WIRELESS TELEGRAPHY.
Arrangements of Simple Circuits of Coil and
Coherer for Receiving and Sending
Messages. The Coherer. Carbon Coherer.
Coherer without Filings. Aluminum
Coherer. Steel Ball Coherer. The
Oscillator. Clarke's Oscillator. Triple
Oscillator. The Coil. Translating
Devices. Air Conductor, etc. 248-265
INDEX 266
Bibliography 270
CONTENTS OF TABLES.
PAGE
GOOD PROPORTIONS OF CORE LENGTHS 7
TABLE OF "SECONDARY" WINDINGS 24
POLARITY TESTS 45
DIMENSIONS FOR DIFFERENT SPARK
LENGTHS 50
TABLE SHOWING RESISTANCES AND FEET
PER POUND OF COPPER AND GERMAN
SILVER WIRE 64
SPECIFIC INDUCTIVE CAPACITY 119
WAVE LENGTHS AND TEMPERATURES 138-139
TABLE OF RELATIVE COSTS OF MATERIALS 191
LIST OF ILLUSTRATIONS.
FRONTISPIECE, THE QUEEN 45″ SPARK COIL.
FIG. PAGE
1. Section of Coil 4
2. Insulating Tube Ends 10
3. Sectional Winding 11
4. Section " First Method 12
5. " " Second Method 13
6. Proportional Diagram of Coil 15
7. Section Winder, End View 17
8. " " Face View 17
9. Assembly of Coils 18
10. Polechanging Switch 31
11. Disruptive Tesla Coil 35
12. Spark Coil for Gas Engine 38
13. Reproduction of a 32-inch Spark 47
14. Simple Medical Coil 53
15. Connections for Simple Medical Coil 55
16. Interchangeable Medical Coil 56
17. Vibrator for Medical Coil 60
18. Simple Contact Breaker 65
19. Imperfect Form of Contact Breaker 67
20. Superior Form of Contact Breaker 67
21. Spotteswoode Contact Breaker 69
22. Polechanging Contact Breaker 74
23. Wehnelt Interrupter 78
24. Ribbon Vibrator 81
25. Queen Contact Breaker 82
26. Adjustable Contact Breaker 86
27. Cone Contact Breaker 88
28. Coil Head Contact Breaker 89
29. Leyden Jar 101
30. Plate Condenser 102
31. Arrangement of Condenser Plates 104
32. Condenser Charging, First Method 110
33. " " Second Method 112
34. Adjustable Condenser 118
35. Spark between Balls 125
36. Short Spark between Balls 125
37. Sparkling Pane 125
38. Luminous Design 128
39. Electric Brush 128
40. Spectrum—Solar 132
41. Spectroscope and Coil 133
42. Simple Air Pump 141
43. Geissler Air Pump 144
44. Sprengel Air Pump 144
45. Solution Tube 150
46. Fluorescent Bulbs 150
47. Ruby Tube—Crookes 150
48. Iridio-platinum Tube—Crookes 151
49. Revolving Wheel 154
50. Tube Holder 157
51. Side View of Wheel 157
52. Geissler Tubes 160
53. Triangle on Disc 161
54. Maltese Cross on Disc 161
55. Gas Lighting Circuit 165
56. Connections for Gas Burners 169
57. Bartholdi Automatic Burner 172
58. Connections for Automatic Burner 174
59. The Grenet Cell 180
60. The Fuller Cell 184
61. The Gethins Cell 193
62. Lead Plate for Storage Cell 201
63. Wooden Separator 201
64. Charging with Rheostat 207
65. Charging with Lamps 207
66. Harrison Electrodes 211
67. Hydrometer 221
68. Hertz Resonator 227
69. Tesla Circuit 229
70. Tesla Cut Out 231
71. Tesla Cut Out, Top Plan 232
72. Circuit for X Ray Apparatus 237
73. Queen's Self-Regulating X Ray Tube 240
74. Transmitter for Wireless Telegraphy 250
75. Receiver for Wireless Telegraphy 252
76. The Branley Coherer 254
77. Clarke's Oscillator 259
78. Triple Oscillator 259
79. Air Wire Insulators 263
CHAPTER I.
COIL CONSTRUCTION.
In commencing a description of the Ruhmkorff coil and its uses, a brief
mention of the fundamental laws of induction directly bearing on its
action will assist in obtaining an intelligent conception of the proper
manner in which it should be constructed and handled.
Any variation or cessation of a current of electricity flowing in one
conductor will induce a momentary current in an adjacent conductor; and
if the second conductor be an insulated wire coiled around the first
conductor, also a coil of insulated wire, the effect is heightened. The
intensity of the secondary or induced current increases with the number
of turns of its conductor, the abruptness and completeness of the
variation of current in the first or primary coil, and the proximity of
the coils. And the insertion of a mass of soft iron within the primary
coil by its consequent magnetization and demagnetization augments still
further the inductive effect. There are other contributing causes which
cannot be treated of here, but are of not so much importance as the
foregoing.
In the Ruhmkorff coil, which is an application of the above laws, the
primary coil is of large wire and the secondary coil of extremely fine
wire, of a length many thousand times greater than the wire of the
primary coil. The current is abruptly broken in the primary circuit
by a suitable device—the contact breaker or rheotome. The current
induced in the secondary at the make of the circuit is in the opposite
direction to that of the primary coil and battery, but the current
at the break of the circuit is in the same direction as that of the
primary. The effect of the current at the break of the circuit is
more powerful than that at the make, which latter is also somewhat
neutralized by the opposing battery current. A condenser or Leyden jar
is connected across the contact breaker to absorb an _extra current_
induced in the primary coil by the break of the circuit, which would
tend to prolong the magnetization of the core beyond the desired limit.
The whole apparatus is mounted on a wood base, having the condenser in
a false bottom for the sake of compactness.
It is not herein intended to describe all the minor operations in the
construction of a Ruhmkorff coil. A sufficient description and review
of the main points to be considered, however, will be given to enable
a person fairly proficient in the use of simple tools to construct a
serviceable instrument.
The parts and their arrangement in relation to one another are shown in
Fig. 1, but are not drawn strictly to scale, although very nearly so.
[Illustration: FIG. 1.]
_C_ is the core, consisting of a bundle of soft iron wires as fine as
can be obtained. The greater the subdivision of the core the quicker
will it respond to the magnetizing current in the primary coil, and
lose its magnetism when the current ceases. It has another advantage,
in that the disadvantageous eddy, or Foucault currents, are lessened,
which fact, however, is of not enough importance to need extended
consideration.
Many coil-makers saturate the core with paraffin or shellac, which
is of slight benefit. This core is wrapped in an insulating layer of
paraffined paper or enclosed in a rubber shell, there not being any
great necessity to use more than ordinary insulation between the core
and the primary coil.
In the majority of induction coils or "transformers" used in the
alternating current system of electric lighting, the iron cores form
a closed magnetic circuit. A closed magnetic circuit in a Ruhmkorff
coil could be obtained by extending the iron core at each end and then
bending and securing the ends together, forming, as it were, a ring
partly inside and partly outside the coil. But although the inductive
effects would be heightened and less battery power required, the
slowness of the circuit to demagnetize would alone be detrimental to
rapid oscillations of current.
There would also be a loss from a greater hysteresis (energy lost in
the magnetization and demagnetization of iron). A core magnetizes
quicker than it demagnetizes, and the latter is rarely complete; a
certain amount of residual magnetism remains, hysteresis being strictly
due to this retention of energy (Sprague). Hysteresis shows itself in
heat, but must not be confounded with Foucault or eddy currents. The
latter are corrected by subdividing the metal, but the former depends
upon the quality of the metal, and increases with its length.
Moreover, a coil with a closed magnetic circuit requires an independent
contact breaker.
In most of the alternating currents used in lighting their rapidity of
alternation is but one hundred and twenty-five periods per second. As
in the simple electromagnet, the proportions of diameter and length
of the primary coil and core will determine its rapidity of action. A
short fat coil and core will act much quicker than a long thin one. But
on a short fat coil the outside turns would be too far removed from the
intensest part of the primary field. A good proportion of core length
is given in the following table:
Spark Length Iron Core.
of Coil.
¼ 4″ × ½″
½ 5″ × 10∕16″
1 7″ × ¾″
2 9″ × 1″
6 12″ × 1⅛″
12 19″ × 1½″
The primary coil _P_ consists of two or not more than three layers of
insulated copper wire of large diameter, being required to carry a
heavy current in a 2-inch spark coil, probably from 8 to 10 amperes.
In designing the primary coil a great advantage arises from using
comparatively few turns but of large wire. Each turn of wire in the
primary has a choking effect upon its neighbor by what is termed
self-induction.
As the primary coil and core may be considered as an electro magnet,
it may not be out of place to notice the rule governing such.
Magnetization of an iron core is mainly dependent upon the ampere turns
of the coil surrounding it—that is, one ampere carried around the core
for one hundred turns (100 ampere-turns) would equal in effect ten
amperes flowing through ten turns. Practically speaking, there would
be certain variations to the rule, for one difficulty would arise in
that the smaller wire used in conveying the smaller current would fit
more compactly and allow more turns to be nearer the core, the active
effect of the turns always decreasing with their distance from the
core. And although a large current and few turns would not have so much
self-induction, there would be trouble at the contact breaker, owing
to the large current it would have to control.
The most suitable sizes of wire for the primary coil are: No. 16 B.
& S. for coils up to 1 inch spark; No. 14 B. & S. up to 4 inches of
spark, and No. 12 B. & S. for a 6-inch spark coil. The coil should be,
say, one-twelfth of the core length shorter than the core.
_I_ is the insulating tube between the primary coil and the secondary
coil _S_. Here great precaution is necessary to prevent any liability
of short circuiting or breaking through of sparks from the secondary
coil. This danger cannot be underestimated, and the tube should be
either of glass or hard rubber, free from flaws, varying in thickness
with the dimensions of the coil. It should extend at least one-tenth
of the total length of the primary coil beyond it at each end. The end
of this tube can be turned down so as to allow of the hard rubber reel
ends being slipped on and held in position by outside hard rubber
rings (Fig. 2).
[Illustration: FIG. 2.]
