In putting the speed indicator in place it may be necessary to
turn handles slightly so as to permit the gears to engage.
In case the vane of the speed indicator comes on the underside
when the indicator is screwed into place, it can be turned into
proper position after loosening the depressed set screw on the
threaded part fitting into the case and then tightening the set screw
again.
In making the set ready for transportation, the speed indicator
should be removed and packed in its proper place in the operating
chest and the opening closed with the brass plug provided.
GEARING.
The gearing is a combination planetary worm-and-spur type of
high efficiency when in proper alignment. The high-speed shafts
have ball bearings and the gears run in grease or oil so as to reduce
the friction as much as possible. The gears should never be taken
apart unless absolutely necessary to replace worn or broken parts,
and then only by an experienced person. If not properly reassem-
bled, or if the driving gear does not run perfectly true with the
108 RADIOTELEGRAPH Y.
worm, undue friction and wear will result, the machine will be
harder to turn than before, and the gears will be speedily destroyed.
The gears and ball-bearings can be lubricated by either a non-
fluid oil or a light, thin oil, such as Medium Monogram, but both
must be free from acid and water to prevent rusting. If oil is
used it should be supplied through a small cap on the opposite side
of the case from the speed indicator. The level should be kept not
more than one-eighth inch above the lower edge of the glass window
at the flywheel end of the gear case ; if kept above this, the oil will
overflow to the lower part of the case and cause trouble and spark-
ing at the commutator and collector rings. The same kind of oil
should be used on the flywheel shaft through the small hole on the
upper side of the bearing.
If nonfluid oil is used it should be supplied through the opening
where the speedometer is screwed into place. Not less than a pint
nor more than a^ quart should be used in the main gear case, but only
a small amount in the spur gear case at the end opposite the flywheel,
as otherwise the machine will turn hard on account of choking the
gears with too much nonfluid oil in the narrow gear case.
With the exception of an occasional addition of oil, the machine
should run for months without attention. If the oil becomes thick
or dirty, the gearing should be washed out with gasoline and refilled
with clean oil without dismantling.
Care must be taken not to start or stop the machine suddenly, as
this may strain or break the gears. The machine must not be stopped
by means of the handles, but only by friction on the flywheel.
CONNECTIONS.
The leads from the armature of the A. C. generator are directly
connected to the transformer primary by means of the heavy pair
of leads, the larger plug of which being put into the socket at the
left-hand end of the operating chest marked " Gen." and the smaller
plug into the socket on the underside of the gear case, also marked
" Gen." The sending key is in the circuit of the alternator fields
and the exciter armature, and is so connected by means of the light
pair of leads, the larger plug of which being put into the socket at
the left end of the chest marked "Fid." and the smaller plug into
the socket on the underside of the case, also marked "Fid." By
the use of these circuits, the electrical load on the machine is
limited to the small one of the exciter field, except when the key
is closed in sending. Experiments have shown that twice the out-
put of the former machines can thus be obtained with practically
no more tiring effects on the men than before.
RADIOTELEGEAPHY. 109
OPERATING CHEST.
In this chest is mounted the transmitting and receiving apparatus,
the diagram of which is shown in figure 79. To put the chest in
condition for sending, connect the double contact plugs of the leads
from the hand generator, field, antenna, and counterpoise to the
receptacles marked "Gen.," "Fid.," "A," and " C," respectively,
and the four variable contact clips on the leads from the condenser,
spark gap, antenna, and hot-wire ammeter, to the four points on the
flat spiral, as indicated on the diagram, making sure that the counter-
poise clip is at the end of the outside turn. Set the control switch
at the " sending " or lower position, Release the indicating needle
of the ammeter by turning the small knurled screw at the left-hand
side of the upper binding post. When the needle is free, adjust to
zero position on the scale by means of the small knurled screw at the
right side of the upper binding post. Set the variable spark-gap
contact on the fifth plate, counted from the left end, so as to put
four gaps in circuit. Start the generator, and when the proper speed
is obtained the set is ready for sending.
QUENCHED-SPARK GAP.
