tages in a high spark frequency which appear both at the trans-
mitting and at the receiving stations. If the closed circuit condenser
is charged 1,000 times per second to a certain potential, it is evident
that more energy will be required than if charged only 120 times, the
formula for the energy being 1/2 C V 2 N, where C is the capacity, V
the potential, and N the number of times per second. If the same
amount of energy is available in the two cases that is, if 1/2 C V 2 N
is constant the smaller the value of N the larger must be the value of
RADIOTELEGRAPH Y. 31
V, other conditions being constant, and, vice versa, the larger the
value of N the smaller may be the value of V. The earlier practice
was to make N small, as 120 per second from a 60-cycle alternator,
and V large, as 30,000 volts. The modern practice is to make N large,
as 1.000 from a 500-cycle alternator, and V small, which in this ex-
ample must be about 10,800 volts. It is evident, then, that the trans-
former secondary and the closed oscillating circuit condenser do not
need to be built to withstand the high voltages formerly used, and
that, therefore, they may be lighter and more compact ; also that the
oscillation transformer and antenna, to be described later, do not need
the very high insulation which was formerly necessary.
The advantages of the high spark frequency at the receiving
station will be mentioned later under that heading.
If suitable constants are used in the formula for the energy, it is
possible to determine the capacity, peak voltage, etc., for any
size of set. Let K. W. be the number of kilowatts that the trans-
former secondary must deliver to the closed oscillating circuit con-
denser ; M. F. the capacity of this condenser in microfarads ; V. the
peak value of the voltage to which the condenser is charged and then
discharged as the spark gap breaks down; and Cycles the number
of cycles per second of the alternator in which there are two dis-
charges per cycle, then
T , W ^(M.F.)X(V 2 )X (Cycles)
10 9
Thus if M. F. is 0.012 mf.; Y. 18,250 volts, peak value; and the
Cycles 500, with two discharges per cycle, then K. W. will be 2.0.
As it is impossible to build a transformer with an efficiency of 100
per cent, it is evident that the armature of the alternator must de-
liver a larger number of kilowatts to the primary of the transformer
than is given by the above formula. The actual number will be
found by dividing the secondary kilowatts by the efficiency of the
transformer. Thus, if the efficiency were 93 per cent or 0.93, then
the alternator armature output or the transformer primary input
C\ r\
would be ^ = 2.15 K. W. By simple changes in the above formula
it is evident that when any three of the quantities are known, the
fourth can be found.
TRANSMITTING CONDENSERS.
A brief description of the three elements, condenser, coil, and
spark gap, will be given.
The functions of the condenser are, by virtue of its capacity, to
store the charge delivered to it by the transformer secondary circuit
until its potential reaches the desired value as determined by the
spark gap, and then to discharge through the gap and the inductance.
32 RADIOTELEGRAPH Y.
An ideal condenser would be one that was perfectly insulating, could
not be punctured, and showed no heating or losses of any kind during
charging and oscillatory discharging.
There are several different types of transmitting condensers used
in the Signal Corps radio stations, varying widely in capacity, size,
voltage, etc., from the small mica ones of the field radio sets to the
4|-foot jars or compressed-air types in the permanent stations. All
types consist essentially of two conducting surfaces, as tin or copper
foil, separated by an insulator or dielectric, as it is often called,
which can withstand without puncturing the high voltage required
to break down the spark gap. Probably the most efficient condenser
is the compressed-air type, which consists of a large number of cir-
cular metal plates mounted on two sets of supports with a small air
space between each plate, the top plate and every alternate plate
being connected together as one set and the remaining plates as the
other set. The whole is contained in an air-tight tank, one set of
plates being connected to the tank as one terminal and the other set
to a terminal brought out through the cover in a porcelain insulator
sealed air-tight by a lead gasket. Air is then pumped into the tank
until a pressure of about 240 pounds per square inch is reached, or
about 16 atmospheres of 15 pounds per square inch, as shown by
a pressure gauge on top of the tank. At this pressure it has been
found that air has an insulating strength many times greater than
at ordinary pressures. Condensers of this type will withstand a
maximum or "peak" voltage of about 20,000 volts under service
conditions. The most serious objection is the excessive weight, a
tank of about 0.006-microfarad capacity weighing about 300 pounds.