The secondary coil consists of many turns of fine insulated copper wire
separated from the primary coil by the insulating tube and a liberal
amount of insulating compound at each end. In coils giving under 1 inch
of spark this coil may be wound in two or more sections.
[Illustration: FIG. 3.]
The usual manner of constructing these sections is to divide up the
space on the insulating tube by means of hard rubber rings placed at
equal distances apart, in number according to the number of sections
desired (Fig. 3). The space between each set of rings, or between
the coil end and a ring, is wound with the wire selected, the filled
sections constituting a number of complete coils, which are finally
connected in series. The sectional method of winding prevents the
liability of the spark jumping through a short circuit, but heightens
its tendency to pass into the primary coil at the ends, where it must
be therefore specially insulated from it.
[Illustration: FIG. 4.]
In winding these sections there is a method now generally adopted which
has many good points, although at first it may seem complicated. The
old way of filling two sections was to wind both in the same direction
as full as desired, then join the outside end of the left-hand coil
to the inside end of the right-hand coil. This necessitated bringing
the outside end down between two disks, or in a vertical hole in the
sectional divider, and thereby rendered it liable to spark through into
its own coil. This is shown in Fig. 4, _A_ and _C_ inside ends, _B_ and
_D_ outside ends, the disk being between _B_ and _C_.
[Illustration: FIG. 5.]
Reference to Fig. 3 shows the new method, and Fig. 5 shows an enlarged
diagram of sections 2 and 3 of Fig. 3.
Sections 1 and 3, Fig. 3, are filled with as many turns as desired; the
spool is then turned end for end, and sections 2 and 4 are wound, being
thus in the opposite direction of winding to sections 1 and 3.
The inside ends of 1 and 2 and 3 and 4 are soldered together, and the
outside ends of 2 and 3 are also soldered together.
The outside ends of 1 and 4 serve as terminals for the coil.
This method of connection leaves all the turns so joined that the
current circulates in the same direction through them all, as will be
seen by an examination of the enlarged diagram, Fig. 5.
Sprague, in his "Electricity: Its Theory, Sources, and Application,"
recommends that the turns of wire in the secondary coil shall gradually
increase in number until the middle of the spool is reached, and then
decrease to the spool end, in order that the greatest number of turns
be in the strongest part of the magnetic field (see Fig. 6). _D D D_
are section dividers, _S_ secondary windings, _P_ primary coil. The
selection of the size of wire to be used depends on the requirements as
to the spark. If a short thick spark be desired, use a thick wire, say
No. 34 B. & S.; if a long thin one, use No. 36 to No. 40 B. & S.
[Illustration: FIG. 6.]
Although it is impossible to lay down rules for determining the exact
amount of wire to be used to obtain a certain sized spark, yet a fair
average is to allow 1¼ pounds No. 36 B. & S. per inch spark for small
coils and slightly less for large ones.
The most satisfactory and perhaps the easiest way for large coils is to
wind the secondary in separate coils, made in a manner similar to that
employed in winding coils for the Thompson reflecting galvanometer.
This method, first described by Mr. F. C. Alsop in his treatise on
"Induction Coils," is somewhat as follows:
A special piece of apparatus (Figs. 7 and 8) is necessary, but presents
no great difficulty in manufacture. A metal disk, _D_, one-sixth of
an inch thick and 7 inches in diameter, is mounted on the shaft _S_.
A second disk is provided with a collar and set screw, _A_, in order
that it may be adjusted on the shaft at any desired distance from the
stationary one. When the diameter of the coil to be wound has been
decided upon, a wooden collar, _W_, with a bevelled surface is slipped
on the shaft, it corresponding in diameter with the desired diameter of
the hole through the centre of the secondary coil. As these coils are
going to be made as flat rings and slipped on over the insulating tube,
a remark here becomes necessary on this diameter. Reference to Fig. 9
will show that it is intended that the coils near the reel ends shall
fit very loosely on the tube _T_ (Fig. 1)—in fact, that there shall be
a clearance of possibly one-half inch in the extreme end, diminishing
gradually to a fifteenth of an inch in the centre coils. Therefore it
becomes necessary to provide a number of wooden rings equal to the
desired diameter of the central hole in the coil. The thickness of
the wood determining the width of the individual coil depends on the
selection of the operator; but the rule may be laid down that the
narrower the coils the better the insulation of the complete coil will
be on completion.
[Illustration: FIG. 7.]
[Illustration: FIG. 8.]
[Illustration: FIG. 9.]
One-sixteenth of an inch is a very fair average, and has been generally
adopted by the writer.
A quantity of paper rings are now cut out of stout writing paper which
has been soaked in melted paraffin. If a block or pad of letter paper
be soaked in paraffin and allowed to become cold under pressure, the
ring may be scratched on the surface of it and the block cut through on
a jig saw. The central apertures of course will vary in size with their
position on the tube _T_ (Fig. 9).
The coil winder is now either mounted in a lathe or fixed in a hand
magnet winder in such manner that it can be steadily and rapidly
rotated. The wire to be wound comes on spools, which can be so
threaded on a piece of metal rod that they turn readily. A metal dish
containing melted paraffin is provided with a round rod, preferably of
glass, fixed under the paraffin surface, so that it can rotate freely
when the wire passes under it through the paraffin. Two paper rings are
slipped on the winder that they may form, as it were, reel ends for the
coil, and if the metal disks have been warmed it is an easy matter to
lay them flat.
The end of the wire is then passed through the paraffin under the
glass rod and through the hole _H_ in the metal disk for a distance
of, say, 6 inches, and held to the disk outside with a dab of paraffin
or beeswax. Then the winder is rotated and the space between the paper
disks is filled with wire. The paraffin, being hot, will adhere to the
wire, and cooling as the wire lays down on the winder, hold the turns
together and at the same time insulate them from each other. It will
not be possible to lay the wire in even layers, as would be necessary
in winding a wider coil, but the spaces can be filled up, using
ordinary care that no radical irregularity occurs—that is, that only
adjacent layers are likely to commingle.
When the space is filled up to the level of the paper disks and the
paraffin is hard, loosen the set screw, and removing the outside disk,
the coil can be slipped off, or a slight warming will loosen it. Any
number of these coils can be made, and there are the advantages in
this mode of construction that a bad coil will not spoil the whole
secondary, and that the wire can be obtained in comparatively small
quantities.
As each coil will not be of very high resistance, the continuity of
the wire can be readily tested by means of a few cells of battery,
connecting one end of the coil to one pole of the battery, and the
other pole of the battery and coil end touched to the tongue. If a
burning sensation is experienced, the connection is not broken. Where
possible the coils should be measured as to their resistance on a
Wheatstone bridge.
When the requisite number of coils has been prepared, they are
assembled in the following manner (Fig. 9): The coils, having their
aperture diameter graded, are placed in order, and starting with the
one having the largest hole, it is slipped over the primary protection
tube _T_, one end being brought out through a hole in the reel end
drilled vertically or between the reel end and the coil. A couple of
paper rings are then slipped on the tube, and another coil placed over
them, having its ends connected as in Fig. 3. This process is continued
until all the coils are in place. The annular space between the coils
and the tube _T_ (Fig. 9) is filled in with melted paraffin and the
coils gently pressed together, so as to form a compact mass, paraffin
being poured over the outside of the whole combination. Before winding
any wire used in this work it must be perfectly dry, which end can be
accomplished by subjecting the whole spool to a short period of baking
in a moderately warm oven.
The accompanying table gives the length of No. 36 silk-covered wire
that will fill a linear space equal to one thickness of the wire in
different-sized rings. This size wire wound tight will give 125 turns
per linear inch. Therefore on a ring having a middle aperture of 1½
inches and an outside diameter of 4 inches, there will be 156 turns, or
a total length of 1347 inches. This is obtained as follows: 1½ inches
× 3.1416 = 4.7124 (or 4.712); 4 inches × 3.1416 = 12.5664 (or 12.56);
(4.712 + 12.56)∕2 = mean circumference—viz., 8.635 inches.
This mean × number of turns in thickness of ring between the two
circumferences—viz., 156 = 1347 inches.
TABLE OF SECONDARY WINDINGS.
————————————————————————-+——————————————————+——————————————————+
| 1½″ | 2″ |
NO. 36 SILK-COVERED WIRE.|Aperture Diameter,|Aperture Diameter,|
125 TURNS PER LINEAR | 4.712″ | 6.283″ |
INCH. 13,306 FEET PER |Aperture |Aperture |
POUND. | Circumference. | Circumference. |
————————————————————————-+——————+————-+————-+——————+————-+————-+
Outside diameter | 4″ | 5″ | 6″ | 4″ | 5″ | 6″ |
Outside circumference |12.56 |15.70|18.84|12.56 |15.70|18.84|
Mean circumference | 8.635|10.20|11.78| 9.421|10.99|12.56|
Turns between | | | | | | |
circumferences | 156 | 219 |282 | 125 | 188 | 250 |
Distance between aperture| | | | | | |
and outside, in inches | 1.25 | 1.75| 2.25| 1 | 1.50| 2 |
Length of wire, in inches| 1347 | 2234| 2650| 1178 | 2066| 3140|
————————————————————————-+——————+————-+————-+——————+————-+————-+
————————————————————————-+——————————————————+
| 2½″ |
NO. 36 SILK-COVERED WIRE.|Aperture Diameter,|
125 TURNS PER LINEAR | 7.854″ |
INCH. 13,306 FEET PER |Aperture |
POUND. | Circumference. |
————————————————————————-+——————+————-+————-+
Outside diameter | 5″ | 6″ | 7″ |
Outside circumference |15.70 |18.84|21.99|
Mean circumference |11.78 |13.35|14.92|
Turns between | | | |
circumferences | 156 | 219 | 282 |
Distance between aperture| | | |
and outside, in inches | 1.25 | 1.75| 2.25|
Length of wire, in inches| 1838 | 2924|4207 |
————————————————————————-+——————+————-+————-+
To obtain the length of wire necessary for a ring occupying more than
the space of one turn on the primary insulating tube, multiply the
length before obtained by the number of turns in the space it occupies.
Thus a flat ring one-tenth of an inch thick would equal 1347 inches ×
12.5.