The spark gap used in this set is made up of several copper disks
separated by mica washers about 0.01 inch thick. Its action is to
allow all of the energy of the closed oscillating circuit to be trans-
ferred to the open or radiating circuit in a few oscillations, after
which the spark is quenched and the circuit is, in effect, open. The
activity in the closed circuit having ceased, the open or radiating
circuit continues to oscillate at its own period, radiating waves of
its own wave length without any retransfer of energy to the closed
oscillating circuit, which continues to remain open until a spark
breaks down the gap again at the peak of the next alternation.
In order to work at maximum efficiency, the quenched-spark gap
should be kept cool. It is for this reason that the plates are pro-
vided with thin cooling flanges having a large surface exposed to
the air, and are blackened, a black body cooling more rapidly than
one highly polished. If the gaps have become too hot, as by keeping
the key closed for a long time, the antenna current will gradually
decrease, the loss at times being as much as 40 per cent, so that it is
ahvays best to allow the gap to cool before using again.
The gap should not be taken apart to clean its sparkling surfaces
like an ordinary type of open gap. In general the more frequently
such a gap is opened the more unsatisfactory may be its operation.
The explanation is that the repeated opening of the gaps introduces
air each time, and that with free exposure to air the sparking sur-
110 RADIOTELEGRAPHY.
faces are corroded or pitted, but that when kept air-tight they are
worn smooth and clean by the sparking action. Sometimes, if there
is a flaw in one of the plates or if air leaks into the gap, there will be
a noticeable drop in the antenna current, and the note will become
poor. When it is believed that the trouble is confined to one or two
gaps it is possible to continue sending without dismounting the
whole gap by short-circuiting the bad gaps by means of clips pro-
vided for the purpose, in which case as many new gaps must be put
into circuit by adjusting the movable clip to the right as were cut
out by the short-circuiting clips.
The gap should be dismounted only when the trouble has been
located in the gap and it has been found impossible to remedy it by
short-circuiting the different gaps in use. The gap should be dis-
mounted only by an experienced man, who should clean the surfaces
by rubbing them face down on fine emery cloth or paper on a flat
surface. It is absolutely necessary that both the bearing surface
and the sparking surface be kept true and plane, as shown by a
straightedge.
Great care should be exercised in reassembling the gap to set the
mica washers accurately on the annular surfaces of the disk and to
put on enough tension with the clamping screws to render all of the
gap spaces air-tight.
TUNING OF SENDING SET.
The tuning of the closed and open circuits to resonance, and the
determination of the correct coupling between them are the two most
important adjustments in a quenched spark transmitter. In the pres-
ent type of directly coupled set with a flat spiral as the oscillation
transformer, these adjustments can be made either with or without the
help of a wave meter. If made without the meter the adjustments
are more difficult and must be found by trial, but they should sat-
isfy the following tests : ( 1 ) The number of turns in the closed cir-
cuit should be chosen so as to give the desired wave length; (2) the
antenna hot-wire ammeter should show the maximum reading that
can be obtained by adjusting the number of turns in the open cir-
cuit according to the table on page 114; and (3) the note as heard
in the telephones of the receiving set should be clear and character-
istic of 500 cycles. These adjustments are, in general, dependent on
each other, an incorrect change in one seriously affecting all the
others; but when obtained the circuits will be in resonance at the
desired wave length, they will be correctly coupled, and the closed-
circuit condenser will be charged and discharged regularly once per
alternation.
KADIOTELEGRAPHY. Ill
The adjustments should be made as follows: Set the closed and
open circuit clips on the turns corresponding to the desired wave
length, as given on page 114. These turns are approximately correct
with the standard antenna and should be used in beginning to make
the adjustments. If the antenna ammeter reads between 2.2 and 3.0
amperes and the note is clear and of 500 cycles, then the adjustments
are correct and the circuits properly tuned. If the ammeter reading
is low and the note low and clear or low and ragged, possibly the
circuits are correctly tuned, but there are too many gaps in circuit,
and the condenser is being charged and discharged either regularly
or irregularly only every second or third alternation. Reduce the
number and see if this change gives a clear 500-cycle note, etc. Simi-
larly if the note is high and hissing, the condenser is being charged
and discharged more than once per alternation. Increase the num-
ber of gaps and see if this change gives a clear 500-cycle note. etc.
