LIGHTING OF LARGE BUILDINGS.
The jump spark system is used where it is desired to light clusters of
gas jets situated in inaccessible places, or a number of them
simultaneously. The spark from a Ruhmkorff coil, being made by a
contact broken at the coil and not at the burner, can be divided up
among a number of simple burners placed in series. One of the burners
used and known as the Smith jump spark burner is shown in Fig. 33. The
wires from the coil are attached to the electrodes shown on each side
of the burner, and the spark jumps across the gap, situated nearly
over the burner orifice. There is a guard-flange of mica round the
lower part.
Fig. 34 shows the manner in which the jump spark is applied to a
Welsbach burner. A small porcelain clip carrying the spark-gap wires
is held on the top of the burner chimney. The electrodes project down
into the chimney so that a draught of air cannot carry the stream of
gas away from the spark-gap.
[Illustration: FIG. 33.]
[Illustration: FIG. 35.]
[Illustration: FIG. 34.]
Fig. 35 shows a burner intended for the stage of a theatre, or where
the lights are located in dangerous and inaccessible places. The
burner is made of porcelain upon which are spun the metal top and
bottom. One electrode is also clamped around it, allowing of
adjustment and better insulation.
[Illustration: FIG. 36.]
These burners are used in series, as shown in Fig. 36. _B B B_ are the
burners; _S S_, the secondary wires from the Ruhmkorff coil, _I_; _P
P_, the primary coil wires from battery, opened and closed by means of
the key, _K_.
It is often possible to place plain burners close enough so that they
can ignite by contagion. In this case one of the plain burners is
removed and replaced by a multiple burner, as above.
It is customary to allow sixteen burners to one inch of spark, in
which case the spark gaps are adjusted about one-sixteenth of an inch
apart. A coil giving a 2-inch spark would operate 32 burners, but
actually it would be found preferable to omit a few, so as to make
allowance for any slight leak. A spark of over 2 inches is hard to
handle, although often used; it is better to make up a number of
circuits of, say, 30 burners each, and operate them alternately by a
suitable switch.
The wire used to connect the burners is generally bare, although an
insulated wire is sometimes used. But the electromotive force of a
2-inch spark is so high that it is better to run the wires so they do
not come near anything liable to cause a leak. The remarkable tendency
of these high-tension currents must be most carefully guarded against;
indeed, it is what makes this style of gas lighting so often
unsuccessful. A damp wall, gilt wall-paper, a gas pipe hidden in the
plaster, will often lead off the current. The wires should be at least
50 per cent. further off from any object than the spark length; that
is, a 2-inch spark circuit should be at least 3 inches away from a
wall, and the further the better. It cannot be too strongly urged that
every precaution be taken to keep the wires away from objects other
than their insulators.
[Illustration: FIG. 37.]
Fig. 37 shows the special form of insulator used. It is made of the
highest grade glaze filled porcelain, and the screw is passed into it
and holds against the lower end as far away from the wire as possible.
Glass tubes should be passed over the wires wherever they come near
any metallic object, that is, within sparking distance.
EDWARDS’ CONDENSER SYSTEM.
This system differs from the foregoing in that the spark-gaps are
connected in multiple, instead of series, and each burner is provided
with a small but efficient condenser.
[Illustration: FIG. 38.]
It prevents trouble should a wire break between burners, in which
event only one burner would be out of commission, whereas in the first
method, the whole number in that series would suffer. It is also more
sure in action and presents less liability of the spark jumping to the
ground. The burner pillars need not be made of porcelain or lava; in
fact, the electrodes can be readily attached to the existing burner.
Fig. 38 is a condenser consisting of a small oval piece of mica, on
each side of which is fastened, with insulating varnish, a
spatula-shaped piece of tinfoil. One foil sheet is attached to the
line, the other to the burner electrode. These condensers must not be
allowed to get wet or their efficiency will be impaired.
[Illustration: FIG. 39.]
[Illustration: FIG. 40.]
Figs. 39 and 40 are the most generally used burners, the wire from the
condenser being attached to the lug or top electrode, which is
insulated from the burner by means of the mica plate to which it is
riveted. The burner pillars are of course grounded through their
being screwed into the gas pipe. The circuit is shown in Fig. 41. _I_
is the induction or Ruhmkorff coil, in the primary circuit of which is
the key, _K_, controlling the current from the battery, _B_, and
across which is bridged the condenser, _C C_. When the coil vibrator
is used, the condenser _C C_ can be omitted, that of the coil itself
serving instead. _S_ is the wire leading from the secondary terminal
of the coil to the burner condenser, _C_, which, in turn, are
connected to the electrodes on the burners, _P P_, as before noted.
The other secondary wire is grounded preferably to the gas pipe
itself.
[Illustration: FIG. 41.]
Where a lot of burners are placed together, as in a ring, it is often
feasible to light them by contagion, one tip only being connected to
the coil circuit, the others lighting from it and conveying the flame
around to the rest. This avoids multiplicity of circuits, or, perhaps,
too many burner gaps on one circuit; but the one burner may fail to
light, whereas, where all are fitted, the chances of failure are less,
especially in the Edwards condenser system.
SWITCH FOR JUMP SPARK-LIGHTING.
