We now come to what is the greatest source of trouble and annoyance in
an induction coil, namely the interrupter. Too much importance cannot be
attached to this instrument, for upon it depends largely the
satisfactory working of the coil. The operation of an induction coil and
the part played by the interrupter were fully explained in the chapter
on induction coils.
An adjustable interrupter is necessary for large coils, that is, one not
only whose speed may be governed, but also the time and duration of the
break.
[Illustration: Fig. 33. Simple Interrupter.]
The rapidity of oscillation of a mechanical interrupter is a very
different thing from the speed of break. The ideal speed of break is
instantaneous. In wireless telegraphy, very faint signals are heard more
distinctly in telephone receivers if the rate of interruption at the
station sending them is high. The human ear is somewhat more sensitive
to sounds higher than those ordinarily produced in the telephone
receivers of a wireless receptor. This seems to argue the use of a
high-speed interrupter to make and break the current. But the effect on
the coil must also be considered.
In the first place, where a condenser is shunted across the terminals of
the secondary as is the case with a wireless transmitter, a high-speed
interrupter would be very likely to set up harmful oscillations in the
secondary of the coil itself.
Second, if too fast, the rise and fall of the secondary currents will be
caused to run into each other, since the break will occur before the
primary current has reached a maximum and the reverse secondary current
has died away.
Third, the diameter of the core of a wireless coil is generally much
larger than that of the ordinary coil, and if a very rapid interrupter
is employed there is not time enough to properly magnetize the core
before the current is broken.
Fourth, the strength of the losses in the core caused by the eddy
currents and hysteresis are proportional to the interruptions in the
primary circuit and therefore a low speed will be the most efficient. A
rapid interrupter requires a higher voltage and amperage than the same
interrupter run at a lower speed.
These are some of the reasons why it is very desirable to use an atomic
interrupter or one so adjustable that the rate of the time and duration
of the "make" and "break" may be closely regulated. An ideal interrupter
is designed to give the longest time possible after contact is
established and before the "break" occurs.
It does not pay to construct an interrupter for an induction coil giving
sparks up to 2 inches in length. The type of interrupter in use on
automobile coils is perfectly well adapted to small coils, and may be
purchased complete with the platinum points for as low a price as $1.50.
The mechanical break described below is designed so that various
adjustments are possible and it may be adapted to almost any coil. Since
it is independent, it need not be mounted directly on the coil, but may
be placed in the position most convenient to the operator for
adjustment. The interrupter will not operate coils well on an
electromotive force above 30 volts, for the excessive voltage causes a
spark at the contacts when the circuit is broken and prolongs the
decadence of the primary current.
[Illustration: Fig. 34. Independent Interrupter.]
*Independent Atomic Interrupter.*—Fig. 34 illustrates two views of the
interrupter. Current is furnished to the electromagnets by a six volt
battery independent of the source supplying the coil. The interrupter is
set in operation by closing the circuit breaker on the aerial switch.
When the primary circuit of the transmitter is then completed by
pressing the key, the coil will respond immediately because the
interrupter is already in vibration.
The electromagnets (Fig. 35) are a pair of four ohm telegraph sounder
magnets. A hole is bored in the center of the top of each magnet core
and threaded with an 8-32 tap so that the pole pieces may be fastened
thereto, The shape and dimensions of these projections, which must be
made of soft iron, are illustrated in Fig. 35.
[Illustration: Fig. 35. Details of Magnets.]
A soft iron yoke _Y_, 2 1/2 x 7/8 x 1/4 inches, connects the bottom of
the magnets and supports them in an upright position. An 8-32 machine
screw passing upward through the base and yoke holds them firmly. The
base is preferably of hard rubber 4 x 3 1/4 x 3/4 inches.
[Illustration: Fig. 36. Details of Moving Parts.]