If none of these changes gives the correct adjustments, then the cir-
cuits are not in resonance, or the coupling is wrong. Move one of
the open-circuit clips to see if the correct adjustments can be ob-
tained; it is impossible to state which clip should be moved or in
which direction. If the change of one clip is not sufficient, move
both open-circuit clips until, by repeated trials, the correct adjust-
ments have been found. If possible, leave the counterpoise clip on
or near the outside turn, so that it will be at ground potential. It
will be found that the character of the note will be changed as these
various changes in coupling and tuning are made, but the clearest
500-cycle note will be obtained when all adjustments are correct and
the circuits properly tuned. After the adjustments have been com-
pleted at this wave length, tabulate the results as shown and repeat
at other wave lengths within the range of the spiral.
Although there is no direct test that can be applied, except with
a wave meter, to determine if a single wave length is being radiated,
yet in general this will be the case if the adjustments satisfy the tests
as indicated on pages 110-111.
If a wave meter is available the adjustments are much easier to
make, and they should satisfy the following tests: (1) A single
sharply defined wave should be radiated of the desired wave length,
(2) the antenna ammeter and the signals in the telephones of the
meter should show the maximum reading and signals obtainable
under the first condition, and (3) the note should be clear and of 500
cycles.
It will probably be best to use the wave meter with a detector or
helium tube, as shown in figure 48, because it will be impossible to
turn the hand generator at a sufficiently constant speed to obtain
112 RADIOTELEGRAPHY.
steady readings on the wattmeter and hence difficult to determine
the resonance point and wave length.
The adjustments for timing should be made as follows: Discon-
nect the open-circuit clips and set the closed-circuit clips on the
turns corresponding to the desired wave length as given on page 116.
Measure the wave length according to the instructions on page 111
to make certain that it is correct. Set the open-circuit clips on the
turns given in the table, and, with the wave meter near the antenna
or counterpoise wires, but not near the spiral, see whether there i^
one or two wave lengths in the meter. If there is a single sharply
defined wave length, and the antenna ammeter reading can not be
increased by slight changes of either or both of the open-circuit
clips and the note is clear and of 500 cycles, then the adjustments are
correct and the circuits properly tuned. If there is only one wave
length, but the antenna ammeter reading is low and can not be in-
creased by slight changes in the open-circuit clips, then the coupling
is too loose and must be tightened. Move the open-circuit turns in
use inward as a whole, by moving both clips inward and slightly in-
creasing the number of turns in circuit to allow for the decrease in
their diameter, until with a single sharply defined wave length the
antenna ammeter reading is a maximum, etc., as before, in which case
the circuits are properly tuned. If the note is low, decrease the num-
ber of gaps; if high and hissing, increase the number as previously
described. If, however, there are two wave lengths, move one or
both of the open-circuit clips, but it is impossible as in the previous
case to state which clip or in which direction, until by repeated trials
it has been found that there is a single sharply defined wave length,
a maximum antenna ammeter reading, etc., as before, in which case
the circuits are properly tuned.
After the adjustments have been completed at this wave length
repeat at other wave lengths as before and tabulate the results as
on page 114.
If the one-eighth or one-fourth kilowatt motor-generator or the
engine-driven one-fourth kilowatt generator supplied by the Signal
Corps, is available, it should be used as the source of the 500-cycle
current because its voltage will be much steadier than that of the
hand generator. When the motor-generator set is used, the A. C.
armature and the D. C. motor should be protected from " kickbacks "'
due to the use of the sending key in the alternator fields by two high-
resistance carbon rods mounted on suitable bases to be connected
as follows : The end terminals of one rod to the two A. C. leads close
to the machine ; the end terminals of the other rod to the two main
line D. C. leads close to the machine, and the middle points of both
KADIOTELEGRAPHY. 113
rods to be connected together and this common point grounded-on
the frame of the machine.