This rule is necessarily only approximate, owing to the way the wires
bed on each other from their cylindrical section. In actual practice,
when the wire is run through the paraffin bath not more than 50 per
cent of the calculated wire will occupy the space. And the thickness of
the paper rings must also be added when figuring the total length of
the coil. In the iron-clad transformers or induction coils of highest
efficiency used in the alternating current electric light system, the
rule for determining the windings of the coils is based on the ratio
of the turns of wire in the primary to the turns in the secondary, the
electromotive force in the primary, and the lines of force cut by the
windings.
The secondary ends can be attached to binding posts mounted on the reel
ends. Unless these reel ends be very high and clear the outside of the
coil considerably, it is better to mount the binding posts on the top
of the hard rubber pillars. A neat plan is to mount on the top of the
coil a hard rubber plate reaching from reel end to reel end, and place
the binding posts on that.
A discharger consists of two sliding metal rods with insulated handles
passing through pillars attached to the secondary coil. The inside ends
of these rods is provided with screw threads for the ready attachment
of the balls, points, etc., which are to be used. The substance to be
acted upon is laid on a rubber or glass table midway between the rod
pillars and slightly below the level of the rods.
By hinging the rod pillars, or using a ball and socket joint, the
discharger can be inclined so as to be better brought near the
substance on the table.
The next important part of the coil is the contact breaker.
The armature _R_ is a piece of soft iron carried at the end of a stiff
spring, in about the middle of which, at _B_, is riveted a small
platinum disk or stud. The adjusting screw _A_ has its point also
furnished with a piece of platinum, which is intended to touch the
platinum on the spring when the latter is in its normal position. The
core _C_ of the coil serves as an electro-magnet. When the current
flows from the battery (represented by the figure at _L_) through the
primary coil and armature spring to the adjusting screw, it causes the
armature to be drawn to the magnetized core, but thereby draws the
platinum disk away from the adjusting screw. In so doing it breaks the
circuit, the magnet loses its power, and the elasticity of the spring
reasserting itself, carries the armature back, thereby reclosing the
circuit. This is repeated many times in a second, the result being a
continual vibration of the spring, and a consequent interruption to the
current.
The condenser or Leyden jar _J_, connected as in the diagram to the
base of the vibrating spring at _K_ and to the adjusting screw wire
_M_, is constructed as follows: On a sheet of insulated paper is laid
a smaller sheet of tinfoil, one edge of which projects an inch or so
over one end of the paper. Another sheet of paper covering this carries
a second sheet of tinfoil, one end of which projects as in the first
sheet, but at the opposite end of the paper. Tinfoil and paper sheets
are laid in this manner alternately until a sufficient number is
attained. The projecting ends are then clamped together and the whole
pile immersed in melted paraffin, as will be described in a subsequent
chapter. Wires are affixed to these clamped ends which serve to connect
the condenser with the contact breaker. The conventional sign for a
condenser is that used at _J_, showing the two series of plates, the
insulation or dielectric, as it is called, being understood.
The size of condenser to use with different-sized coils varies
according to the winding of the primary and the battery used. A primary
coil of few turns would not necessitate as large a condenser as one of
a large number of turns. At the same time, a condenser may be made of
too great a capacity, and thereby weaken the action of the coil.
The base upon which the coil and its parts are mounted may be of dried
polished wood. But where the coil is designed to give large sparks—over
2 inches—it is an advantage to use hard rubber one quarter of an inch
and upward in thickness. Glass, were it not for the difficulty of
drilling it and its brittleness, would be a desirable material for a
coil base in a dry atmosphere. Hard red or black fibre coated with
shellac varnish is also serviceable, and, moreover, is extremely easy
to work. Slate must never be used; there is too much liability of
iron veins being found in it, which in such high tension experiments
as will be described would seriously impair the usefulness of the
apparatus. The material selected for the base must be one that will
not absorb moisture. A paraffined surface collects moisture up to a
certain point in isolated drops, whereas a glass and even a hard rubber
surface condenses the moisture as a film, which latter is extremely
undesirable. But unfortunately the fact that a paraffined surface
does not present a pleasing appearance would probably result in its
rejection. And lastly, by mounting the coil on hard rubber blocks, or
extending the reel ends to raise the coil body, a high insulation can
be obtained at the sacrifice perhaps of appearance or height. From
the care taken to insulate the secondary coil, it may be considered a
superfluous precaution to so carefully select a base, but practical
work with the instrument at some important crisis will demonstrate
the necessity of extreme care in the smallest details relating to
insulation. It may be well to note here that hard rubber is acted upon
by ozone, and is thereby impaired as an insulator.
[Illustration: FIG. 10.]
The base forms the top of a flat box in which the condenser lies; but
there are a few points worth considering right here. As the connections
of the coil will probably be under the base, a sufficient space must
intervene between the base and the top of the condenser. It is a
good plan to lay the condenser at least one half inch below the top
of this box, and fill up to, say, one eighth of an inch with melted
paraffin, leaving the condenser wires projecting for attachment. The
connections of the primary coil and contact breaker should by all means
be soldered, not simply wires held under screw nuts. And, moreover,
all wires under the base should be so run that they do not cross one
another, which precaution only requires a little planning. Then, when
the connections are all made and the base laid on top of the box, it
can be pressed down if the paraffin be warm, so that the screw heads
and wires mark out their own channels and cavities in which to lie.
A commutator or pole-changing switch is often added to change the
polarity of the battery current. The diagram of connection is shown in
Fig. 10. When the levers are as in the figure, the circuit is broken
and no current flows through the coil.
COILS IN SERIES.
Ruhmkorff coils can be connected in series, but it is not to be
recommended. When it becomes necessary, however, the cores should be
removed, and one long core inserted, extending through each primary.
This will bring the time constants of each primary coil together and
prevent the interference otherwise present. The primary coils and
secondary coils are connected in series by assuming that they are
but adjacent sections of one complete instrument. Of course, as the
resistance of the primary is raised, the electromotive force of the
battery must be raised also.
OIL IMMERSED COIL.
A highly satisfactory induction coil can be made without much labor and
few tools, and will prove useful in many experiments which would not
warrant a more expensive instrument.
Make a bundle of soft iron wires, No. 22 B W G, for the core, ten
inches in length and one inch or more in diameter. Wrap this with
insulating tape or even ordinary tape to prevent the primary coil from
coming in contact with the iron. Now, wind on a primary of two layers
No. 14 B & S gauge cotton-covered copper wire, and insert the coil into
a hard rubber (or glass preferred) tube large enough to hold the coil
tight and to project an inch or so beyond the core ends.
A secondary coil of about one pound No. 36 cotton-covered magnet wire
should now be made on a hard rubber spool, the hole through centre
of this spool must be at least one inch larger in diameter than the
diameter of the primary cover. This spool should not exceed four
inches in length, and is to be slipped over the primary coil and held
suspended by blocks of wood in such a manner that it does not touch
the primary coil or cover. The whole outfit is now immersed in an
earthenware or glass vessel filled with linseed or heavy paraffin oil.
The contact breaker and condenser will be mounted independently; the
condenser for the two-inch spark coil will be suitable (see Table on
page—7).
"TESLA" COIL.
The coil just described, without contact breaker or iron core, can be
connected up and used in place of a "Tesla coil," which it resembles.
The coils used by Nikola Tesla are so many and varied that it becomes a
difficult task to describe a mode of construction which will meet the
wants of those who ask for "Tesla" coils. The _American Electrician_
gives a description of one wherein a glass battery jar, 6 inches × 8
inches, is wound with 60 to 80 turns of No. 18 B & S magnet wire. Into
this is slipped a primary, consisting of 8 to 10 turns of No. 6 B & S
wire, and the whole combination immersed in a vessel containing linseed
or mineral oil.
[Illustration: FIG. 11.]
DISRUPTIVE "TESLA" COIL.
For Fig. 11 the specification is as follows: Secondary, 300 turns of
No. 30 B & S silk-covered magnet wire, wound on rubber tube or rod,
and the ends encased in glass or rubber tubes. This is inserted _into_
the primary, which consists of two coils, each of 20 turns No. 16 B &
S rubber-covered wire, wound separately on a long rubber tube not less
than ⅛ inch thick. The last tube must be large enough to be very loose
when the secondary coil is inserted in it, and it must project at least
two inches over each end of the secondary. A hard rubber division must
be placed between these primary coils. The four ends of the latter
coils are connected _C C_ to two condensers and _D D_ to two discharger
balls, the secondary wires going to the exhibitive apparatus. A further
description of these connections is to be found in Chapter XII., also
notes upon the use of the disruptive coil.
_Coils for Gas Engines._
These are either primary only or primary and secondary. Two to three
pounds of No. 14 B & S magnet wire are wound on an iron wire core eight
to ten inches in length by one inch in diameter. The contact is made
and broken in the igniter of the engine as at the wipe spring of a
ratchet gas burner. Four to eight large cells of dry battery are used,
or eight cells Edison-Lalande—iron-clad type. Number of cells varies
with size of coil needed, some classes of engines require a heavier
spark than others to ignite the vapor.
When a primary and secondary are used, the primary should be made of
two or three layers No. 14 B & S magnet wire, and a secondary of one
pound No. 34 B & S magnet wire. There can be an independent contact
breaker or the coil can be made up similar to a one-half inch spark
Ruhmkorff coil (see Chapter I.).
[Illustration: FIG. 12.]
The method of connecting up a coil of the latter description is shown
in Fig. 12, which is self-explanatory. It shows a form of cam-shaft
switch which is operated by the engine, and which opens and closes the
primary circuit of the induction coil, the sparks from the secondary
winding passing between the points of the igniter in the engine
cylinder. As shown in Fig. 12, the igniter or ignition plug is similar
in operation to a coil discharger, the two terminals being, however,
insulated from each other by the use of porcelain. To ensure a good
insulation under the severe working conditions has been somewhat of a
task, but it seems to have been attained in the types of igniters known
as the Splitdorf and the Roche or New Standard.
The Splitdorf gas-engine coil is the result of much experiment and
careful design. It is built to stand hard usage, and the insulation
used has been adopted only after exhaustive test. In automobile work,
where a heavy strain is made upon the engine, as in climbing heavy
grades, it has been found that a stronger spark gives surer results.
