supplied with
instruments called ammeters, an abbreviation of amperemeters, for the
purpose of measuring the current passing out from the dynamos. Each of
these consists of a coil of wire through which the current passes. In
some there is a piece of iron near by, which is attracted more or less
as the current varies, the iron being pulled back by a spring and its
movement against the tension of the spring being indicated by a pointer
on a dial.
In others the coil itself is free to swing in the neighbourhood of a
powerful steel magnet, the interaction between the electro-magnet, or
coil, and the permanent magnet being such that they approach each other
or recede from each other as the current varies. A pointer on a dial
records the movements as before.
In yet another kind the permanent magnet gives way to a second coil, the
current passing through both in succession, the result being very much
the same, the two coils attracting each other more or less according to
the current.
Another kind of ammeter known as a thermo-ammeter works on quite a
different principle. It consists of a piece of fine platinum wire which
is arranged as a "shunt"--that is to say, a certain small but definite
proportion of the current to be measured passes through it. Now, being
fine, the current has considerable difficulty in forcing its way through
this wire and the energy so expended becomes turned into heat in the
wire. It is indeed a mild form of what we see in the filament of an
incandescent lamp, where the energy expended in forcing the current
through makes the filament white-hot. The same principle is at work when
we rub out a pencil mark with india-rubber, whereby the rubber becomes
heated, as most of us have observed. The wire, then, is heated by the
current passing through it, and accordingly expands, the amount of
expansion forming an indication of the current passing. The elongation
of the wire is made to turn a pointer.
A simple modification makes any of these instruments into a voltmeter.
This instrument is intended to measure the force or pressure in the
current as it leaves the dynamo.
A short branch circuit is constructed, leading from the positive wire
near the dynamo to the negative wire, or to the earth, where the
pressure is zero. In this circuit is placed the instrument, together
with a coil made of a very long length of fine wire so that it has a
very great resistance. Very little current will flow through the branch
circuit because of the high resistance of the coil, but what there is
will be in exact proportion to the pressure. The voltmeter is therefore
the same as the ammeter, except that its dial is marked for volts
instead of for amperes, and it has to be provided with the resistance
coil.
Instruments of the ammeter type can also be used as ohmmeters. In this
case what is wanted is to test the resistance of a circuit, and it is
done by applying a battery, the voltage of which is known, and seeing
how much current flows.
All the voltmeters and ohmmeters mentioned owe their method of working
to what is known as Ohm's law. One of the greatest steps in the
development of electrical science was taken when Dr Ohm put forward the
law which he had discovered whereby pressure, current and resistance are
related. The reader will probably have noticed from what has already
been said about the units of measurement--the volt, the ampere and the
ohm--that the current varies directly as the pressure and inversely as
the resistance. That is the famous and important "Ohm's law" and anyone
who has once grasped that has gone a long way towards understanding many
of the principal phenomena of electric currents.
But the instruments so far referred to are of the big, clumsy type,
suitable for measuring large currents and great pressures. They are like
the great railway weigh-bridges, which weigh a whole truck-load at a
time and are good enough if they are true to a quarter of a
hundredweight. The instruments about to be described are more comparable
with the delicate balance of the chemist, which can detect the added
weight when a pencil mark is made upon a piece of paper. Indeed beside
them such a balance is quite crude and clumsy. They may be said to be
the most delicate measuring instruments in existence.
We will commence with the galvanometer. The simplest form of this is a
needle like that of a mariner's compass very delicately suspended by a
thin fibre in the neighbourhood of a coil of wire. The magnetic field
produced by the current flowing in the wire tends to turn the needle,
which movement is resisted by its natural tendency to point north and
south. Thus the current only turns the needle a certain distance, which
distance will be in proportion to its strength. The deflection of the
needle, therefore, gives us a measure of the strength of the current.
But such an instrument is not delicate enough for the most refined
experiments, and the improved form generally used is due to that prince
of inventors, the late Lord Kelvin. He originally devised it, it is
interesting to note, not for laboratory experiments, but for practical
use as a telegraph instrument in connection with the early Atlantic
cables.