There are many types of condensers using glass as the dielectric,
such as plates or jars covered w y ith foil or plated with copper. When
these condensers are used at high potential, such as 25,000 volts or
more, there is developed at the sharp edges of the foil or plating a
discharge (sometimes called brush discharge}, which spreads out over
the surface of the glass, is accompanied by a hissing sound and con-
siderable heating of the glass close to the edges, and in a dark room
shows a pink light at the edges. The puncturing of the glass and
the breaking down of the condenser often takes place close to the
edges, due probably to the brush discharge and the local heating of
the glass. These discharges represent losses which, in part at least,
can be prevented by covering the edges of the foil with an insulating
coating, such as asphaltum, and more completely by immersing the
condensers in an insulating oil, such as castor oil, etc.
The capacity of these condensers and the voltage which they can
withstand depend so much on the quality of glass, the manner in
which it was annealed, its thickness, etc., that it is impracticable to
give figures except for condensers that have actually been tested.
EADIOTELEGRAPHY.
33
[
T
FIG. 23.
The capacity of one glass plate about T 3 e inch thick and with the
foil 15 inches square is about 0.0020 to 0.0025 microfarad. The capac-
ity of a jar with glass -J inch thick, 4f inches in diameter, and height
of foil of 10 inches is about 0.002 M. F. In the case of a good grade
of plate glass about -fg inch thick, free from scratches, bubbles, etc.,
a potential of 20,000 volts, peak value, can be safely used.
In figure 23 is shown a closed oscillating circuit with three con-
denser jars connected in parallel; that is, the three outside coatings
are connected to-
g-ether as one ter-
minal and the three
inside coatings as
the other, and with
a potential of 20,000
volts between the
terminals. When
condensers are thus
connected in paral-
lel the total capacity is the sum of all the capacities; if the con-
densers are all of equal capacity, the total capacity is the capacity
of any one condenser multiplied by the number. Thus in figure 23
if each condenser were a jar of capacity 0.002 M. F., the total
capacity would be 0.006 M. F., or three times 0.002 M. F.
If the condensers break down at this potential or if higher poten-
tials, such as 30,000 volts, are to be used, two banks, each of three
jars in parallel should be connected in series, as shown in figure 24.
It is to be noted
that this connection
requires twice as
many jars as be-
fore, but if the
total potential is
30,000 volts, the po-
tential across each
FIG. 24. J ar i g now on ly
15,000 volts instead
of 20,000 as before. Whenever condensers are connected in series,
the total capacity is always reduced ; if two equal condensers are so
connected, the total capacity is one-half the capacity of either; if
three equal condensers are so connected, the total capacity is one-
third, etc. As the connections shown in figure 24 rdeuce the capacity
to one-half the desired value in figure 23, two banks each of six jars
must be connected in series-parallel^ as shown in figure 25, thus
requiring four times as many jars as the first circuit.
66536 17 3
t
W5000 V.+I5000 VM
I* 30000 Volts->|
34
RADIOTELEGRAPHY.
Another type of condenser having some advantages is the Moscicki
jar, which consists essentially of a glass tube or jar Avith 'inside and
outside coatings, as in the other types, but at the edges of the coatings
where the puncture usually takes place the glass is thickened to give
increased strength, and at the same time the edges are covered with
an insulating liquid to stop the brush discharge. The whole is con-
tained in a brass tube to which the outside, coating is connected, the
inside coating being brought out to a binding post through a sealed
porcelain insulator. The case and the binding post thus become the
two terminals. These tubes are made in two sizes, the larger of which
is in more general use, has capacity of about 0.005 M. F., and is capa-
ble of withstanding 20,000 volts.
There are many other types of condensers using such dielectrics as
mica, paper, and various molded insulating compounds. In a IV \v
cases oil is used as the dielectric, in which case metal plates are
mounted on insu-
lating supports a
short distance apart
in tanks filled with
a suitable insulat-
ing oil, such as cas-
tor oil, etc.