The moving parts are illustrated in Fig. 36. The main spring, _D_, is a
strip of spring steel, 2 1/8 inches long, 1/2 inch wide, and 1/32 inch
thick. The soft iron armature, _A_, is fastened to the spring by means
of two small 4-36 machine screws. _M_ is a piece of brass rod, 1 1/2
inches long, bent in the form of a hook and threaded with a 4-36 die to
screw in a similarly threaded hole in the back of the armature _A_. The
hooked portion of _M_ is fitted with a small piece of hard rubber rod,
_R_, to insulate it where it comes into contact with the spring, _G_.
The spring, _D_, carries a second hook, _E_, riveted to the center of
the spring 1 5/8 inches from the lower end. The hook, which is about 3/8
inch long, passes through a hole in the top of the spring, _F_, and
engages it so that it is set in operation by the vibratory motion of the
spring, _D_. The spring, _F_, is 1 3/4 inches long, 5/16 inch wide, and
ir 1/64 inch thick. It carries a platinum rivet 3/4 inch from its lower
end. The spring, _G_, is 2 1/2 inches long, 5/16 inch wide and 1/64 inch
thick. A heavy platinum rivet is fastened 2 1/4 inches from the lower
end. An elongated hole, 1/4 inch long and 3/16 inch wide, permits the
hook, _M_, to pass through the opening. A 5/32 inch hole, 1 1/4 inches
from the bottom, allows the adjusting screw to pass through and make
contact with the platinum rivet on the spring, _F_.
Two rectangular pieces of brass, _O_, 1 1/4 x 1/2 x 5/16 inches are
fastened to the base to support the springs.
[Illustration: Fig. 37. Details of Standard and Screws.]
The standard, _U_, supporting the adjusting thumbscrews is a piece of
3/8-inch brass, 2 1/2 inches high. It tapers from 1 1/2 inches at the
bottom to 3/4 inch at the top. A hole 2 1/4 inches from the bottom is
threaded with a 10-32 tap to receive the thumbscrew, _B_. A second hole
1 1/4 inches from the base is threaded with an 8-32 tap to fit the
adjusting screw, _S_.
Both of the adjusting thumbscrews carry heavy platinum points. The
standard is held upright to the base by means of two machine screws
passing through the base.
A 3/32 inch brass rod 1 3/4 inches long is threaded to fit a hole in the
top of the armature. A sliding weight, _W_, may be clamped in any
position on the rod by means of a thumb-screw. Raising or lowering the
position of the weight decreases or increases the natural period of
vibration of the interrupter. Screwing the hook, _M_, in or out so as to
shorten or lengthen it, decreases or increases the ratio of the make to
the break.
[Illustration: Fig. 38. Diagram of Connections for an Independent
Interrupter.]
Fig. 38 shows a diagram of the connections of the interrupter. The
standard, the thumbscrew, _B_, and the spring, _G_, form part of the
primary circuit of the induction coil. The standard, the thumbscrew,
_S_, the spring, _F_, and the electromagnets are placed in series with a
six-volt battery and connected to the circuit breaker on the aerial
switch, so that when the switch is thrown in position for transmitting,
the interrupter will be set in operation.
A condenser must be shunted across the larger contacts of the
interrupter in order to hasten the demagnetization of the core of the
induction coil and create a higher e.m.f. in the secondary. The
condenser must be suited in size to the induction coil with which the
interrupter is to be used and so the following table is appended to
serve as a guide.
The condensers are built up of alternate sheets of tin foil and
paraffined paper. Connections are made to the sheets by means of tin
foil strips which project out alternately from opposite sides as in the
illustration.
The paper should be about two inches larger each way so as to leave a
one inch margin on all sides of the tin foil.
[Illustration: Fig. 39. Construction of a Paper Condenser.]
When the alternate sheets of tin foil and paper have all been assembled,
the condenser is warmed so as to soften the paraffin. It is then placed
between two flat boards and subjected to great pressure in a letter
press or a vice. The capacity of a pressed condenser is often several
times that of a condenser of the same dimensions but not pressed.