Before describing it, it may sharpen the reader's interest to mention a
wonderful experiment which was made by Varley, the famous electrician,
on the first successful Atlantic cable. He formed a minute battery of a
brass gun-cap, with a scrap of zinc and a single drop of acidulated
water. This he connected up to the cable. Probably there is not one
reader of this book but would have thought, if he had been present, that
the man was mad. What conceivable good could come of connecting such a
feeble source of electrical pressure to the two thousand miles of wire
spanning the great ocean; the very idea seems fantastic in the extreme.
Yet that tiny battery was able to make its power felt even over that
great distance, for the Thomson Mirror Galvanometer was there to detect
it. Two thousand miles away, the galvanometer felt and was operated by
the force generated in a battery about the size of one of the capital
letters on this page.
This wonderful instrument consisted of a magnet made of a small fragment
of watch-spring, suspended in a horizontal position by means of a thread
of fine silk, close to a coil of fine wire. When current flowed through
the coil the magnetic field caused the watch-spring magnet to swing
round, but when the current ceased the untwisting of the silk brought it
back to its original position again.
So far it seems to differ very little from the ordinary galvanometer
previously mentioned, but the stroke of genius was in the method of
reading it. With a small current the movement of the magnet was too
small to be observed by the unaided eye, so it was attached to a minute
mirror made of one of those little circles of glass used for covering
microscope slides, silvered on the back. The magnet was cemented to the
back of this, yet both were so small that together their weight was
supported by a single thread of cocoon silk. Light from a lamp was made
to fall upon this mirror, thereby throwing a spot of light upon a
distant screen. Thus the slightest movement of the magnet was magnified
into a considerable movement of the spot of light. The beam of light
from the mirror to the screen became, in fact, a long lever or pointer,
without weight and without friction.
The task of watching the rocking to and fro of the spot of light was
found to be too nerve-racking for the telegraph operators, and so Lord
Kelvin improved upon his galvanometer in two ways. He first of all
managed to give it greater turning-power, so that, actuated by the same
current, the new instrument would work much more strongly than the older
one. Then he utilised this added power to move a pen whereby the signals
were recorded automatically upon a piece of paper. The new instrument is
known as the Siphon Recorder.
The added power was obtained by turning the instrument inside out, as it
were, making the coil the moving part and the permanent magnet the fixed
part. This enabled him to employ a very powerful permanent magnet in
place of the minute one made of watch-spring. The interaction of two
magnets is the result of their combined strength, and that of the coil
being limited by the strength of the minute current the only way to
increase the combined power of the two was to substitute a large
powerful magnet for the small magnetised watch-spring. This large magnet
would, of course, have been too heavy to swing easily and therefore the
positions had to be reversed.
So now we have two types of galvanometer, both due originally to the
inventions of Lord Kelvin. For some purposes the Thomson type (his name
was Thomson before he became Lord Kelvin) are still used, but in a
slightly elaborated form. Its sensitiveness is such that a current of a
thousandth of a micro-ampere will move the spot of light appreciably.
And when one comes to consider that a micro-ampere is a millionth part
of an ampere this is perfectly astounding.
But there is a more wonderful story still to come, of an instrument
which can detect a millionth of a micro-ampere, or one millionth of a
millionth of an ampere. It is not generally known that we are all
possessors of an electric generator in the form of the human heart, but
it is so, and Professor Einthoven, of Leyden, wishing to investigate
these currents from the heart, found himself in need of a galvanometer
exceeding in sensitiveness anything then known. Even the tiny needles or
coils with their minute mirrors have some weight and so possess in an
appreciable degree the property of inertia, in virtue of which they are
loath to start movement and, having started, are reluctant to stop. This
inertia, it is easy to see, militates against the accurate recording of
rapid variations in minute currents, so the energetic Professor set
about devising a new galvanometer which should answer his purpose. This
is known as the "String Galvanometer."
[Illustration: FIG. 1.-This shows the principle of this wonderful
Galvanometer invented by Lord Kelvin in its latest form. Current enters
at _a_, passes round the coils, as shown by the arrows, and away at _b_.
A light rod, _c_, is suspended by the fine fibre, _d_, so that the eight
little magnets hang in the centres of the coils--four in each. The
current deflects these magnets and so turns the mirror, _m_, at the
bottom of the rod. At _e_ are two large magnets which give the little
ones the necessary tendency to keep at "zero."]