I
1
U| 5000 V*l+f 5000 vJ
k- 30 000 Volts ->|
TRANSMITTING
INDUCTANCES.
FIG. 25.
The function . of
the inductance is to
form one of the two elements, the condenser being the other, neces-
sary for developing and maintaining the oscillations, and to serve as
a mans of transferring energy from one circuit to another. An ideal
coil would be one having the desired inductance but with a zero
resistance to the oscillating currents.
The inductance coil L, which has been shown in the various figures,
may be any one of several different types, such as a helix of heavy
copper wire, thin-walled copper tubing, or flat strips, or a flat spiral
of copper ribbon, such as the linking coil of the early Signal Corps
field radio sets, etc. These are generally provided with clips so as to
be able to vary continuously the number of turns, and hence the
inductance in circuit. In any single coil, the fewer the number of
the turns the less will be the inductance, and vice versa, the larger
the number of turns the greater will be the inductance. In some
cases the coil may be provided with plugs and sockets to vary the
inductance by steps and other means provided elsewhere in the circuit
to get all adjustments between the steps.
Curves showing how the inductance of a coil varies with the num-
bers of the turns in circuit is called a calibration curve of the indue-
KADIOTELEGEAPHY.
35
tance. In figure 26 is shown such a curve for a helix, with square
turns wound with copper tubing about one-fourth inch in diameter,
the length of each side being 21| inches and the spacing of the turns
being 1 inch between centers. In figure 27, A and B, are shown two
calibration curves of a flat spiral, similar to the one used in the field
radio sets, in the first of which (A) the turns are counted from the
outside inward, and in the second (B) they are counted from the
inside outward. Thus it is seen that in using different numbers of
turns in a flat spiral care must be taken to state how the turns are
counted. The explanation of the difference between the two curves
.150
.100
$'
1
5
!
050
10
15
Number of turns
FIG. 26.
is that, other things being equal, the greater the diameter of the turn
the larger will be the inductance; and hence the inductance will be
the larger for a few turns in that curve in which the turns are counted
from the outside inward.
There is another useful type of inductance called the variometer,
which consists essentially of two coils connected in series or parallel,
as desired, one of which is movable with respect to the other. In
some cases one coil is arranged to slide past the other in a plane
parallel to its windings, as indicated in figure 28 ; in other cases one
coil is rotated inside the windings of the other, as indicated in figure
36
RADIOTELEGRAPHY.
29. In the second type, when the coils are in the same plane and the
windings are connected so that the current is circulating through
.120
.110
.100
.090
0.080
^.070
|.060
*.050
* -040
1 .030
|.020
^ .010
2 46 8 10 12 14 16 18 20 22 24 26 28 30
Number of turns
FIG. 27.
them in the same direction, the two magnetic fields are helping each
other and the inductance is a maximum ; if, now, one coil is rotated
FIG. 28.
through an angle of 180 degrees the two fields are opposing and the
inductance is a minimum; for intermediate angles the inductance
will have some intermediate value. The variometer thus has the
RADIOTELEGRAPH Y. 3 7
advantage of giving a continuous change of inductance without
moving clips or contacts, but has what may be under certain condi-
tions the disadvantages of not giving zero inductance at its minimum
position and of always having the resistance of all its wire in circuit.
A variometer is generally used in connection with a helix or coil,
variable only by steps, to give intermediate values of the inductance
as mentioned above, and shown in figure 76.
The earlier types of closed circuit inductance were wound with
wire or tubing, the resistance of which to direct current was very
low. Both theory and experiment have shown, however, that the
resistance to high-frequency currents may be comparatively large.
The explanation is that these high-frequency currents tend to travel
almost wholly on the surface of the conductor and do not penetrate
to any considerable distance into the wire. Thus a thin-walled tube
will have practically the same resistance to high-frequency currents
as a solid wire of the same diameter, the inside of the wire carrying
no current at all.
This tendency of the current to flow only on the outer surface is
sometimes called the "skin effect" and the distance to which the
current penetrates the thickness of the skin. The higher the fre-
quency the more marked is the skin effect and the thinner is the
skin ; in other words, the higher the frequency the larger will be the
38
RADIOTELEGRAPHY.
resistance for the same size and length of wire. In figure 30 is given
the curve showing the increase in resistance for Xo. copper wire,
B. & S. gauge (about 325 mils in diameter), as the frequency changes
from zero or a steady current up to 1,000,000 cycles per second.