This would indicate more battery current through the coil, and it is a
wise precaution to have a few extra cells attached that can be switched
on if necessary.
In constructing spark coils for gas engines particular care must
be given to the contact breaker. In most types of gas or oil vapor
engines it is absolutely necessary to have the spark pass with uniform
regularity, and immediately and surely when required. For automobiles
or where the apparatus is subject to jar, a heavy iron vibrating
armature would become unreliable by reason of its inertia and its
responding to shock. At every jolt of the vehicle it would jar and get
out of rhythm, and it certainly seems preferable to use a mechanical
contact apparatus whenever feasible. In the older type of gas engine
the spark is made by mechanism breaking contact right in the vapor. The
actual arrangement of these devices is detailed and illustrated in the
later works on gas and oil engines.
RESISTANCE COILS.
Although foreign to the title of this book, these coils will be
mentioned, being often necessary as accessories to the operation of
coils, wireless telegraphy, etc. These are coils of insulated German
silver wire, wound to a specified resistance. The main feature about
those designed for testing is that they are wound non-inductively—that
is, the wire is wound double in such manner that the current flows
both ways around the turns, and so neutralizes the inductive action.
In cases where dynamo current is to be used, as in telegraphs operated
from dynamo current, the coils are wound on tin tubes to make them
fireproof and yet radiate the heat. As the resistance of German silver
varies very largely, only approximate figures can be given. The table
(page 64) has been made up from the best averages obtainable. The
carrying capacity of resistance coils varies with their construction,
the better they can radiate heat, the more current they can safely
carry.
GENERAL REMARKS ON COILS, ETC.
Ruhmkorff induction coils should always be fitted with a switch to
open, close, or reverse the power circuit, a double throw, double
pole, baby knife switch, mounted on a separate porcelain base, is very
suitable. Such a switch is open when the handle is vertical, and it
should always be left so when changing connections, fixing battery,
etc. A large, well-finished coil will have the secondary wires brought
in rubber tubes to binding posts mounted on hard rubber pillars, or
to binding posts mounted considerably above the coil cover level.
A very neat mode is shown in the frontispiece on the large 45-inch
spark coil. Here the secondary wires go to hard rubber pillars, which
also carry adjustable rod dischargers. These rods are movable towards
or away from each other by means of the large hard rubber handle to
which they are connected by a simple system of levers. In this coil
the secondary is moulded on a flexible tube, which fits loosely over
the primary tube in order to compensate for changes of temperature
and consequent expansions and contractions. All well-designed coils
should be so arranged that the primary coil and core can be readily
removed from the secondary, or _vice versa_. It is sometimes desirable
to use a different primary. This arrangement will greatly facilitate
any necessary repairs. It must be always remembered that the working
of a coil depends on the insulation between primary and secondary.
_Spare no pains to have perfect insulation_; it is a hopeless task to
reinsulate a broken-down secondary, although the sectional method of
winding facilitates repairs. In large winding rooms it is customary
to have a revolution counter connected to the spindle, so that the
number of turns can be seen at all times. A bicycle cyclometer can be
readily fitted up for this purpose, and will be found of considerable
assistance where a number of sections are needed, each with a similar
number of turns. In the commercial construction of telephone coils and
magnet spools it is often the rule to specify only the number of turns
of the requisite size wire, the ampere turns of the coils being thus
regulated.
THE TESTING OF A COIL FOR POLARITY.
This is often necessary, and may be done in a variety of ways. When the
coil is working, and sparks be passed between fine wires mounted on the
discharger, the positive wire tip will be cold, whereas the negative
end will be quite hot. In vacuo, the positive shows a purple red when
the negative glows with a bluish violet. The decomposition of water,
which consists of oxygen and hydrogen in the formula H_{2}O, is readily
accomplished by the secondary current, and the greatest volume of gas
(hydrogen) will be evolved at the _negative pole_. For ready reference
a summary of these facts is given below:
Positive | Negative
|
Cold wire, | Hot wire,
Anode, | Cathode,
+ sign, | - sign,
Purple red, | Bluish violet,
Zinc plate, | Carbon plate,
(Carbon) pole, | Zinc pole,
Oxygen gas. | Hydrogen gas.
Although it is customary to use bundles of fine, soft iron wire for
coil cores, very excellent results have been obtained with cores made
up of soft iron filings. These filings should be tightly packed in the
core tube and have a soft iron head at the contact breaker end. Filings
demagnetize very quickly and prevent the formation of destructive eddy
currents, which have been previously discussed (Chapter I.).
Modern practice tends towards a lengthening of the core and primary, in
some cases fully 20 per cent of the core length projects from each end
of the coil. One result must be as in electromagnets, the longer the
core, the longer it takes to magnetize or demagnetize. But even here
it is a matter of individual construction.
The common practice is to make coils to be in a horizontal position;
there is no reason why they cannot be made to stand on end. In fact,
this position to an extent takes off some of the strain on the primary.
It is mostly a matter of choice or convenience.
As to the possible output of an induction coil, it depends upon design
and construction; but S. P. Thompson gives the following law in his
work on Electricity and Magnetism: The electromotive force generated in
the secondary circuit is to that employed in the primary nearly in the
same proportion as the relative turns of the two coils.[1]
[1] We do not attempt to reconcile this quotation with the enormous
estimates of spark potential.
In selecting a Ruhmkorff coil, it must be remembered that the rating
in spark length is subject to question. Supposing two similar coils
be operated, one with a rapid vibrator and the other with a slow
vibrator, other things being equal, the slow vibrator will give the
greatest spark length. Again, the appearance of the spark is of vast
importance. Although two coils might be sparking across the same
length air-gap, the one giving the whitest and thickest continuous
succession of sparks is the better. Fig. 13 shows a reproduction from
a photograph of a spark 32 inches long, generated by the coil shown on
the frontispiece.
[Illustration: FIG. 13.]
It is easy to take a coil, and by snapping the vibrator contacts
together a few times a spark of thin bluish character will jump across
a gap, of length far exceeding the spark gap when vibrator is working
at normal speed. But this spark only passes at irregular intervals,
seemingly gathering strength for its forced leap. It must not be
considered in rating the coil.
In winding primary coils it is proposed to reduce the self-induction
or inductance of its adjacent coils by means of similar methods used
in winding electromagnets. The primary winding, instead of being
composed of a number of turns of one large wire, is made up of a
multiple winding of small wires, aggregating the conductivity of the
large wire. This materially reduces sparking at the contact breaker,
and certainly allows of a closer bedding of wire nearer the core, also
giving a greater percentage of ampere turns. Another scheme which uses
the Dessauer contact breaker provides two separate primary windings,
opening one when the other closes. Such schemes as these come well
within the scope of the experimenter, and it is highly possible that
valuable improvements will be made in coil design during the coming
years.
FAILURE TO WORK.
The following are the commonest causes of coils not working to their
best limit: Contact breaker contacts dirty, burned, stuck, too small,
not in good parallel relation face to face of platinum.
Secondary wires crossed outside coil, often happens that the secondary
is quietly sparking away into or through some object touching it,
particularly when long wire connections are run from secondary to place
of desired sparking.
Condenser too small, burned out, badly insulated (see other pages on
this subject).
Battery too small—too high internal resistance or wires leading from
battery to coil too small—for ordinary coil work, distance of, perhaps,
ten feet, use No. 10 to 12 B & S flexible lamp cord or solid wire.
Ruhmkorff coils require plenty of current to produce large sparks.
DIMENSIONS FOR DIFFERENT SPARK LENGTHS.
——————————————————————+————————-+——————-+————————-+——————-+———————
| ½ | 1 | 2 | 6 | 12
| inch | inch | inches | inches| inches
——————————————————————+————————-+——————-+————————-+——————-+———————
Foil sheets | 5½ × 4 | 6 × 4 | 6 × 6 | 10 × 5| 12 × 8
Number | 40 | 40 | 60 | 60 | 60
Paper sheets | 6½ × 5 | 9 × 5 | 8½ × 7 | 12 × 7| 14 × 10
Number | 60 | 60 | 80 | 80 | 80
Core length | 5 | 7 | 9 | 12 | 19
Core diameter | ⅝ | ¾ | 1″ | 1⅛ | 1½
Primary size B & S | 16 | 14 | 14 | 12 | 10
Secondary size B & S. | 36 | 36 | 36 | 36 | 38
Core wire size B W G. | 22 | 22 | 22 | 22 | 22
Quantity in pounds of | | | | |
secondary wire | ¾ | 1¼ | 2½ | 7 | 12
Layers of primary | 3 | 3 | 2 | 2 | 2
Area of paper, sq. in.| 2,000 | 2,700 | 4,800 | 6,600 |11,000
Area of foil, sq. in. | 880 | 960 | 2,100 | 3,000 | 5,760
——————————————————————+————————-+——————-+————————-+——————-+———————
As it is not always convenient to procure paper and foil in set sizes,
the area of material needed for condensers is also given. The above
table is approximate. It represents data collected from the best modern
practice. The gauge above given for copper wire is that of Brown &
Sharpe, and is used throughout these pages.
MEDICAL COILS.
The main points of difference between coils for electrotherapeutics and
Ruhmkorff coils is that the former are devoid of condensers, are rarely
insulated to a high degree, and are arranged for current strength
regulation. The modes of regulation are many, briefly the principal
are: (_a_) In coils with independent circuit breakers, sliding both
core and primary coil out of the secondary together or independently.
(_b_) Moving a metal tube over or off the primary coil or core or both.
Many combinations of these methods are practised. Attempts have been
made to regulate battery current by rheostat, but it is not feasible,
except in large stationary outfits. Cheap medical coils are wound with
bare wire, with layers of thread between adjacent turns, or even only
bedding the wire turns in paraffined paper. It is not intended to
convey the idea that winding bare wire coils is a makeshift; far from
it. This method is being very generally adopted in telephone work. But
it requires special and delicate machinery, and is unsuited to amateur
work, where slight differences of cost or labor are insignificant.
Others for specific purposes consist of a primary coil only. The best
and most complete made are so arranged that independent secondary
coils of different sized wires can be used with the one primary, being
readily slipped on or off as required. There is another scheme of
regulation, where the coil is wound in sections and these sections cut
in or out by means of a switch, but it is not desirable.