[Illustration: FIG. 2.--Here we see the working parts of the "String
Galvanometer," by which the beating of the heart can be registered
electrically. The current flows down the fine silvered fibre, between
the poles, _a_ and _b_, of a powerful magnet. As the current varies, the
fibre bends more or less.]
The main body of the instrument is a large, powerful electro-magnet, in
shape like a large pair of jaws nearly shut. Energised by a strong
current, this magnet produces an exceedingly strong magnetic field in
the small space between the "teeth" as it were. In this space there is
stretched a fine thread of quartz which is almost perfectly elastic. It
is a non-conductor, however, so it is covered with a fine coating of
silver. Silver wire is sometimes used, but no way has yet been found of
drawing any metallic wire so thin as the quartz fibre, which is
sometimes as thin as two thousandths of a millimetre, or about a
twelve-thousandth of an inch. A hundred pages of this book make up a
thickness of about an inch, so that one leaf is about a fiftieth of an
inch. Consequently the fibre in question could be multiplied 240 times
before it became as stout as the paper on which these words are printed.
The current to be measured, then, is passed through the stretched fibre
and the interaction of the magnetic field by which the fibre is then
surrounded, with the magnetic field in which it is immersed, causes it
to be deflected to one side. Of course the deflection is exceedingly
small in amount, and as it is undesirable to hamper its movements by the
weight of a mirror, no matter how small, some other means of reading the
instrument had to be devised. This is a microscope which is fixed to one
of the jaws, through a fine hole in which the movements of the fibre can
be viewed. Or what is often better still, a picture of the wire can be
projected through the microscope on to a screen or on to a moving
photographic plate or strip of photographic paper. In the latter case a
permanent record is made of the changes in the flowing current.
An electric picture can thus be made of the working of a man's heart. He
holds in his hands two metal handles or is in some other way connected
to the two ends of the fibre by wires just as the handles of a shocking
coil are connected to the ends of the coil. The faint currents caused by
the beating of his heart are thus set down in the form of a wavy line.
Such a diagram is called a "cardiogram," and it seems that each of us
has a particular form of cardiogram peculiar to himself, so that a man
could almost be recognised and distinguished from his fellows by the
electrical action of his heart.
The galvanometer has a near relative, the electrometer, the astounding
delicacy of which renders it equally interesting. It is particularly
valuable in certain important investigations as to the nature and
construction of atoms.
The galvanometer, it will be remembered, measures minute currents; the
electrometer measures minute pressures, particularly those of small
electrically charged bodies.
Every conductor (and all things are conductors, more or less) can be
given a charge of electricity. Any insulated wire, for example, if
connected to a battery will become charged--current will flow into it
and there remain stationary. And that is what we mean by a charge as
opposed to a current.
Air compressed into a closed vessel is a charge. Air, however
compressed, flowing along a pipe would be better described as a current.
Imagine one of those cylinders used for the conveyance of gas under
pressure and suppose that we desire to find the pressure of the gas with
which it is charged. We connect a pressure-gauge to it, and see what the
finger of the gauge has to say. What happens is that gas from the
cylinder flows into the little vessel which constitutes the gauge and
there records its own pressure.
And just the same applies with electrometers. Precisely as the
pressure-gauge measures the pressure of air or gas in some vessel, so
the electrometer measures the electrical pressure in a charged body.
Further, some of the charged bodies with which the student of physics is
much concerned are far smaller than can be seen with the most powerful
microscope. How wonderfully minute and delicate, therefore, must be the
instrument which can be influenced by the tiny charge which so small a
body can carry.
It will be interesting here to describe an experiment performed with an
electrometer by Professor Rutherford, by which he determined how many
molecules there are in a centimetre of gas, a number very important to
know but very difficult to ascertain, since molecules are too small to
be seen. This number, by the way, is known to science as "Avogadro's
Constant."
Everyone has heard of radium, and knows that it is in a state which can
best be described as a long-drawn-out explosion. It is always shooting
off tiny particles. Night and day, year in and year out, it is firing
off these exceedingly minute projectiles, of which there are two kinds,
one of which appears to be atoms of helium.