Thus at 500,000 cycles it is seen that the resistance has been increased
about 22 times the D. C. value. The scale of such a curve will differ
with the different sizes of wire, the increase being greater than here
shown for wires larger than Xo. and less for smaller sizes. In fig-
ure 31 is given the curve showing the increase in resistance for the
various sizes of copper wire in the B. & S. gauge at a frequency of
A/o. O yy/re, B. & S. Gauge
A/umber of Times of Increase in Resistance
r _' r\> ru GJ o $
9UIOUOWOUIC
^
<1
^
^
/
^
/
/
/
01 234-567
Frequency Jn Hundred thousand cyc/es
FIG. 30.
10
500,000 cycles per second. Thus a wire as small as Xo. 35, B. & S.,
has very nearly the same resistance at this frequency as at a steady
current, or, in other words, the thickness of the skin at this fre-
quency is about equal to the radius of the wire. In order to be able
to include all sizes of wire at all frequencies it is evident that a large
number of curves or an extensive table of resistance and frequency
would be necessary.
If a large number of wires, the diameter of which is such that the
current just penetrates to the center at any given frequency, is used
in parallel in the form of a compactly stranded wire or cable it is
evident that all the copper is in use and that the current-carrying
KADIOTELEGBAPHY.
39
surface of such a cable is very much greater than that of a solid wire
of the same outside diameter, and hence the resistance is very much
lower. Each wire must, however, be separately insulated, as other-
wise the current will immediately seek the outer surfaces of the outer
wires on account of the skin effect, and the resistance will not be much
decreased from that of a solid wire. Such a stranded wire or cable,
with its individual wires separately insulated, as with enamel, is
sometimes called litzendraht, from the German word. The size of
the insulated wire depends upon the frequencies at which it is to be
used. If the highest frequency should be 500,000 cycles per second,
then from figure 31 it is evident that there would be but little gain
40
35
30
25
20
15
10
1.0
Frequency 5OOOOO
5 10 15 20 25
Sizes of wires: B. & S. Gauqe
FIG. 31.
30
35
40
in using a wire smaller than No. 34 or No. 35 on B. & S. gauge. The
number of wires depends upon the current to be carried and the re-
sistance desired. For small currents it is generally a multiple of 7,
as TX^I or 49 wires, but for heavy currents the number may be in
the hundreds or even in thousands.
It is evidently impossible to get a continuously variable inductance
by a sliding clip or contact on all the wires of a litzendraht coil, so
that when such an inductance of low resistance is desired it is gen-
erally made in the form of a variometer wound with litzendraht.
Many modern sets, particularly those of the quenched-spark type of
the Telefunken Co., use such coils.
40 RADIOTELEGRAPHY.
The use of litzendraht is not confined to transmitting coils, but
is also used in receiving sets to get low -resistance circuits.
SPARK GAPS.
The function of the gap is to serve as a trigger in starting the oscil-
lations and to limit the potential applied to the condensers by the
transformer secondary. An ideal gap would be one having an infinite
resistance during the charging of the condensers and a zero resistance
during each wave train of the discharge.
The types of spark gaps in use differ nearly as much as the other
parts of the closed-circuit elements. In small-sized sets the electrodes
or terminals are generally made of zinc or brass, the sparkling sur-
faces being either balls of one-half inch diameter or more, or else
rounded surfaces. Sharp points are not used, as at small separations
the potential required to break down the gap is too small to allow any
considerable power to be used, and if the gap is opened to increase
the potential and power the gap resistance becomes too high. As the
power delivered to the transformer is increased it is soon found that
the discharge at the gap becomes flaming in character and has a
hissing sound, seeming to be more like an arc than a spark, and the
gap terminals become very hot. The reason for this is, that owing
to the great quantity of electricity discharged across the gap the
resistance becomes so low that a high-potential alternating-current
arc, which is almost a short circuit, is maintained at the transformer
secondary terminals. This arc is formed in the heated air and the
vapor of the metals forming the gap terminals. Experiment has
shown that a blast of air across or through the gap will blow out
the arc but not the spark. By thus removing the short circuit the
condenser can be charged to the full potential of the secondary and the
power of the set increased in some cases it may be nearly doubled.