MEDICAL COIL WITH TUBE REGULATION.
[Illustration: FIG. 14.]
Figure 14 shows a coil with tube mode of regulation. The core _C_
consists of a piece of iron tube, very thin, 4 inches long by ⅜ inch
diameter, and filled with soft iron wires. One end of this core is
firmly fixed in the left-hand bobbin head. The object of the iron
tube is to prevent the sliding tube from catching in the iron wires,
otherwise it can be dispensed with. Over this tube is slipped a brass
tube _T_, ending in a handle _H_ at the right-hand end; this must work
easily over the core tube. The spool for the primary is now made up by
fixing the other bobbin head on a paper or fibre tube and fastening
its free end to the left-hand bobbin head, or the spool can be made in
the usual way by glueing up two spool ends on a fibre or paper tube and
securing the iron core firmly in one end, allowing room, of course, for
the brass tube to slide in at the right-hand end. The primary winding
is three or four layers of No. 20 B & S gauge cotton-covered magnet
wire, the ends being brought out for future connection. Over this is
now laid a few layers of paraffined paper, and ten or twelve layers of
No. 36 B & S cotton-covered magnet wire is wound on for the secondary
coil.
The contact breaker _R_ is in no way different from the simple form
described in Chapter II. Its construction can be readily seen from the
figure.
A layer of cloth of the kind used in covering electromagnets is laid on
over the secondary, and the coil is ready to be attached to the base.
The base is seven inches long by three wide, and has little feet at its
four corners to elevate it from the table and prevent abrasion of the
connections underneath.
[Illustration: FIG. 15.]
The connections are as given in Fig. 15. When in operation, the
electrode cords being attached to binding posts, Nos. 1 and 2 are in
circuit with the secondary coil only. When at Nos. 2 and 3 they receive
the induced current or extra current in the primary, caused by the
break of the battery circuit (see page 3).
MEDICAL COIL WITH INTERCHANGEABLE SECONDARIES.
This form of coil is the only one for practical medical work, and more
space will be given to its construction than to the foregoing, which
is suited only for limited use.
[Illustration: FIG. 16.]
Fig. 16 shows side elevation of coil on base. The design can be largely
varied, also it can be used either for a wall board, a cabinet top,
or made to be carried in a case containing battery, electrodes, etc.
_S_ is one of the secondary coils, of which at least three should be
provided. The dimensions are, of course, the same—namely, four inches
long by 3½ inches wide over all. The spool ends are furnished with heel
pieces, which slide under the brass track bar _T_. This accurately
centres the coil and prevents it from working loose.
WINDINGS FOR SECONDARY.
The following windings for removable or interchangeable secondary coils
are those most in use.
Coil No. 1. 4500 feet (.375 pound) No. 36 B & S, approximating 1800
ohms. This may be led out in three divisions by means of switch on coil
head. First division, 4500 feet; second division, 3000 feet; third
division, 1500 feet.
Coil No. 2. 2400 feet (.6 pound) No. 31 B & S, about 350 ohms, divided
into 2400 feet, 1500 feet, and 900 feet.
Coil No. 3. 750 feet (1 pound) No. 22 B & S in one coil, or two
divisions of 500 and 750 feet, respectively; approximate resistance of
wire, 125 ohms.
Coil No. 4. It may be necessary to obtain currents of extremely high
tension, in which case a coil may be prepared of 5000 feet No. 38 B &
S, or No. 40 B & S preferably.
The finer the wire, the less current and the most sedative effect; the
coarser the wire, the more current with corresponding increased painful
action.
The spools, in fact as much of the framework as possible, should be
made of hard rubber, to which a fine finish can be given, although
mahogany, rosewood, or even stained oak can be used. On each side of
the right-hand spool heads a flat brass spring is screwed, making the
contact for the secondary wires on brass strips screwed on top of
the track rods. These secondary connections can be made by means of
flexible cords to binding posts, but the sliding contact is preferable.
The primary coil _P_ is firmly held in the left spool head, and
consists of a core of No. 22 B W G soft iron wires, insulated and wound
with three layers of No. 20 B & S magnet wire. The outside of this coil
is neatly enclosed in a hard rubber tube to permit of the secondary
coils sliding freely upon it. It is better, however, for the secondary
coils not to touch the primary tube. The vibrator, or contact breaker,
should be of the adjustable form shown in Fig. 17. The adjusting screw
for the contact breaker can be mounted in a brass lug carried by the
spool head.
Connections of this coil are substantially the same as those of
the first-described medical coil. This apparatus is well worthy of
elaboration; it should be fitted with a ribbon vibrator as well as an
adjustable speed slow vibrator, a switch controlling either. A great
variety of secondary coils can be made, those of coarse wire taking
the place of the current from the contact breaker. The vibrators
should be operated from an independent battery, although in the last
coil described the magnet may be wound with the same size wire as the
primary and then be in series with it. The secondary spools can be
made of stained hard wood ends fitted on to fibre tube, which latter
is easily procurable. Particular attention should always be paid to
the spools and heads; if not properly made, they may come apart, and a
disastrous unravelling of the wires ensues.
[Illustration: FIG. 17.]
BATH COILS.
A coil much used for electric baths has a primary winding only,
regulated by the sliding in and out of the iron core, which
necessitates the use of an independent vibrator, or else by varying
the current strength with a rheostat. The general directions given
before will answer in the present case, the only data necessary being
the size of wire, which should be about six to ten layers of No. 20 B
& S. The coil with movable secondaries here comes into service. Strong
currents are needed for bath work, and any variety of winding can be
used with this make of coil. There are so many descriptions of bath and
small medical coils in the electrical magazines published for amateur
workers, that it is hardly necessary here to give more than a mention
of the principal ones.
HINTS IN CARING FOR MEDICAL COILS.
A few remarks on medical coils and their diseases may not be amiss;
often a very little defect, if remedied in time, will prevent costly
repairs.
The main care in medical electrical apparatus is the battery (see
Chapter X. for descriptions of coil batteries and their operation).
Clean, fresh solutions and clean contacts are essential. Keep zincs
well amalgamated, remove wires from binding posts, and scrape bright
the metal where the wires make connection; see no fluid is splashed on
contacts, clean all contact springs periodically. The Edison-Lalande
battery is probably the best for medical use, but even this requires
occasional attention as to contacts, new zincs, fresh solution, etc.
Poor adjustment at contact breaker, dirty or corroded contacts, loose
wires, loose binding posts, corroded binding posts, are often the only
trouble in a coil refusing to work.
Flexible cords are fruitful of trouble: the tinsel breaks, and there is
no circuit; gets wet and crosses or causes a leak; cord tips get loose
and alternately open and close a contact; one minute all is well, next
minute no current can be obtained. Another trouble in acid batteries
is caused by leaving the zincs in the fluid. It is easy to do it in
most cases, although the ingenuity of the leading medical electrical
apparatus makers to-day is directed to this point. Cleanliness and
careful inspection of all contacts is well repaid; carelessness surely
brings its evils.
It is very desirable in medical work to eliminate the noise attendant
upon the working of the coil vibrator. This jarring or humming is often
in itself a source of irritation to a nervous patient. The sound can be
deadened in various ways, for instance, by placing over the vibrator
a temporary wood cover, lined with felt, resting upon a soft rubber
gasket; or in any other manner that may suggest itself to the operator.
TABLE SHOWING RESISTANCES AND FEET PER POUND OF COPPER AND GERMAN
SILVER WIRES.
——————————+————————-+——————-++——————————-++——————————-
| | || COPPER. || GERMAN
| | || || SILVER.
Gauge, | | ++——————————-++——————————-
Browne |Diameter.| Feet || || ONLY
& Sharpe. | |per lb.|| Ohms ||APPROXIMATE.
| | || per ++——————————-
| | || 1,000 ft. || Ohms
| | || ||per 1,000 ft.
——————————+————————-+——————-++——————————-++——————————-
8 | .1285 | 20 || .62881 || 11.77
9 | .1144 | 25 || .79281 || 11.83
10 | .1019 | 32 || 1 || 18.72
11 | .09074 | 40 || 1.2607 || 25.59
12 | .08081 | 51 || 1.5898 || 29.75
13 | .07196 | 64 || 1.995 || 37.51
14 | .06408 | 81 || 2.504 || 47.30
15 | .05707 | 102 || 3.172 || 59.65
16 | .05082 | 129 || 4.001 || 75.22
17 | .04525 | 162 || 5.04 || 94.84
18 | .0403 | 204 || 6.36 || 119.61
19 | .03539 | 264 || 8.25 || 155.10
20 | .03196 | 325 || 10.12 || 190.18
21 | .02846 | 409 || 12.76 || 239.81
22 | .02535 | 517 || 16.25 || 302.38
23 | .02257 | 660 || 20.30 || 381.33
24 | .0201 | 823 || 25.60 || 480.83
25 | .0179 | 1039 || 32.20 || 606.31
26 | .01594 | 1310 || 40.70 || 764.59
27 | .01419 | 1650 || 51.30 || 964.13
28 | .01264 | 2082 || 64.80 || 1215.76
29 | .01126 | 2623 || 81.60 || 1533.06
30 | .01002 | 3311 || 103 || 1933.03
31 | .00893 | 4165 || 130 || 2437.23
32 | .00795 | 5263 || 164 || 3073.77
33 | .00708 | 6636 || 206 || 3875.61
34 | .0063 | 8381 || 260 || 4888.49
35 | .00561 | 10560 || 328 || 6163.97
36 | .005 | 13306 || 414 || 7770.81
——————————+————————-+——————-++——————————-++——————————-
CHAPTER II.
CONTACT BREAKERS.
[Illustration: FIG. 18.]
