James Erskine Murray.

A handbook of wireless telegraphy; online

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in a telephone receiver in the local circuit.

From microscopic observations, Dr de Forest discovered
that the treelike crystals, which are formed by electrolysis
from various metallic solutions, are broken up by the action
of current jigs, but are immediately reformed by the current
from the local battery. It seems probable that the tips of



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136



WIRELESS TELEGRAPHY.




the lead trees are melted or deflagrated by the oscillatory
currents. The leaves of the trees are extremely thin plates,
coming to very sharp points ; as therefore the resistance
must be almost entirely localised at the tips of the leaves,
a very moderate amount of current will be sufficient to fuse

them and make the
circuit discontinuous.
The rapidity with
which the circuit
recovers continuity
proves that the
breaks in it must be
very minute, the
crystals growing
again immediately
under the influence
of the constant volt-
age applied by the
local battery. This
detector has been
found to be exceed-
ingly sensitive and
quite reliable.

The second prin-
cipal type of electro-
lytic detector has
only a liquid be-

FiG. 63.— Fessknden's Electrolytic Receiver, twecn the eleCtrOdeS,

OK barreter. one at least of which

A, Aerial ; B, Barreter ; £, Earth ; Ji, Battery ; - ,,

C, Galvanometer or Telephone. mUSt be VCry Small.

The best electrolyte
for signalling purposes appears to be nitric acid. A battery
and a telephone receiver are put in series with the detector.
If the terminals of the detector are also connected to air and
earth wires, signals may be received as with other forms of
detector. The invention appears to have been made inde-
pendently by Fessenden, Ferric, Schloemilch, and Vreeland.



o



^('H



61



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ELECTROLYTIC DETECTORS. 1 37

The thing is practically a small Wehnelt interrupter, in
which the applied voltage of the battery is too small to
maintain an appreciable current without the assistance of
the electric jigs. A Wehnelt break will not start work un-
less there is a certain minimum inductance in circuit This
condition means that the break will not start unless the
electricity is surging to and fro, since the effect of put-
ting inductance in a circuit is to make such revibration pos-
sible. Under these conditions quiet electrolysis of the
liquid gives place to a much larger but intermittent current,
suitable for use in the primary of an induction coil. Per-
haps the theory which best suits both cases is, that the
surges destroy the " polarisation " of the cell, and thus allow
a sudden rush of current from the battery which shortly
fails again owing to reformation of the film which constitutes
" polarisation.*' The sudden stoppage produces again elec-
trical surges in the circuit if the total resistance be low, as
in the Wehnelt break, which again destroy the polarisation.
In the detector, or barreter, as Professor Fessenden calls it,
the applied voltage is not high enough to keep up the inter-
mittent current by causing surges in the circuit when it
stops and starts, the current therefore only flows when surges
are impressed upon it by an outside agency, />., the jigs
transmitted to the detector by the aerial and earth wires.

There has been considerable difference of opinion as to
the mode of action of these detectors. Professor Fessenden
holding that it is due to variation of the resistance of the
electrolyte through heating by the current, while others
speak of " polarisation " as the only cause. One difficulty
in deciding between the claims of the rival theories is the
vagueness of the knowledge which we have of the actual
mechanism of polarisation ; also it must be observed that a
thin film of vapour produced by heat, and not electrolysis,
would appear to be equally effective in increasing the resist-
ance, and may account for the action of the Wehnelt break.
It seems possible that the actual molecular process in the
barreter may be otherwise explained by taking into account



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138



WIRELESS TELEGRAPHY.



Dr H. S. Sand's observations on the rapid increase of resist-
ance which takes place at an electrode very shortly after a
direct current is started in an electrolyte. Dr Sand * has
shown that even in a cell in which polarisation does not,
under ordinary conditions, occur, the resistance of the layer
of electrolyte nearest to the electrode is much less at the
commencement than it is when a steady state has been
attained, and he has proved that this rise of resistance is
due to the layer next the electrode becoming partially




Fig. 64.— Vacuum Valve used to Rectify Electric Oscillations and render

THBM DbTECTABLE IIY AN ORDINARY GALVANOMETER, G.