*Mercury Interrupters.*—The mercury turbine interrupter is one of the
most convenient and successful breaks in use. The construction is such
that a stream of mercury is made to play against a number of saw shaped
metal teeth. A spiral worm terminating in a nozzle at the top is rapidly
revolved by an electric motor. The lower end of the tubular worm dips in
a mercury reservoir, so that when the spiral is revolved the mercury is
caused by centrifugal action to rise in the tube and be thrown out in
the form of a jet at the upper end. When the revolving jet strikes one
of the metal teeth, the circuit is closed and the current flows from the
mercury jet into the teeth. When the mercury jet passes between the
openings between the teeth, the circuit is interrupted. By raising and
lowering the saw teeth so that the mercury strikes either the lower or
upper part of them, the ratio between the make and break may be made
smaller or larger. By regulating the speed of the motor driving the jet,
the number of interruptions may be varied from 10 to 10,000 per second.
The bottom and sides of the mercury reservoir are ribbed to prevent the
mercury from attaining a rotary motion.
A somewhat simpler and more easily constructed type of mercury
interrupter consists of a hard rubber disk having a brass rod running
through from the periphery to the center, where it connects with the
shaft. The lower edge of the disk dips at an angle in a mercury bath and
is rapidly revolved by an electric motor. When the rod is under the
surface of the mercury, the circuit is made through the mercury to the
rod. The circuit is broken when the rod is above the surface. The
mercury is covered with a layer of alcohol, which prevents excessive
sparking and makes a quicker break. An interrupter of this kind when run
by a motor of the magnetic attraction type is exceedingly simple.
The break of any of the mercury type interrupters when properly adjusted
is much quicker than the hammer spring break and gives thicker sparks.
After the mercury has been in use awhile it becomes churned up into
small globules of a black color, but may be easily cleaned and restored
for use by shaking up with some strong sulphuric acid. Care must be
taken that the mercury is perfectly dry and free from acid before
replacing in the interrupter.
*Electrolytic Interrupters.*—Fig. 40 shows a diagram of a Wehnelt
interrupter. The cathode or negative electrode is a lead plate immersed
in dilute sulphuric acid. The anode is a piece of platinum wire placed
in a porcelain tube and projecting through a small hole in the bottom,
so that only a very small surface of the wire is exposed to the liquid.
When a strong electrical current is passed through the acid electrolyte,
the current is very rapidly interrupted by the formation of gases on the
small platinum electrode. The speed of the interrupter is variable
through great ranges by moving the platinum electrode up or down and
changing the amount of surface exposed to the liquid. The only
disadvantage of this interrupter is that the electrolyte soon becomes
heated, and unless the interrupter is provided with a water jacket or
some device for cooling, the bubbles of gas do not form freely. A
potential of at least 40 volts is required to operate a Wehnelt or other
electrolytic break.
[Illustration: Fig. 40. Wenhelt and Simon Electrolytic Interrupters.]
A Wehnelt interrupter may easily be made by sealing a platinum wire in a
glass tube. It is well to make several such tubes with the platinum
projecting from one-sixteenth to one-quarter of an inch. The different
tubes will each have a different speed of interruption, and one should
be picked out which seems to be most suitable for the coil upon testing.
Connection to the platinum wire is established by filling the tube with
mercury and dipping a wire in it.
Fig. 40 also shows a diagram of a Simon electrolytic interrupter. It
consists of a vessel containing dilute sulphuric acid and divided into
two parts by a thin porcelain diaphragm having a small hole in the
center. A lead electrode dips into each of the divisions. The
interruption is caused by exceeding a certain current density in the
small hole at the diaphragm. Upon the passage of the current the liquid
is so heated that it becomes vaporized. The vapor is a poor conductor to
low voltages and so the current is broken. Immediately upon the
cessation of the current, the vapor condenses and the circuit is
established again. This cycle repeats itself with a speed depending upon
the size of the aperture and the amount of current flowing.