The simple form of contact breaker already described is useful up to a
certain point, but it has disadvantages. Its rate of vibration is only
variable through narrow limits, and it is not suitable for very heavy
currents. But as it stands it has done long service, and will be used
probably wherever the requirements from it are not exacting. The most
desirable form of this simple spring break is shown in Fig. 18. _R_
is the soft iron armature; _S_, the spring; _C_, check-nut which holds
the adjusting screw _A_ from becoming loose; _T_, a second adjusting
screw used to tighten the spring and so raise its rate of vibration;
_K_ is the base to which one wire of the coil is attached; _L_, base of
adjusting device to which battery wire runs at _I_. Where tightening
screw T passes through the pillar of the adjusting screw, the hole
therein is bushed with rubber to prevent accidental contact. Both _A_
and _T_ are provided with insulating heads of rubber or ivory. At _B_
are the platinum contacts, which should be fully ⅛ inch in diameter.
One serious defect in the action of the simple spring vibrator (Fig.
19) is the tendency of the spring to vibrate, as it were, sinusoidally.
This causes an irregularity in the rate of the vibrations, which
affects the discharge of the coil very considerably. By far the better
plan is to use a very short thick spring riveted to an arm carrying
the armature at its end (Fig. 20). _R_ is the armature, _S_ the piece
of spring, and _K_ the point of attachment to the base. The actual
width of the portion of the spring which vibrates—the hinge portion, it
might be called—should not be over ⅛ inch.
[Illustration: FIG. 19.]
[Illustration: FIG. 20.]
The rate of motion is high; but an erroneous notion has been taken of
its performance by many persons in the knowledge of the writer. The
rate of vibration is _not_ wholly dependent on the size, or, rather,
smallness of its spring; the arm and armature considerably alter this,
although they are not pliable, by reason of their mass and the momentum
consequent on their mass.
A word here on the size of the armature. It should be somewhat larger
than the face of the electro-magnet core, and should be thick—that is,
in a circular form—say one half its diameter. Of course this does not
apply to the steel lever armature before mentioned. It is impossible to
lay down arbitrary rules where the conditions are not determined, but a
very small amount of experimenting will demonstrate the correct lines
on which to build.
When in action, all rapid rheotomes give out a definite musical note
whereby the rate of vibration can be determined. Reference to any work
on acoustics will show a table of the number of vibrations necessary to
produce any stated musical note. The foregoing style of rheotome forms
the basis of very nearly all those which are in use. The shorter and
stouter a spring the more rapidly will it vibrate, and _vice-versa_.
Carrying out this rule, we can manufacture an instrument which will
give as high as 2500 vibrations per second (Fig. 21).
[Illustration: FIG. 21.]
The armature _A_ is a piece of flat hard steel bar ¼ × ½ inch, held
rigidly on the metal support _S_ and just clearing the upper surfaces
of the magnet cores _C_. The adjusting screw _P_ should be provided
with an arm, _B B_, whereby the rotation of it can be delicately
varied. This screw must also be firmly held or the high speed of the
armature will jar it loose. A check-nut on each side of the frame
carrying it should be provided in every case. The necessary platinum
contact can be hammered into a hole drilled before the armature is
hardened. The proper place for this contact is about one fourth of
the total length of the armature from its support, although in the
simple contact breaker it can be placed at the distance of one third
if desired. The reason is that the concussion of the adjusting screw
dampens the free vibration, and the amplitude thereof is lessened in
addition to the counter vibrations of the screw disturbing the regular
vibrationary series.
Owing to the fact that the amplitude of the armature vibration is so
small, a very delicate adjustment is necessary. The adjusting screw
can be placed nearer the free end, but for the reasons given it is not
to be desired. The metal bridge should be a solid casting, and the
armature clamped by more than one screw.
The mercury vibrator, which is applied to almost every large coil, is
as follows:
A pivoted arm carries on one end a soft iron armature, which is
attracted by the coil core. The other end is provided with a platinum
point adjustable by a set screw. This platinum point dips into a
mercury cup—a glass cup containing mercury, with a thin layer of
spirits of turpentine. The object of the spirits of turpentine, which
is a non-conductor, is to help choke off the spark which would ensue
whenever the platinum point was raised from the mercury.
A form of contact breaker which will admit of great variation of speed,
and which is adapted to carry large currents, is the wheel-break,
constructed in the following manner:
A brass or copper disk 3 inches or more in diameter and upward of ½
inch thick has its periphery divided by a number of saw cuts, which
divisions are often filled in with plugs of hard rubber or fibre. This
disk is mounted on a shaft, which latter is either the shaft of an
electro-motor, or is provided with a pulley by which it can be rapidly
rotated. A strip of spring copper on each side of the disk presses
upon the toothed surface, one strip being connected to the coil and
the other to the battery or other current source. It will now be seen
that when the disk rotates the slits or pieces of hard rubber cause the
break in the circuit through the brushes or copper strips, the rapidity
of the breaks depending upon the rate of rotation of the disk, and the
number of slits in the wheel.
The slits or rubber pieces should be one-half the width of the
intervening brass, but must be at least one sixteenth of an inch in
width, especially where a high voltage is used in the primary coil.
The shaft of the machine may serve as one point of connection in place
of one of the copper brushes; but in this event either a wide journal
must be used, or else some conducting substance, as plumbago, replace
the lubricating oil in the bearings.
POLE CHANGING BREAKER.
Fig. 22 shows a diagram of a pole changing contact breaker which will
allow of rapid alternations of current. It is operated by an electric
motor by preference, although any motive power can be applied to it.
[Illustration: FIG. 22.]
_W a W b_ are two brass wheels, the peripheries of which are broken by
the insertion of insulating blocks _I I_, shown black in the sketch. _S
S_ are the shafts on which the wheels are mounted, the two wheels being
necessarily insulated from each other. 1, 2, 3, 4 are four brushes of
copper pressing on the rim of the wheel and leading in the current
from the battery _B_. The primary coil is attached to the brass
body of the wheel or to the shafts. When the wheel is in the position
shown, the coil and battery are on an open circuit; but on the wheel
commencing to revolve, the brushes 1 and 2 bear on the brass, and the
current flows from the positive pole of the battery to 2 through the
wheel _W a_ to the coil _P_, up through wheel _W b_ and out at 1 back
to the battery. The next position of the brushes 1 and 2 will be on
the insulations, and 3 and 4 will come into action. Then the positive
current will reach _W b_ by means of brush 3, and after traversing
the primary coil and wheel _W a_, emerge at 4 to the battery, thus
reversing the current through _P_ as many times as there are sets of
segments, which latter can be multiplied according to requirements. The
main point to be considered after that of good connections is that the
brushes 1 and 3 and 2 and 4 do not at any time touch any part of the
brass wheel at the same time, as this would short circuit the battery.
This is avoided by making the insulating space longer than the brass
surface, and adjusting the brushes as in the sketch, that each pair of
them is a fraction further apart than the length of the brass tooth.
Accordingly, a wheel may be constructed with many segments and rotated
at a high speed and rapid reversals of current produced, the uses of
which are manifold.
As will be described in the notes on the Tesla effects, an
electro-magnet, the poles of which are brought near the sparking point
of the contact breaker, will help wipe out the spark, and so assist the
suddenness of the break.
An extremely successful expedient in operating contact breakers is
to employ a high-pressure air blast directed point blank against the
contact point. The effect of this air blast when the contact is made is
of course null, but on the platinum surfaces becoming separated, the
high air pressure produced forms a path of extremely high resistance,
and tends to blow off the spark as soon as it is generated. The stream
of air should issue from an insulated nozzle of glass or rubber, and
should not contain moisture.
WEHNELT INTERRUPTER.
One of the most important inventions in coil work is the electrolytic
interrupter of Wehnelt. Briefly, the apparatus consists of a vessel
containing a solution of acid, into which dip two electrodes connected
in series with the source of power and the primary of the coil. Upon
passing a current through the combination the fluid becomes agitated at
the electrodes and a rapid make and break of the current ensues (Fig.
23).
[Illustration: FIG. 23.]
It requires considerable electromotive force for operation, a minimum
of 40 volts being desirable. Its rapidity of action varies up to and at
times exceeding 4000 interruptions per second. A Wehnelt interrupter
can be made as follows: Procure a glass jar _J_ holding about one
quart or a little less, also a cover for same _C_, a piece of sheet
lead _L_ large enough to fit loosely across the jar and yet not touch
the bottom, eight inches of one-quarter-inch glass tube _M_, a few
inches of No. 20 platinum wire _P_, and two ounces of mercury. Heat
the end of the glass tube in a gas flame, and bend an inch or less at
a right angle; at the same time seal in the platinum wire by means of
a blowpipe, so that the tip just projects from the bent end of the
tube. This sealing can be accomplished readily by one unused to working
glass, but almost any philosophical instrument maker will have it done
at small cost. Holes being bored through the cover, the lead plate
and the glass tube are fitted in, the platinum point almost touching
the lead. Adjustment is, however, easy, as the tube, being turned,
will retract or advance the platinum point from or towards the lead
electrode. Nearly fill the jar with a solution composed of one part
sulphuric acid to eight parts water, and fill up the glass tube with
mercury. The connections can then be made by means of a clamp on the
lead and a wire dipping into the mercury. Connect the lead plate _L_ to
one pole of the battery or source of energy, and the platinum-mercury
electrode _F_ to one post of primary. The other side of battery and
coil being closed, the apparatus will begin to work. No condenser is
needed with this interrupter.
DESSAUER CONTACT BREAKER.
This is a modification of the spring hammer-head type, but has a
platinum contact on both sides of the spring. It thus obtains double
vibrations, but is liable to stick. The elasticity of the spring
normally prevents the circuit remaining closed on the forward movement
of the hammer head, but this combination requires attention.
STEEL RIBBON INTERRUPTER.
For light currents and rapid vibrations, such as are employed in
electrotherapy, the steel ribbon interrupter is suitable. It consists
of a steel ribbon _V_ one-half inch wide by six or eight inches long
and the thickness of a stout visiting-card. Near the end is riveted
a platinum contact. One end of the ribbon is held by a brass upright
_R_, to which connection is made to circuit; the other end is riveted
to a threaded rod, which passes through a brass pillar, and is held
by a thumb-screw and check nut _S_. Turning the thumb-screw either
way tightens or loosens the ribbon and so raises or lowers the rate of
vibration (Fig. 24).
CONTACT BREAKERS IN VACUO.
Contact breakers in vacuo, as applied to Ruhmkorff coils, are by no
means of recent date. Poggendorff made use of such prior to 1859, and
noted the diminished sparking at the contact breaker and increased
effect in the secondary circuit.