(From " Electric Wave Telegraphy," by kind permission of Prof. Fleming and Messrs
Longmans, Green, & Ca)

depleted of the ions which were in it before the E.M.F. was
applied. In the barreter this action must be superposed on
any others which are taking place, and may be the most
important of them all. The jig voltage, by creating ions
may cause increased conductivity, which is immediately
lost by continued application of the steady voltage of the
local circuit It is thus possible that the phenomenon may
be explainable without reference to films of gas or even
temperature variations of resistance.

* Proc, Faradtiy Soc, vol. i. p. i, 1904,



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ELECTROLYTIC DETECTORS.



139



This type of detector is very sensitive, and is in use in
many wireless telegraph stations in all parts of the world.



FlemingTs Hot
Carbon Rectifier-
Professor Fleming
discovered in 1890
that a current will
pass with ease from
the glowing filament
of an incandescent
lamp to another con-
ductor in the bulb.
If the second conduc-
tor be a piece of metal
the current will pass
from the metal to the
carbon through the
space only when the
positive terminal is
connected to the
metal and the nega-
tive terminal of the
cell to the negative
end of the battery
which heats the car-
bon filament The
arrangement is thus
a rectifier, ;>., if an
alternating voltage be
applied to the termi-
nals, an intermittent
current in one direc-
tion only will be
transmitted. The ap-
paratus may be used to detect oscillatory currents by means
of an ordinary galvanometer, as under favourable circum-



J


1


Si


— 1


U-


|~~==^


F
B


A Ijl

- 175 "


-« °
>






> z


'<M


k


1


1






1



Fig. 65.— Orling and Armstrong Capillary
Receiver :— Adapted Form of Lippmann's
Electrombtbr.

At Glass tube endinK in fine point, filled with mer>
cary ; B, U*tube with roercuiy : — dilute acid at
D ;^ Cf C, Platinum wires connecting mercory with
aerial and earth ; F, F, rods for regulating mer*
cury level



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I40 WIRELESS TELEGRAPHY.

stances, e.g,^ when the carbon filament is giving light at the
rate of about one candle per three watts and the metal con-
ductor is cool, the rectification is stated to be from 80 to 85
per cent.

Armstrong and Orling Electro-capillary Detectors.

— Messrs Armstrong and Orling have shown that a capillary
electrometer of Lippmann's invention may be used as a
detector of jigs. A glass tube, drawn down to a very fine
bore at one end, dips into a wider tube. Mercury is put in
the wide tube with dilute sulphuric acid on top of it
Mercury is also put in the upper and wider end of the fine
tube, but does not run out at the bottom because of its sur-
face tension. The lower end of this tube dips into the sul-
phuric acid in the wide tube, and by forcing the mercury
down the upper tube and allowing it to retract, the air in
the upper tube is driven out and the acid electrolyte be-
comes continuous between the mercury electrodes. Very
minute changes of voltage will cause an alteration of the
surface tension of the mercury, and produce a motion of the
column in the fine tube, which is easily observable by means
of a magnifying glass, and may be recorded photographi-
cally or otherwise.

I have included the above descriptions in order to show
by what a large variety of means it is possible to detect
electric jigs. Success in wireless telegraphy, then, depends
much more on the proper adaptation of the detector, of
whatever kind it may be, to the rest of the apparatus, than
on the actual species of detector used.



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CHAPTER VIII.

THE MARCONI SYSTEM.

Without going into questions of priority of invention, we
shall comnnence our description with Marconi's system, as
it was probably the first open-circuit telegraph in com-
mercial use. Already, in Chapter II., the general principle
of the original invention has been described ; we must now
go into details of the more modern apparatus.

The more important improvements embodied in recent
installations are (i) the receiving transformer, or jigger ; (2)
the adoption of Tesla's method of producing long trains of
oscillations of high frequency and voltage; and (3) the
magnetic detector. Other improvements consist in methods
for the employment of an alternator of considerable size in
place of the modest induction coil, and in various combina-
tions of inductances and capacities for the purpose of render-
ing the receiver responsive to jigs of a definite frequency
only. Similar additions have been made to the apparatus
employed in almost every other system. The questions of
priority of invention have not in most cases been settled by
the courts as yet. Many difficulties have also occurred on
account of the enormous power of the spark in a large
station, for though the total amount of energy sent out at
each spark may be comparatively small, perhaps only a few
foot-pounds, the time is so short that the activity during the
existence of the spark is very great. In a small station
which sends out about one foot-pound of energy per spark,
the horse-power during the spark is about 300 ; in a larger



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142



WIRELESS TELEGRAPHY.