The air blast may be obtained from a blower or compressor driven,
for example, by an electric motor or directly by the rotating of the
gap terminals themselves, in which case it is known as a rotating
gap. There are two general types of rotating gaps, in the first of
which the rotation is simply a convenient means of giving the neces-
sary ventilation and cooling. It is not necessary that it be provided
with rotating terminals, although it may be so provided. In one of
the early types used in the Signal Corps, shown in figure 32, a rotating
disk is used between two fixed terminals. In this case the sparks shift
from place to place on the edges of the disk as it turns, the ventila-
tion being by means of fans on the face of the disk, which blow the
air away from the gaps. As no attempt is made to secure any
special time relation between the discharges and the alternator fre-
quency this type of gap is often called a nonsynchronom yap.
KADIOTELEGEAPHY.
41
In the second type of rotating gap one set of electrodes is attached
to the alternator shaft, preferably insulated from it, and thus rotates
at the same speed as the armature; the other terminal is mounted
so as to be capable of adjustment, both in the direction of rotation
and in a radial direction. If the spacing of the revolving terminals
is such that there are as many terminals pass the fixed terminal per
second as there are alternations per second, and, further, if the ad-
justments of potential, etc., are such that the discharge is at the peak
of each alternation, then there will be as many sparks per second as
there are alternations, and the gap is called a synchronous gap.
In order to secure the correct adjustments of a synchronous gap
the fixed terminal should be adjusted radially to give only a small
clearance, as ^ inch or less, and then adjusted in the direction of
rotation as follows: If the rotating terminals are watched by the
light of the sparks themselves, they will appear either to be waver-
ing back and forth or else to be nearly fixed in position. In the
former case the discharge does not occur at the peak of the wave, but
FIG. 32.
perhaps before the peak in one alternation and after in the next,
and hence the wavering appearance ; in the latter case the discharge
is at the peak of the wave as shown by the apparent steadiness of
position. At the same time that this correct adjustment is secured
the note of the spark as heard either in the station itself or at a
distant receiving station will become much clearer, the advantages of
which will be mentioned later.
As it is generally best not to have long leads from the spark gap
to the other elements of the closed circuit, it may be necessary to
have all of the closed circuit as well as the open circuit in the room
with the alternator, in which case the operator and the receiving set
should be in another room. In some cases it may be possible to mount
the alternator and gap so that short leads can be brought out from
the latter through well-insulated bushings into the next room, which
should be sound proof, and thus all the circuits be contained in the
same room with the operator for convenience and promptness in
making changes in wave length and other adjustments, etc.
42
EADIOTELEGRAPHY.
QUENCHED SPARK GAPS.
Most modern sets use the quenched spark gap, a brief description
of which will be given here and the theory of the quenched spark
transmitter later. The gap is essentially a series gap consisting of a
number of plates with small separations between the sparking sur-
faces, which are inclosed in air-tight chambers formed between the
plates themselves.
In figure 33 is shown a section of a gap where P are the plates
often made of copper, which, on account of good conductivity for
heat, will carry off the heat of the spark; F are the flanges, which
Fio. 33.
help the cooling by exposing a large area to the air or to the air
blast to be mentioned later; S are the sparking surfaces between
which the sparks pass, which may be of the same copper stock as the
rest of the plate or of heavy silver plate fastened in place at S; M
the separators or insulating rings, also called gaskets, between the
plates, often made of mica, about 0.010 inch thick (10 mils), the
thickness of which determines the distances between the sparking
surfaces. In some cases the separators are made of rubber or other
insulating materials which are somewhat compressible, and then the
Compressible
GasAet
bearing surfaces are often corrugated, as shown in figure 34, so that
the material may be pressed down into the annular spaces. What-
ever the type of separator, the gap as a whole must be put under
strong mechanical pressure so that the air shall be excluded from the