Some years ago, when radium was being much talked about and the names of
M. and Madame Curie were in everyone's mouth, little toys were sold, the
invention, I believe, of Sir William Crookes, called spinthariscopes.
Each of these consisted of a short brass tube with a small lens in one
end. Looking through the lens in a dark room, one could see little
splashes of light on the walls of the tube. Those splashes were caused
by a tiny speck of radium in the middle of the tube, the helium atoms
from which, by bombarding the inner surface of the tube, produced the
sparks.
Now if we can count those splashes we can tell how many atoms of helium
are being given off per minute. And if then we reckon how many minutes
it takes to accumulate a cubic centimetre of helium we can easily reckon
how many atoms go to the cubic centimetre. But the difficulty is to
count them.
So the learned Professor called in the aid of the electrometer. He could
not count all the atoms shot off, so he put the piece of radium at one
end of a tube and an electrometer at the other. Every now and then an
atom shot right through the tube and out at the farther end. And since
each of these atoms from radium is charged with electricity, each as it
emerged operated the electrometer. By simply watching the twitching of
the instrument, therefore, it was possible to count how many atoms shot
through the tube--one atom one twitch. And from the size and position of
the tube it was possible to reckon what proportion of the whole number
shot off would pass that way.
The result of the experiment showed that there are in a cubic centimetre
of helium a number of atoms represented by 256 followed by seventeen
noughts. And as helium is one of the few substances in which the
molecule is formed of but one atom, that is also the number of
molecules.
And now consider this, please. A cubic centimetre is about the size of a
boy's marble. That contains the vast number of molecules just mentioned.
And the electrometer was able to detect the presence of those _one at a
time_. Need one add another word as to the inconceivable delicacy of the
instrument.
In its simplest form the electrometer is called the "electroscope." Two
strips of gold-leaf are suspended by their ends under a glass or metal
shade. As they hang normally they are in close proximity. Their upper
ends are, in fact, in contact and are attached to a small vertical
conductor. A charge imparted to the small conductor will pass down into
the leaves, and since it will charge them both they will repel each
other so that their lower ends will swing apart. Such an instrument is
very delicate, but because of the extreme thinness of the leaves it is
very difficult to read accurately the amount of their movement and so to
determine the charge which has been given to them.
In a more recent improvement, therefore, only one strip of gold-leaf is
used, the place of the other being taken by a copper strip. The whole of
the movement is thus in the single gold-leaf, as the copper strip is
comparatively stiff, and it is possible to arrange for the movement of
this one piece of gold-leaf to be measured by a microscope.
The other principal kind of electrometer we owe, as we do the
galvanometers, to the wonderful ingenuity of Lord Kelvin. In this the
moving part is a strip of thin aluminium, which is suspended in a
horizontal position by means, generally, of a fine quartz fibre. Since
it is necessary that this fibre should be a conductor, which quartz is
not, it is electro-plated with silver. Thus a charge communicated to the
upper end of the fibre, where it is attached to the case, passes down to
the aluminium "needle," as it is called. Now the needle is free to swing
to and fro, with a rotating motion, between two metal plates carefully
insulated. Each plate is cut into four quadrants, the opposite ones
being electrically connected, while all are insulated from their nearest
neighbours. One set of quadrants is charged positively, and one set
negatively, by a battery, but these charges have no effect upon the
needle until it is itself charged. As soon as that occurs, however, they
pull it round, and the amount of its movement indicates the amount of
the charge upon the needle, and therefore the pressure existing upon the
charged body to which it is connected. The direction of its movement
shows, moreover, whether the charge be positive or negative.
A little mirror is attached to the needle, so that its slightest motion
is revealed by the movement of a spot of light, as in the case of the
mirror galvanometers. Instruments such as these are called "Quadrant
Electrometers."
My readers will remember, too, the "String Galvanometer" already
mentioned. The same idea has been adapted to this purpose. A fine fibre
is stretched between two charged conductors while the fibre is itself
connected to the body whose charge is being measured. The charge which
it derives from the body causes it to be deflected, which deflection is
measured by a microscope.
In all cases of transmission of electricity over long distances for
lighting or power purposes the currents are "alternating." They flow
first one way and then the other, reversing perhaps twenty times a
second, or it may be two hundred, or even more times in that short
period. Some electric railways are worked with alternating current, and
it is used for lighting quite as much as direct current and is equally
satisfactory.