Station it is proportionately greater, probably amounting to
tens, or even hundreds, of thousands of horse-power. This
high rate of working, though only momentary, requires that




Fig. 66.— Arrangement of Receiving Apparatus in Marconi System of
Syntonic Wireless Telegraphy.



A, Antenna ; £, Earth-Plate ; /-', e*i Tuning Inductance ; y , y**, Jigger ; j^. Jigger Con-
denser; c\ c*, Choking Coils; 7*, Sensitive Tube, or Coh
Battery.






elay; B^



(Note.— Figs. 66 and 67 are inserted by kind permission of Prof. Fleming and Messrs
Longmans, Green, & Ca)

conductors and insulators alike should be of special designs
best suited to the new conditions. The determination of
the proper forms for the multifarious apparatus necessary
has been a cause of delay in the completion of some of the



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THE MARCONI SYSTEM.



Hi



larger stations to an extent which was not foreseen. The
preliminary difficulties have, however, been got over satis-
factorily, and for some time past the transmission of mes-
sages over distances of at least two thousand miles has been
carried on with regularity. Fig. 66 shows diagrammatically
the arrangement of the receiver in a modern long-distance
station on the Marconi system.




3^ ^ ^^f^^MeD"' ^



Transmitting Station.



Receiving Station.



Fig. 67.— Arrangement op Transmitting and Receiving Apparatus in Marconi
System op Multiple Syntonic Wireless Telkgrai'hy.



Among other details of the apparatus an important one
is the sending key patented by the Marconi Company, in
conjunction with Mr Andrew Gray. In dealing with the
large alternating currents in use at the larger stations it
was clearly impossible to use an ordinary Morse key for
transmitting on account of the arc which may be formed
between the contacts. To obviate this difficulty the key
shown in Fig. 68 was designed. It is constructed so as
to allow of the contact being broken only at a moment
when the instantaneous value of the current is zero. As
this occurs perhaps a hundred times every second the actual



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144



WIRELESS TELEGRAPHY.



break does not lag appreciably behind the movement of
the handle by the operator. The contacts are held together
by an electro-magnet actuated lyy the alternating current,
and therefore are only free to separate at the moment when
the value of the current is zero, which happens twice
during every alternation.

The spark itself was at first the cause of much trouble
in high power stations, the great heat generated producing
pits in the metal of the discharging balls at the point where
a spark took place. The deafening rattle of the discharge
was also very trying to the operators. To obviate both




7J»=C



L4T



-nHnHPt-



M



Fig. 68. — Gray's Sending Key for ALiEKNAroR Circuit (Marconi Company Patent).

A, Alternator; ^.Electro-magnet; C, Circuit-making Contacts; A', Handle of Key.
The contact C is only broken by the action of the spring at the moment when the
ma^etic attraction of M, and therefore the main current, h zero, thus avoiding
arcing.



difficulties Professor Fleming designed a cast-iron box in
which the spark balls were placed, being mounted so that
they could be kept in rotation by a motor outside the box.
The box was filled with compressed air to increase the
efficiency of the spark in producing oscillations. The
principle of the apparatus, except in regard to the revolv-
ing balls, is the same as that of Mr Fessenden's earlier
American patent.

The outside gear at a large station is shown in Fig. 69.
The pyramidal aerial depicted, though of large capacity
and convenient shape, has the disadvantage that the waves



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THE MARCONI SYSTEM. I4S

from one side interfere to some extent with those from the
opposite side, since their phases on arriving at the receiver
are slightly different. This was pointed out by Dr de
Forest, who found that a fan-shaped aerial radiates much



Fig. 69.— Marconi Station at Glace Bav,

more strongly in a line perpendicular to its plane than in
the plane itself, and that the same is true as regards absorp-
tion of jigs by a fan-shaped aerial at a receiving station.
We shall return to this matter in Chapter XV.

K



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Fui. 70.



a




Fig. 71.




Fic. 79.

Diagram Showing the Connections in Marconi's X«Stopper.

fr , Receiving Antenna ; ;*, Inductance Coil with Sliding Contact, h ; f , Condenser ;

<". <i% J'Kger »n Cymoscope ;/, /, Earth-Plates.