The constant speed of the motor generator makes it possible to get
steady readings on the wattmeter of the wavemeter, and hence easy
to find the resonance point and wave length. It may also be more
convenient than a detector because it is often difficult to keep a
detector point in sensitive adjustment on account of the nearness of
the spark gap and to determine the resonance point on account of
the continuous note in the telephone. However, the detector and
helium tube can be used if desired; the circuits will be correctly
tuned no matter what means are used for determining resonance.
The circuits should be adjusted to resonance, etc., as described in
previous paragraphs.
In some cases it may be convenient to use the following slight
modification of the method described on page 111. Disconnect the
transformer secondary from the closed circuit and connect it to the
two terminals of a small zinc or brass spark gap, one of which is
connected to the counterpoise and the other to the standard antenna.
Measure this wave length, which will be the fundamental wave
length. Next insert, say, two turns of the spiral, Nos. 28 to 30,
counting the turns from the inside turn outward, in series with the
antenna, and measure this wave length. Continue in this manner
until all wave lengths are measured within the range of the spiral
and tabulate as shoAvn on page 114. Next make the standard con-
nections, setting the open-circuit clips on the turns corresponding
to the desired wave length, as just obtained. Set the closed-circuit
clips on the turns given in the table on page 116, and make the neces-
sary adjustments by moving these clips until it has been found by
trial with the wavemeter that there is a single sharply defined wave
length, maximum current in the antenna, etc., as before. Tabulate
these results and repeat for wave lengths within the range of spiral.
In this case the closed circuit is tuned to the open circuit, whereas
in the previous case the open circuit was tuned to the closed circuit,
but the same tuning points will be found for the same wave length,
whichever method of tuning is used.
It is impossible to use exactly the same method as on page 116, in
which the number of turns for a given wave length is determined
for both the primary and secondary circuits, after which the princi-
pal adjustment is one of coupling, because the number of turns in the
primary circuit of the spiral at any wave length will depend on the
part of the spiral which is included in the circuit, and hence it will
vary with every combination of turns. It is for this reason that the
primary tuning and coupling must both be found by trial,
66536 17 8
114
RADIOTELEGRAPHY.
OPEN-CIRCUIT AND CLOSED-CIRCUIT TUNING.
Wave length.
Antenna.
Counterpoise.
Meters.
300
325
350
375
400
425
Turn No.
26?
24 1
20J
Turn No.
30
30
30
30
30
30
Turns to be counted from the Inside turn outward.
Wavelength.
Closed-circuit
clips.
Open-circuit
clips.
Meters.
300
325
3,%
375
400
425
Turns.
8 and 12*
8 and 131
8 and 138
8 and 144
Sand 111
8 and lo|
5
26j
24
22
20-
18:
1&
rums.
and 30
and 30
and 30
and 30
and 30
and 30
Turns countedf rom the inside turn outward.
Although a transmitting set using the flat spiral oscillation trans-
former is not as easily tuned as some other types, yet when the ad-
justments have once been made and tabulated it is practically as
efficient as other types. It has the advantage of being one of the
simplest, most rugged, and compact forms which can be installed in a
field set.
RECEIVING SET. TYPE B.
The receiving set consists of an inductively connected transformer
with broadly tuned secondary circuits, as mentioned on page 73,
galena, or other similar detector, high-resistance telephones, etc.,
provided with the necessary switches for tuning to different wave
lengths. The primary circuit includes the antenna, primary coil,
series condenser or not as may be needed, and counterpoise. The
antenna is connected to the primary coil through switches which
put into circuit a variable number of turns, steps of 10 turns being
inserted by one dial switch and single turns by the other. The total
number of primary turns is thus the sum of the numbers on the two
dials indicated by the two switch arms, which can be varied by
single turns from one to the whole number in the coil. For wave
lengths shorter than the fundamental wave length of the antenna, a
fixed condenser is inserted in series with the primary coil by throwing
the switch near the binding post marked " G " to the position " In,"
as shown in figure 79. For the longer wave lengths the switch is
thrown to the other position, short-circuiting the condenser, and thus
leaving only the coil in circuit. The secondary circuit includes the
RADIOTELEGRAPHY.