[Illustration: FIG. 24.]
[Illustration: FIG. 25.]
Mr. D. McFarlan Moore, whose experiments in vacuum tube lighting
have proven so interesting, was granted patents upon various forms
of contact breakers, in which the chief merit was that the contacts
were broken in a vacuum. The sparking was almost eliminated, and the
suddenness of the break of contact so accentuated as to materially
improve the output of an induction coil. A perusal of his patents,
copies of which may be procured through almost any bookseller, will
prove profitable to the coil constructor.
QUEEN CONTACT BREAKER.
The most important advantage of this arrangement is the abrupt break,
owing to a collar in the vibrator striking a movable contact while at
full speed. Reference to Fig. 25 will show that the movable platinum
contact is carried on a small vertical spring behind the vibrator
spring, and projects through a collar on the vibrator spring. When
the contact is made, the movement of the vibrator is not arrested,
but continues at its full amplitude, thus allowing a long "make."
The vibrator is kept moving at a constant amplitude by means of the
small coil shown in the illustration, which is in shunt with the main
circuit. In the old forms there has always been a liability of the
platinum contacts sticking (or welding together). In the new form, as
the break is made when the vibrator is in the middle of its swing, the
sudden blow with the entire momentum of the iron hammer head is always
sufficient to break the platinums apart. This form of contact breaker
is very efficient on electric-light circuits, and operates with the
utmost regularity.
THE QUEEN CONTACT BREAKER FOR LARGE COILS.
This is a device where the actual break is made in alcohol between
large studs of platinum nearly one-quarter inch in diameter. The bottom
contact can be raised or lowered by means of an adjusting screw. The
top contact is secured into the bottom end of a rod passing down a
guide tube into the alcohol to meet the lower contact. By means of
an electric motor and a cam motion, the top contact and plunger are
made to work up and down in the alcohol, thus making and breaking the
current flow. One of the commendable features of this contact breaker
is that the platinum studs are caused to revolve while in operation,
thus presenting new faces to each other after each blow. The apparatus
is not adapted for rapid action, but for the handling of heavy currents.
ADJUSTABLE CONTACT BREAKER FOR MEDICAL COILS.
An adjustable contact breaker for medical coils is shown in Fig. 26.
_M M_ are the magnet coils, _A_ is the armature, carrying a platinum
contact, which vibrates against the adjusting screw _P_. The armature
is pivoted at _J_, but is held at a distance from the magnets by the
springs _S S_. The other end of the armature carries a ball _B_, which
can be slid up and down on the rod and set at any point by a set-screw.
When the ball is at the end of the armature rod most remote from the
magnets, the vibrations are slowest; when moved towards the magnets,
the vibrations become more rapid. Adjustment of the two springs _S S_
at _R R_ enables the contact breaker to operate on varying current
strength, and also tends to lessen the jerkiness of gravity contact
breakers. A flat spring, however, can be substituted for the spiral
springs, in which case the pivot would be dispensed with and the spring
riveted, as in the hammer form of vibrator. The illustration shows this
arranged for a wall board, but it can readily be adapted for table work.
[Illustration: FIG. 26.]
ADJUSTABLE CONE VIBRATOR.
Fig. 27 shows a form of contact breaker much used in portable medical
coils for slow speeds. It consists of a cone of iron _H_, mounted on
the vibrator spring, and furnished with adjustable contact spring
and screw _A_. Its amplitude of vibration is limited by the two pins
mounted on the disc, between which the cone vibrates. The disc is
turned by hand, thus moving the pins, and so varying the travel of
the cone _H_ to and from the core _C_. It does not give good results
from the fact that the rhythmical movements are disturbed every time
the cone strikes against the pins, also at the contact spring striking
the contact screw. As we showed before, a really satisfactory contact
breaker should have a spring, which allows of no sinusoidal movement.
Where a pivoted armature is governed by a spiral spring, the result is
a series of steady, rhythmical shocks, provided the adjustments are
satisfactory.
[Illustration: FIG. 27.]
COIL HEAD CONTACT BREAKER.
Fig. 28 shows the details of a contact breaker to be attached to
the coil head direct. It is often used on very small coils, which,
together with a miniature dry cell, is slipped into a pocket case.
An important detail in small coils is to use a contact breaker of
sufficient size. Most of them are not large enough to stand ordinary
usage, the adjusting screw is not of sufficient diameter and the thread
soon strips. There is no reason why the adjusting screw, its platinum
tip, and the pillar or lug which holds it should not be solidly built,
it would certainly require less adjustment. Either single or double
check-nuts can be fitted to the adjustment screws of nearly all the
forms of contact breakers described.
[Illustration: FIG. 28.]
CONTACTS.
It is absolutely essential that the _diameter_ of contacts for all
contact breakers should be as large as possible and their faces filed
truly parallel to enable them to easily carry all the current required.
One of the main causes of failure of coil is burning of the platinum
point and platinum burr, the current being then materially reduced.
Large sparks at point of rupture are often indications that the
condenser is not working properly—perhaps has broken down or is not
large enough. The contacts will sometimes fuse together; at any rate,
the excessive sparking is an evidence of waste as much as in a dynamo
generator.
The adjustable method of arranging condensers (see Chapter IV.) is
here of great value, but it is easy to attach more condenser sections
to the contact screw pillar and vibrator pillar and notice result.
In the construction of Ruhmkorff coils it is a good plan to make all
connections possible on the coil base, instead of inside the condenser
chamber. This is done either by means of rubber-covered wires or neat
strips of brass, screwed down on the base from points of connection,
and, of course, carefully bent over or well insulated from all other
leads which they have to cross.
The best makers of induction coils construct their instruments so
that they can be readily taken apart with as little detachment of
connections as possible.
CHAPTER III.
INSULATIONS AND CEMENTS.
In selecting an insulating compound for apparatus designed to be
under the influence of high tension currents, a glance at some of the
peculiarities of such currents will not be out of place. Mineral oil is
used in many of the converters employed to transform the high voltage
currents on the mains of the alternating electric-light systems to the
comparatively low voltage used at the points of consumption. Professor
Elihu Thomson, in a series of experiments, noticed some interesting
facts in the sparking distances of high potentials in oils.
He found that discharges of low frequencies, as 125 alternations per
second, were capable of puncturing mineral oils at one third to one
half the thickness of an air layer sufficient to just resist punctures
by the same discharge; but with frequencies of 50,000 to 100,000
per second, an oil thickness of one thirtieth to one sixtieth was a
sufficient barrier.
At a frequency of 125 per second, a half-inch spark in the air
penetrated one third to one fourth inch of oil; but at frequencies of
50,000 to 100,000 per second, a layer of oil one fourth of an inch
successfully resisted the passage of a spark which freely passed
through 8 inches of air.
The effect of drying an oil improved its insulating qualities. (Tesla
uses boiled-out linseed-oil.)
He also noted that pointed electrodes could be brought nearer together
under oil than balls without allowing a discharge. Flat plates allowed
of still greater sparking distances. Tesla notes that oil through which
sparks have passed must be discarded, probably owing to particles of
carbon being formed.
Paraffin wax has a higher resistance than oil, providing it has not
been heated over 135° C. It will stand alternate heating up to 100°
C. and cooling, being of lower resistance when hot than when cold.
But a serious permanent deterioration takes place when it has been
heated over 100°C.; its color, from the normal pure white, changes to a
yellowish tint when its insulation is impaired. Paraffin also undergoes
a deterioration when heated for a long time even at 100° C., and should
never be used for fine work when it is at all yellow. It is always best
to melt it in a hot-water bath, not permitting, however, any steam or
moisture to come near it. In this climate (United States) it is not
so necessary to mix in any tallow to obviate brittleness, the average
temperature of most workshops being sufficiently high to keep it from
becoming brittle.
Resin oils do not suffer permanent injury from being heated, as does
paraffin, but their insulating properties diminish much more rapidly
on becoming even warm, the initial resistance of resin oils being lower
than that of paraffin.
Paraffin has a fault—its tendency to absorb a slight degree of
moisture. It has been found in telephone and telegraph cables
saturated with paraffin that this is a very important cause of their
deterioration. In Ruhmkorff coils, however, which are intended for
operation in enclosed places free from damp atmospheres, the absorption
of moisture would be probably reduced to its minimum.
There is one substance which, were it not for its cost, would be far
preferable to paraffin for coil work, and that is beeswax. Its cost,
however, is generally five times that of paraffin, even when purchased
in quantities. It never becomes brittle enough to be damaged in careful
handling, its melting point is low, and it does not absorb moisture.
But it must be unquestionably pure and clear.
In foreign practice a variety of resinous mixtures are used to
insulate the turns of the wire in Ruhmkorff coils.
Equal parts of resin and beeswax used hot, paraffin, resin and tallow,
and shellac and resin are employed.
Shellac—that is, the yellow lac—is much used as a varnish for
electrical instruments, being dissolved in alcohol to saturation. For
dynamo armatures and similar apparatus the shellac varnish is of great
service, and many good compounds of shellac, such as insullac and
armalac, have been prepared for ready use. But (excluding beeswax) for
our purposes paraffin stands pre-eminently at the head of the list.
In using shellac varnish, in high tension work more particularly, care
must be taken that the moisture has entirely evaporated. Although a
piece of shellacked apparatus may appear perfectly dry, yet when the
current is allowed to flow unlooked-for results may appear—it takes
hours in a dry atmosphere for shellac varnish to dry. Baking the
apparatus in a warm oven is a necessary expedient whenever feasible,
care being taken not to burn or decompose the shellac. The proportions
most generally used are 1 ounce shellac to 5 ounces alcohol. Stand the
vessel containing the mixture in a warm place, and shake it frequently;
filtration improves the varnish somewhat.
A ready and efficient varnish for silk is prepared by mixing 6 ounces
of boiled linseed-oil and 2 ounces of rectified spirits of turpentine.