In wireless telegraphy it is essential. In that case, however, the
reversals may take place _millions_ of times per second. Consequently,
to distinguish the comparatively slowly changing currents of a
"frequency" or "periodicity" of a few hundreds per second from these
much more rapid ones, the latter are more often spoken of as electrical
oscillations. And these alternating and oscillating currents need to be
measured just as the direct currents do. Yet in many cases the same
instruments will not answer. There has therefore grown up a class of
wonderful measuring instruments specially designed for this purpose, by
which not only does the station engineer know what his alternating
current dynamos are doing, but the wireless operator can tell what is
happening in his apparatus, the investigator can probe the subtleties of
the currents which he is working with, and apparatus for all purposes
can be designed and worked with a system and reason which would be
impossible but for the possibility of being able to measure the
behaviour of the subtle current under all conditions.
One trouble in connection with measuring these alternating currents is
that they are very reluctant to pass through a coil.
One method by which this difficulty can be overcome has been mentioned
incidentally already. I refer to the heating of a wire through which
current is passing. This is just the same whether the current be
alternating or direct.
One of the simplest instruments of this class has been appropriated by
the Germans, who have named it the "Reiss Electrical Thermometer,"
although it was really invented nearly a century ago by Sir William Snow
Harris. It consists of a glass bulb on one end of a glass tube. The
current is passed through a fine wire inside this bulb, and as the wire
becomes heated it expands the air inside the bulb. This expansion moves
a little globule of mercury which lies in the tube, and which forms the
pointer or indicator by which the instrument is read. As the temperature
of the wire rises the mercury is forced away from the bulb, as the
temperature falls it returns. And as the temperature is varied by the
passage of the current, so the movement of the mercury is a measure of
the current.
Another way is to employ a "Rectifier." This is a conductor which has
the peculiar property of allowing current to pass one way but not the
other. It thus eliminates every alternate current and changes the
alternating current into a series of intermittent currents all in the
same direction. Rectified current is thus hardly described by the term
continuous, but still it is "continuous current" in the sense that the
flow is always in the same direction, and so it can be measured by the
ordinary continuous current instruments. The difficulty about it is that
there is some doubt as to the relation between the quantity of rectified
current which the galvanometer registers and the quantity of alternating
current, which after all is the quantity which is really to be measured.
How the rectification is accomplished will be referred to again in the
chapter on Wireless Telegraphy.
But to return to the thermo-galvanometers, as those are termed which
ascertain the strength of a current by the heat which it produces, the
simple little contrivance of Sir William Snow Harris has more elaborate
successors, of which perhaps the most interesting are those associated
with the name of Mr W. Duddell, who has made the subject largely his
own. Besides their interest as wonderfully delicate measuring
instruments, these have an added interest, since they introduce us to
another strange phenomenon in electricity. We have just noted the fact
that electricity causes heat. Now we shall see the exact opposite, in
which heat produces electrical pressure and current. And the feature of
Mr Duddell's instruments is the way in which these two things are
combined. By a roundabout but very effective way he rectifies the
current to be measured, for he first converts some of the alternating
current into heat and then converts that heat into continuous current.
If two pieces of dissimilar metals be connected together by their ends,
so as to form a circuit, and one of the joints be heated, an electrical
pressure will be generated which will cause a current to flow round the
circuit. The direction in which it will flow will depend upon the metals
employed. The amount of the pressure will also depend upon the metals
used, combined with the temperature of the junctions. With any given
pair of metals, however, the force, and therefore the volume of current,
will vary as the temperature. Really it will be the difference in
temperature between the hot junction and the cold junction, but if we so
arrange things that the cold junction shall always remain about the
same, the current which flows will vary as the temperature of the hot
one. The volume of that current will therefore be a measure of the
temperature. Such an arrangement is known as a thermo-couple, and is
becoming of great use in many manufacturing processes as a means of
measuring temperatures.
In the Duddell Thermo-galvanometers, therefore, the alternating current
is first led to a "heater" consisting of fine platinised quartz fibre or
thin metal wires. Just above the heater there hangs a thermo-couple,
consisting of two little bars, one of bismuth and the other of antimony.