115
secondary coil, detector, and the stopping condenser shunting the tele-
phones. The coil is variable only by sections, marked " 100," " 200,"
etc., the smaller numbers to be used at the shorter wave lengths and
the larger ones at the longer wave lengths. The position of the sec-
FIELD RADIO PACK SET, MODEL 1913
DIAGRAM
FIG. 79.
ondary coil within the primary that is, the coupling is variable,
and for the sake of convenience a scale is provided so as to be able to
note the different adjustments. The coupling is closest when the
secondary is inside the primary, in which case the scale reading is 0,
116
RADIOTELEGRAPHY.
and vice versa, the coupling is loosest when the secondary is drawn
outside the primary and the scale reading is 40.
SHORT WAVES.
Primary condenser in series.
Switch on "In" contact.]
Wave
length (in
meters).
Primary
turns.
Secondary
turns.
Coupling
scale.
200
300
400
500
600
700
800
Etc.
18
26
36
47
60
74
88
Etc.
100
200
200
300
300
400
400
Etc.
20
20
20
20
20
25
30
Etc.
LONG WAVES.
Primary condenser short-circuited.
[Switch not on "In" contact.]
Wave
length (in
meters).
Primary
turns.
Secondary
turns.
Coupling
scale.
300
24
200
20
400
30
200
20
500
38
300
20
600
46
300
20
700
56
400
25
800
65
400
30
900
76
400
30
1,000
91
400
25
1,100
107
400
25
1,200
125
400
30
1,300
144
400
25
1,400
Etc.
162
Etc.
400
Etc.
25
Etc.
TUNING OF THE RECEIVING SET.
First, the detector must be adjusted to a sensitive point by means
of the test buzzer, the note of which should be clearly heard in the
receiving telephones when it is held near the antenna or counterpoise
wires or the coil windings. When the wave length of the sending
station is known, the number of turns in the primary and secondary
coils and the coupling should be set according to the values in the
above table, which will be approximately correct for all sets using
the standard antenna. When the wave length is unknown, then
signals can be found only by repeated trials of different combinations
of turns and couplings, in which, however, consistent sets of values
may be taken from the table. When once the signals have been heard
such further adjustments of primary and secondary turns and cou-
RADIOTELEGRAPH Y. 117
pling should be made as will give the maximum sound in the tele-
phones. In general it will be found that when there is interference
or static troubles the sharpest tuning and the best protection from
interference will be obtained when the loosest coupling is used ; that
is, when the secondary is pulled out as far as possible and still hear
the desired station. It will be noticed that for some wave lengths
there are two different possible combinations in the primary circuit,
either without a condenser and a few primary turns or with a con-
denser and more primary turns. It is impossible to tell which combi-
nation is the better without actual trial. In general the best coupling
between the circuits will vary with the damping of the transmitting
station, close coupling being possible with highly damped transmit-
ters, and loose coupling necessary with feebly damped transmitters.
In changing the coupling between the two circuits by means of the
handle on the secondary coil care must be taken to see that the con-
tacts on the various studs are not loosened, as otherwise the signals
may be lost entirely or the tuning made much broader on account
of high resistance that may be introduced at these contacts.
If the receiver is used with the standard antenna and signals are
being received from an unknown station, the table of wave length
can be used to determine approximately the wave length of the un-
known station.
SHELTER TENT.
This tent is similar in dimensions and construction to the standard
" common " wall tent issued by the Quartermaster's Department, but
is made of lighter material and is not provided with ridge pole or
uprights. In erecting the tent the extra sections furnished with the
mast should be used as the ridge pole and uprights as follows : One
hollow section, one plug, and one extension piece for the ridge, and
one section, one extension piece with spike for each upright. The
method of erection is illustrated in figure 80.
INSULATING DEVICE.
A device is provided for use in insulating the antenna when the
shelter tent is used in damp weather, consisting of a square piece of
sheet rubber with small marginal holes for lacing into the ventilator
at either end of the tent, and a tube attached to the center for ad-
mitting the antenna lead. When in use, sufficient slack should be left
in the antenna lead to form a drip loop outside of the tent, and if
found necessary a piece of heavy insulated wire can be used as a