A crude form of this type of interrupter may be made by heating the end
of a test tube in a pin flame, and then blowing on the open end of the
tube so as to burst the soft glass and form a small hole. Several such
test tubes should be prepared having holes varying from 1/32 to 1/8 of
an inch in diameter. The one which gives the best results upon trial is
selected for use. A number of holes in a single tube, if not too many or
too large in diameter, increases the efficiency and the speed of
interruption. The tube should be immersed in a glass jar containing
dilute sulphuric acid. One lead electrode is placed inside of the test
tube and the other outside. It makes no difference which way the current
flows through this interrupter.
The Caldwell interrupter is a modification of the Simon type in which
the size of the aperture is made adjustable by means of a pointed glass
rod which may be raised or lowered in the hole and the speed of
interruption varied. An interrupter of the test tube type as described
above may be modified to this form by locating the hole directly in the
center of the bottom of the tube and inserting in it a hard glass rod
which has been drawn out to a point.
Electrolytic interrupters do not require any condenser connected across
the break.
[Illustration: Fig. 41. Construction of Electrolytic Interrupters.]
Fig. 41 shows in section more substantial forms of both the Wehnelt and
Simon-Caldwell interrupters. The containers are ordinary 5 x 7 inch
battery jars. They are fitted with covers made of two thicknesses, _C_
and _B_, of 3/4-inch wood. The upper piece, _C_, is 6 inches in
diameter, while the under one should fit snugly into the interior of the
jar. The wood must be boiled in paraffin to protect it from the action
of the acid. A slit is made in the left-hand side of both covers for the
passage of a lead electrode, _L_, 1 inch wide and 1/4 inch thick. The
upper end of the electrode is bent over and fitted with a binding post.
[Illustration: Fig. 42. Details of Electrolytic Interrupters.]
The mechanism for adjusting the interrupters is the same in both cases.
The dimensions are indicated in Fig. 42. A brass yoke, _Y_, is mounted
on the cover in the position shown. A 1/2-inch hole is bored through the
upper part of the yoke and a piece of brass tubing, _S_, 1 inch long
soldered in a vertical position in the hole. A 1/4-inch threaded brass
rod passes through the tube, 5. A groove is milled in A along its entire
length and engages a pin in the wall of _S_. The rod is thus enabled to
slide up and down in the tube but is prevented from revolving. A fiber
head, _H_, is fitted with a brass sleeve or bushing in its center. The
bushing should fit tightly into the fiber head and is threaded to fit
the rod, _A_. The electrode may then be carefully raised or lowered by
revolving the head. The tube, _V_, for the Simon-Caldwell interrupter is
a hard glass test tube. A 1/8-inch hole is blown in the bottom of the
tube. A hard glass rod, _G_, is drawn out to a point and fastened to the
lower end of the rod, _A_, by means of a short length of flexible rubber
tubing, _R_.
The tube, N, for the Wehnelt break is made from a piece of hard rubber
tubing 6 inches long, having a bore of J inch.
[Illustration: Fig. 43. Electrolytic Interrupter.]
The lower end is fitted with a spark plug porcelain. The porcelain must
fit the tube tightly and not leak. The electrode, _P_, is a piece of
brass wire which will just pass through the hole in the porcelain. The
upper end of the electrode, _P_, is soldered or fastened otherwise to
the lower end of the rod, _A_.
A small hole, _h_, should be made in the tubes, _N_ and _V_, above the
level of the electrolyte in the jar. When the interrupter is in
operation the electrolyte gradually rises in the tubes, and would
corrode the lower end of _A_ if it were not able to pass out through the
vents.
In the Simon-Caldwell interrupter, a strip of lead passes from the
binding post mounted on the foot of the yoke down inside of the test
tube. The size of the hole in the tube is regulated by revolving the
fiber head so that the glass pointed rod will be inserted in or
withdrawn from the hole.
The frequency of the interruption will also depend somewhat on the
concentration of the acid solution. It is therefore best to start with a
weak solution and add acid slowly until it is of the proper strength.