These two are connected together at their lower end, where they nearly
touch the heater, but their upper ends are kept a little apart, being
joined, however, by a loop formed of silver strip. This arrangement will
be quite clear from the accompanying sketch, and it will be observed
that the loop is so shaped that the whole thing can be easily suspended
by a delicate fibre which will permit it to swing easily, like the coil
in a mirror galvanometer.
It is indeed a swinging coil of a galvanometer formed with a single turn
instead of the many turns usual in the ordinary instruments, and it will
be noticed from the sketch that there is a mirror fixed just above the
top of the loop.
This coil, then, with the thermo-couple at its lower extremity, is hung
between the ends of a powerful magnet much as the fibre of the Einthoven
Galvanometer is situated. The alternating current to be measured comes
along through the heater. The heater rises in temperature. That warms
the lower end of the thermo-couple. Instantly a steady, continuous
current begins to circulate round the silver strip which forms the coil,
and that, acting just as the current does in the ordinary galvanometer,
causes the coil to swing round more or less, which movement is indicated
by the spot of light from the mirror. A current as small as twenty
micro-amperes (or twenty millionths of an ampere) can be measured in
this way.
Mr Duddell has also perfected a wonderful instrument called an
Oscillograph, for the strange purpose of making actual pictures of the
rise and fall in volume of current in alternating circuits.
[Illustration: FIG. 3.--The "Duddell" Thermo-galvanometer.
In this remarkable instrument _alternating_ current enters at _a_,
passes through the fine wire and leaves at _b_. In doing this it heats
the wire, which in turn heats the lower end of the bismuth and antimony
bars. This generates _continuous_ current, which circulates through the
loop of silver wire, _c_, which, since it hangs between the poles, _d_
and _e_, of a magnet, is thereby turned more or less. The amount of the
turning indicates the strength of the _alternating_ current.]
To realise the almost miraculous delicacy of these wonderful instruments
we need first of all to construct a mental picture of what takes place
in a circuit through which alternating current is passing. The current
begins to flow: it gradually increases in volume until it reaches its
maximum: then it begins to die away until it becomes nil: then it begins
to grow in the opposite direction, increases to its maximum and dies
away once more. That cycle of events occurs over and over again at the
rate it may be of hundreds of times per second. Now for the actual
efficient operation of electrical machinery working on alternating
current it is very necessary to know exactly how those changes take
place--do they occur gradually, the current growing and increasing in
volume regularly and steadily, or irregularly in a jumpy manner?
Engineers have a great fancy for setting out such changes in the form of
diagrams, in which case the alternations are represented by a wavy line,
and it is of much importance to obtain an actual diagram showing not
what the changes should be according to theory, but what they really are
in practice. It is then possible to see whether the "wave-form" of the
current is what it ought to be.
Once again we must turn our thoughts back to the string galvanometer. In
that case, it will be remembered, there is a conducting fibre passing
between the ends or poles of a powerful magnet, the result of which
arrangement is that as the current passes through the fibre it is bent
by the action of the magnetic forces produced around it. If the current
pass one way, downwards let us say, the fibre will be bent one way,
while if it pass upwards it will be bent the opposite way. Suppose then
that we have two fibres instead of one, and that we send the current up
one and down the other. One will be bent inwards and the other outwards.
Then suppose that we fix a little mirror to the centre of the fibres,
one side of it being attached to one fibre and the other to the other.
As one fibre advances and the other recedes the mirror will be turned
more or less. Consequently, as the current flowing in the fibres
increases or decreases, or changes in direction, the mirror will be
slewed round more or less in one direction or the other.
The spot of light thrown by the mirror will then dance from side to side
with every variation, and if it be made to fall upon a rapidly moving
strip of photograph paper a wavy line will be drawn upon the paper which
will faithfully represent the changes in the current.
In its action, of course, it is not unlike an ordinary mirror
galvanometer, but its special feature is in the mechanical arrangement
of its parts which enable it to move with sufficient rapidity to follow
the rapidly succeeding changes which need to be investigated. It is far
less sensitive than, say, a Thomson Galvanometer, but the latter could
not respond quickly enough for this particular purpose.