For paper, 1 part of Canada balsam and 2 parts of spirits of turpentine
dissolved in a warm place and filtered before being used. A good
insulating cement for Leyden jars and insulating stands is prepared
from sulphur, 100 parts; tallow, 2 parts, and resin, 2 parts, melted
together until of the consistence of syrup, and sufficient powdered
glass added to make a paste. To be heated when applied, this will
resist most acids. The resin and beeswax compound is handy when making
experimental mercurial air pumps of glass tubes, as it has a fair
tenacity, is not too brittle, and is easily used.
CHAPTER IV.
CONDENSERS.
A condenser is an apparatus whereby a charge of electrical energy
may be temporarily stored, the amount of energy it will hold
determining its "capacity." The capacity of a condenser is measured
in micro-farads, the commercial unit representing one millionth of a
farad. A farad equals the capacity of a body raised to the potential of
one volt by a charge of one ampere for one second at one volt—_i.e._ =
one coulomb.
The measurement of the capacity of a condenser is accomplished by
the use of a ballistic galvanometer. The latter instrument has a
bell-shaped magnet suspended in a coil of fine wire. When a momentary
current is passed through this coil the magnet hardly commences to
rotate until the current has practically ceased. A beam of light is
reflected from a mirror fixed to the magnet on to a scale. The degree
of deflection is compared with that obtained by the discharge of a
condenser of known capacity, and the capacity of the condenser being
measured is deduced by a simple rule. The farad, which is the unit of
capacity requiring a condenser of an immense size, is replaced by a
commercial unit, the micro-farad—that is, one millionth of a farad.
The original form of the condenser was the Leyden jar, which owes its
name from the town of Leyden in Europe.
[Illustration: FIG. 29.]
The Leyden jar is made as follows (Fig. 29): A clean uncracked glass
jar with a wide mouth is coated on the inside and outside with tinfoil;
sometimes loose tinfoil is filled inside, the tinfoil, however, not
reaching more than two thirds of the jar's length from the bottom.
A cork is fitted, and through the middle of it a wire is passed
touching the inside coating of tinfoil and terminating in a metal
sphere outside. A simple Leyden jar can be made in a few moments by
half filling a glass bottle with water and wetting the lower half of
the outside; a wire run through the cork into the water finishes the
job. But this is at least only a makeshift, although a fair amount of
current has been collected from a leather engine belt in motion in one
thus made.
A condenser can be easily made as follows (Fig. 30):
[Illustration: FIG. 30.]
Procure a clear glass plate, _G_, free from flaws, 11 inches square
by 3∕32 inch thick. Give this a good coating of shellac varnish all
over, sides and edges. Cut out of smooth tinfoil two sheets, _T_, 8
inches square, and round off the corners with a pair of shears. There
must be no sharp corners, projections, or angles to induce leakage.
Lay the glass plate on a sheet of paper, and mark its outline thereon
with a pencil; then remove it and substitute a sheet of the tinfoil,
and mark that. This will enable you to centre the foil. Give one side
of the glass plate another coat of varnish, and so lay it on the paper
that its outline coincides with the pencil outline. When the varnish
has partly dried take a sheet of the trimmed foil, and by observing
the pencilled marks you can lay it on the varnished plate exactly in
the centre. Lay down the top edge first along this line, and carefully
deposit the remainder of the foil in place. Next, with a flat brush
full of varnish go over the plate, pressing out any air bubbles, and
ensuring both a flat and a well-varnished surface. When this is dry,
turn over the plate and repeat the operation on the other side.
If desired, a metal hemisphere of at least an inch in diameter may be
attached with varnish, first scraping the foil to make a contact. The
whole plate can be swung in a cradle of two silk threads, laid on a
glass tumbler, or mounted on end in a shellacked block of wood.
A strip of tinfoil, _S_, attached at the corner can be used as a
connector. The plates must be joined in the following manner when two
or more are used in conjunction, and a quantity of current is desired.
They should be placed so the connecting strips project alternately from
each side (Fig. 31), and all on each side joined so as to leave two
terminals, one to the 1, 3, 5 plates, the other to the 2, 4, 6 plates,
and so on, which, when joined, will have the same effect as would
result from the use of two large plates of the same total area. The
nearer the plates are together the greater capacity they will have,
always supposing the insulation is good, the insulation being known
as the dielectric. Another good method, when a high quality of glass
can be procured, is to lay the tinfoil on the plates without varnish,
piling one on top of the other, tinfoil and glass alternately, and
clamping the whole securely, laying a piece of cloth top and bottom
to avoid cracking the glass from the pressure. This must be kept from
moisture; a strip of paraffined paper stuck along the edges and extra
paraffin run on will answer very well.
[Illustration: FIG. 31.]
In constructing these glass condensers, they must be designed to
correspond with the coil with which they are to be charged. In the
foregoing description we have allowed a margin of 1½ inches of glass
around the foil coatings. This will make 3 inches as the maximum
distance between the coatings. Although a 2-inch spark from the coil
would not jump this interval, a certain discharge will take place,
and the less this occurs, the more serviceable the condenser will be.
Therefore a greater margin should be allowed for a longer spark than 2
inches.
In the commercial condenser for telephone and telegraph use, paraffin
and paper are substituted for glass, as will be described later.
Heavy paraffin oil gives excellent results, but its fluidity is
disadvantageous.
There is no valid reason why paraffin could not be used on the glass
plate condensers, care being observed that it is free from dirt and
metallic chips. In fact, the space between the glass plates of the
multiplate condenser may be filled in with paraffin, and thereby
exclude the air. Only a condenser so built up is not convenient to take
apart for experimental purposes.
The foregoing description of a glass insulated condenser was written
with the assumption that a good quality of glass be used. But the
ordinary window glass is generally useless, and paraffined paper is
preferable. The quality of glass known as "hard flint glass" is best,
the superior qualities being imported from Europe. This latter is used
in the manufacture of the standard Leyden jar for lecture purposes.
Were it not for its cost, the finest dielectric we could use would be
sheet mica. Unfortunately sheet mica over 3 inches square is expensive,
and becomes rapidly more so as it becomes larger.
Standard condensers for testing are made with mica carefully selected,
and retain the charge for the maximum length of time. The built-up mica
condenser is immersed in molten paraffin until the same has permeated
the sheets, and then the complete mass is put under a pressure until
the paraffin is well set.
PAPER CONDENSER.
The paper used in the manufacture of the commercial form is a special
thin, tough linen paper carefully selected, sheet by sheet, to avoid
pin-holes or flaws, and kept in an oven until used to ensure absolute
dryness.
When this cannot be procured, use thin unsized writing paper of a
good quality, well dried, and absolutely clean. As an example of
the necessity of cleanliness, a light lead-pencil mark would serve
to conduct the current entirely from a charged sheet to wherever it
terminated, and if suitably located, utterly destroy the usefulness
of the apparatus. Ink, which most generally contains iron, will cause
trouble, and although some cheap foreign condensers are built up of old
ledger pages, yet their efficiency is very uncertain.
The paper used in commercial condensers is from four to seven
thousandths of an inch in thickness.
SERIES.
The smaller the amount of surface the less will be the capacity, but
the quicker the discharge. The apparatus heretofore mentioned has had
the alternate plates connected together in two series, presenting a
large surface and rendering a large amount of current. A condenser
so made will have a low voltage or potential, but is not so liable
to leakage as one made to render a high potential. The multiple
condenser of a large capacity will hardly discharge and spark over an
air gap requiring a contact of the two electrodes. But a smaller one,
consisting only of a single pair of small plates, will spark across
quite a considerable air gap.
A number of charged condensers may be put in series, and the resultant
potential thereby increased. Cut a number of pieces of paper of the
desired size, say 6 inches square, and a number of sheets of foil 3
inches square. Round off the corners of the foil and build up first a
sheet of paper, then a sheet of foil in its centre, then another paper
and another foil sheet, and so on. There is to be no connection from
sheet to sheet, only the inductive action of one on its neighbor. The
foil must be considerably smaller than the paper in this construction,
owing to the greater tendency to discharge round the edges of the
sheets, owing to the greater potential of the current.
When the requisite number of sheets have been built up, leave a sheet
of foil top and bottom for connection, tie between two pieces of stout
card or board, and immerse in the molten paraffin. When thoroughly
soaked, remove and put under pressure until cold. It will be found
undesirable to make these with more than a dozen pairs of sheets, but
to make a number of blocks of that number for ready service.
[Illustration: FIG. 32.]
Fig. 32 shows the arrangement of the apparatus to charge a Leyden
jar, the plate form being connected in a similar manner. The jar is
stood upon an insulating support—a dry tumbler will answer—with the
ball _B_ connected to one pole of the coil. From the outside tinfoil
coating _T_ a wire runs to the discharger _D D_, which is in circuit
with the secondary coil, _S_. The discharger balls _D D_ are carefully
approximated until the spark just passes, this latter point being
of great importance. Were the discharger balls too near the spark
would probably pierce the dielectric of the condenser, therefore the
balls should be carefully _brought near_ to each other until the
exact distance is found. Even if the insulation of the condenser were
not pierced, yet a path would probably be opened through which some
succeeding discharge would pass, and ruin the instrument.
Another method of charging is to leave an air gap at _B_; then there
is not much liability of the condenser discharging back through the
coil—an undesirable event, as it would most likely perforate the
insulation of the coil.
[Illustration: FIG. 33.]
In designing or using any apparatus intended to hold a charge of high
potential, it must be kept in mind how readily points or sharp edges
serve to allow the current to pass off—we might almost say evaporate.
Given two bodies, one a globe and the other a rectangular block, each
well insulated from the earth or any other large body, and the globe
would be found to hold its charge long after the block had dissipated
all trace of the charge given to it. Therefore round off every edge and
angle, projection or point.
In making handles, supports, or any work requiring an intervening high
insulation, hard rubber is preferable to glass where there is liability
to moisture. When the apparatus is as shown in Fig. 33, the condenser
is alternately charged and discharged with a loud noise, the vivid
sparks passing across the discharger balls _D D_ possessing great
deflagratory powers.
In experimenting with a Ruhmkorff coil it is not advisable to leave the
instrument working while the secondary terminals are beyond sparking
distance, as there is a great strain on the secondary insulation. Nor
is it wise to use only one electrode in an experiment, unless the other
is connected to some apparatus of an approximate capacity to that at
the other, for the foregoing reason.
ROLLED-UP CONDENSERS.