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THE MARCONI SYSTEM. I47

Another improvement in the Marconi system is the
X-stopper, an instrument which is intended to transmit
only jigs of a definite frequency from the aerial and earth
terminals of the receiver to the detector. This is accom-
plished by making two earth connections to the aerial.
One of these, which contains much capacity and inductance,
and has therefore a definite frequency of vibration, is the
receiver circuit ; the other earth wire has very little capacity
or inductance, and will therefore conduct jigs of any period
as forced vibrations. If jigs having the proper frequency
are received on the earth and aerial wires, they cause the
receiver circuit to revibrate, and are recorded in the in-
struments ; if disturbances of any other frequency arrive
they do not go through the receiver circuit, but pass between
the earth and aerial by the second wire whose electrical
inertia is so much smaller.

The best results are obtainable by using a number of
the capacity and inductance vibrators in series, each with
its proper earth wire, the receiver being on the last vibrator,
as in Figs. 70-72.

The X-stopper (which means the stopper of stray waves
or X's) should be compared with Slaby's aerial and syn-
tonic horizontal wire with two earth-plates, which it re-
sembles. Other arrangements of capacities and inductances
for a similar purpose have been patented by Professor
Fessenden and almost every inventor in wireless tele-
graphy. The magnetic detector and other improvements
now embodied in Mr Marconi's system have been already
described.*

♦ For further details of the Marconi system the reader is referred
to "The Principles of Electric Wave Telegraphy,** by Prof. J. A.
Fleming (Longmans).



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CHAPTER IX.

THE LODGE-MUIRHEAD SYSTEM,

Sir Oliver Lodge, realising more fully than other
inventors the importance of having the transmitter and
receiver of the same natural frequency, patented in 1897
a system of syntonic wireless telegraphy. The apparatus
was based directly on Hertz's radiation experiments, and
on his own resonating circuits and point coherer.

In Chapter II. a sketch has been given of the earlier
forms. I shall now describe very fully the system now in
use commercially in many parts of the world, availing
myself (through the courtesy of the Lodge - Muirhead
Syndicate and the proprietors of the Electrician) of the
subjoined description by Mr H. C. Marillier, which appeared
(in substance) in the Electrician of 27th March 1903.

Mr Marillier commences his paper with a brief reference
to the preliminary tests to which the system of wireless
telegraphy devised by Sir Oliver Lodge and Dr Alexander
Muirhead had been subjected, with the result (he says)
that the inventors now feel justified in offering it as a
real advance upon any of the systems hitherto put forward,
as regards clearness and accuracy of signalling.

" This claim," he goes on to say, " is well borne out by
the accompanying specimen of script (Fig. 73), taken under
ordinary working conditions from a siphon recorder coupled
directly to a new pattern of coherer without any relay and
without any tapping back apparatus, a simplification which
in itself constitutes a marked and definite improvement.



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THE LODGE-MUIRHEAD SYSTEM. I49



^



"The fundamental patents on which the
Lodge-Muirhead system depends were taken
out for the most part in 1897, and provide
for the following essentials : —

" I. The combination, in the transmitting
and receiving circuits, of two capacity areas
and an inductance coil, as a vital element
in a syntonic system of wireless telegraphy.
The capacity areas may be regarded as the
two coats of a Ley den jar spread out in
•^ S space, one of them being suspended in the
i air and the other near the earth, in fact,
^ the earth itself if desired. Between them is
S situated the spark-gap, and between the

spark-gap and the lower capacity area is

1 the inductance coil as well as a condenser,
I? both being adjustable (Fig. 74).* The pur-
1: pose of these is to prolong the electrical
I oscillations, and by means of their adjustment
> to tune the radiator to any desired frequency
S or pitch, and thus render syntony in the
I receiver possible. Sir Oliver Lodge's main
* idea in regard to transmission has always
J been to obtain a succession of true waves
i of definite frequency, the cumulative influ-
g ence of which will produce a perceptible
M effect on a suitably tuned receiver, no matter
< how feeble the waves may be, after the
i, well-known principle of sympathetic reson-
J ance. This action is illustrated electrically
^ in Sir Oliver Lodge's old experiment of

the syntonic Leyden jars, in which the
closed circuit of one jar responds to the
other, so that the jar overflows or gives
Ki other signs of having become charged when-

ever discharge occurs in the second through
its own distinct, similarly tuned circuit The
adjustability of the inductance and con-



.Ni



♦ In Figs. 74-77, and 83-85, the top connection a
leads to the upper, and that from x or ^, to the lower
insulated capacity area of the oscillator.



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ISO



WIRELESS TELEGRAPHY.



denser render it further possible to tune the two circuits
in such a way that they shall be protected against certain
kinds of specified outside interference ; but for very close
tuning of this kind more elaborate devices are necessary,
which will not be entered into in the present article.



c




KiG. 74.— Oi'EN Sending Circuit.



Fig. 75.— Closed Sending Circuit.



7%



tf



^ rii



il







inx,



?



Fio. 76. — Open Receiving Circuit.



Fig. 77.— Oi'EN Receiving Circuit.



" 2. The second point claimed by the inventors is the use
of a transformer, or ironless induction coil, in the receiving
circuit (see //, tSy Fig. 76). This device is probably essential
in all long-distance wireless telegraphy, and should disputes
arise at any future time as to the claims of individual
inventors, it is a feature which might conceivably be fought
over. Its purpose may be described briefly as tending
to magnify the E.M.F., and putting the coherer into a
second circuit, instead of in direct series with the vertical
collecting wire and the lower capacity. The passage in



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THE LODGE-MUIRHEAD SYSTEM. I5I

which this transformer claim occurs is as follows, and may
be found in Sir Oliver Lodge's patent, No. 11, 575 of
1897:—

" • In some cases I may . . . surround the syntonising coil
of the resonator with another or secondary coil (constituting a
sort of transformer), and makes this latter coil part of the
coherer circuit, so that it shall be secondarily affected by the
alternating currents excited in the conductor of the resonator,
and thus the coherer be stimulated by the currents in
this secondary coil, rather than primarily by the currents in
the syntonising coil itself; the idea being thus to leave the
resonator freer to vibrate electrically without disturbance from
attached wires.'

" The illustration which accompanies this description in
the specification definitely indicates the transforming-up
and magnification of the E.M.F.

" 3. The use of a condenser shunt (x^ in Fig. 76) in the
coherer circuit, such as will enable that circuit to have a
definite time period, as described in the Lodge-Muirhead
patent, No. 18,644 of 1897. The addition of the condenser
as a shunt eliminates the.battery and receiving instrument
so far as oscillations are concerned, and is regarded by the
inventors as a feature of great importance. This, as well
as the transformer, has been adopted in principle by other
workers, and a common modification of it is to divide the
secondary of the transformer in two parts and place the
condenser between them. In the Lodge-Muirhead patent,
No. 29,069 of 1897, the condenser is shown placed between
the lower or earth capacity and the syntonising inductance
coil described in i. In practice, when a transformer is
used, it has been found best to have two condensers, the
one described, and another in the secondary transformer
circuit (see Fig. 76).

" The last-mentioned patent also provides for enclosing
the coherer in a completely closed metal case, a precaution
which, in the practical development of the system, is supple-
mented by a device which automatically short-circuits the
coherer whenever the connections are switched from the
receiving on to the sending apparatus.

"Besides these more or less essential features of the
system. Sir Oliver Lodge and Dr Muirhead have introduced
a number of detailed changes into the apparatus which tend



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1 52 WIRELESS TELEGRAPHY.

to simplify or to improve the working. The most important
of these is the new coherer. Up to a short time ago the
inventors thought that they had reached the desired point
of perfection in a filings-tube coherer fitted with two needle
points in place of the usual plugs or knots, the tube being
tapped by a constantly working clockwork tapper as shown
by Sir Oliver Lodge in 1894, There are certain disadvan-
tages, however, attendant upon all coherers in which metal
filings are employed. They require a special and carefully
adjusted apparatus for tapping back, and many of them are
capricious in working and liable to fatigue. The new
coherer introduced by Lodge and Muirhead requires no
tapper, but is kept perpetually sensitive by the rotation of
a small steel disc just separated from a column of mercury
by a film of mineral oil. The impulse of electric oscillations
breaks down the oil film and establishes momentary
cohesion between the steel disc and the mercury. The
coherer, without its metal case, is shown in Fig. 78, and its
construction in Figs. 79 and 80, in which a is the rotating
steel wheel, and b the mercury in its trough d, with an
amalgamated spiral of platinum wire r, connecting it to the
terminal or binding screw h, ^ is a copper brush making


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Online LibraryJames Erskine MurrayA handbook of wireless telegraphy; → online text (page 11 of 26)