In a switch for controlling the current of the secondary coil it will
be evident that the utmost attention must be paid to matters of
insulation. The object of such a switch is to control a number of
circuits; for example, as it is not advisable to put more than 20 to
25 burners on one circuit, a case requiring the lighting of 100
burners would necessitate some means of passing the current to each
circuit in turn. This is shown in Fig. 42, in which _S_ is a hard
rubber plate, provided with a revolving metal arm and handle, _H_, and
four contact points, _P_, which latter receive the ends of the wires
from the groups of burner condensers _B_ by means of nuts or binding
posts. The wire from the secondary of the coil is run to the
switch-handle, _H_, great care being taken that it does not pass near
to the circuit wires, or contact points. Revolving the switch-handle
connects the secondary wire to each circuit in turn. It will be
noticed that, unlike a battery switch, this one has a large base, long
switch-arm, contact points situated far apart, and every precaution
taken to control the passage of the high-tension current. The base
should always be of rubber or glass. Shellacked-wood, or such
substitutes, are productive of trouble.
[Illustration: FIG. 42.]
When it is desired to light automatically a number of burners from a
distance, the Trailer (Fig. 43), is used. This is a switch similar to
above, but the arm is revolved by means of toothed wheels by the
electro-magnet shown on the back. As it is never desirable to
unnecessarily prolong the secondary wires, this device admits of the
switch being put near the circuits, and yet being operated from afar.
[Illustration: FIG. 43.]
RUHMKORFF COIL.
Fig. 44 shows a diagram of a Ruhmkorff coil, the letters referring as
follows:
_C_ the iron core, _P_ the primary coil wires, _I_ the insulating tube
between primary _P_ and the secondary coil _S_. In small coils this
may be dispensed with, and a heavy layer of paraffin wax laid over the
primary coil. _D D_ are the ends of the secondary, showing sparking
taking place between a pair of balls (or between the electrodes of a
gas burner); _R_ is a stiff spring fastened to the coil base and
carrying a soft iron hammer, which is attracted toward the iron core,
_C_, when current passes through the primary coil and magnetizes it.
_L_ is a battery, _J_, a condenser, to be more fully described later
on. When the spring _R_ touches the adjustment screw _A_ at _B_, as
they are insulated from each other, contact is made and reference to
circuit will show that the current from battery _L_ flows through
primary coil, magnetizing the core and attracting soft iron hammer on
_R_. As this bends forward, it breaks contact at _B_, the core loses
its magnetism and the spring flies back, to again make contact. This
is repeated many times per second.
[Illustration: FIG. 44.]
As a heavy spark occurs at _B_ on the break of contact, the condenser,
_J_, is attached at _M K_. This is a series of insulated tinfoil
sheets, which has the property of nullifying the spark at _B_, and so
preventing the waste of platinum with which both adjustment screw _A_
and spring _R_ are equipped.
A Ruhmkorff coil differs from a simple primary coil in three main
points. Two separate coils instead of one; high insulation, and a
primary coil of few turns. In the simple coil we desired
self-induction; here, we desire to avoid it as much as possible.
The average size Ruhmkorff coil, for jump spark work, would be one
giving a 2-inch spark, specifications for which are as follows:
_Spool_--Nine inches long by one inch in diameter. End cheeks 4 inches
high by 3 inches wide.
_Core_--Sufficient soft iron wires, 9 inches long by No. 22 B. W.
gauge as will fill the spool tube.
_Primary_--Two layers No. 14 B. & S. gauge cotton-covered copper wire.
_Secondary_--Two and one-half pounds No. 36 B. & S. gauge double
cotton or silk-covered magnet wire wound in four sections (or more
than four sections, if feasible).
_Condenser_--Seventy sheets tinfoil 4 by 7-1/2 inches; 80 sheets
condenser paper 5 by 8 inches.
SPOOL.
This should be made up of a fibre tube 9 inches in length by about
1/16 inch thick, and should be firmly fixed into the spool ends. If it
be glued in it should also be pinned as well; it is easily possible to
drive in a few screws passing through the tube into the spool ends,
particularly as the soft iron core, being of loose wires, will adapt
itself to the slightly projecting screw-heads. Remember that this
spool must be made strong; if it comes apart during the winding
process, much trouble will ensue, and perhaps all the wires lost or
ruined. For reasons to be seen later, do not affix the right-hand
spool end yet, but have it ready. The core consists of as many fine
iron wires, say of No. 22 B. W. gauge, as can be forced into the tube,
but the core can better be added after the windings are all in; that
is, in such cases where a rigid spool tube is used.
PRIMARY WINDING.
This consists of two layers of No. 14 B. & S. gauge cotton (or silk)
covered copper magnet wire, and should be evenly and tightly laid on.
For winding coils, a lathe is a most handy machine, or the spool can
be mounted on a spindle and rotated by hand. It is not feasible here
to give all details of coil-construction; reference should be made to
the many excellent works on the subject. The two ends are brought out
through holes in the spool ends, as indicated for the simple primary
coil before described. After winding, the wire is to be well basted
with melted paraffin wax until it is saturated, any excess being
scraped off so as to leave a smooth cylindrical surface for the
secondary coil. Half a dozen turns of stout paper or oiled silk is now
to be wound on, and enough paraffin wax added to leave an insulation
at least one-quarter of an inch around the outside of the winding. The
right-hand end of spool, where the end was not attached, will require
a little care that the wire does not run off; but, as only two layers
are to be wound, it is an easy thing to do.
When the primary coil is finished off, cut three pieces of hard rubber
four inches square, with a central hole just big enough that they may
be slipped on over the primary coil to form divisions into which the
secondary wire goes. These can be fixed equal distances apart by means
of removable wooden blocks, which are taken off as each section is
wound.
SECONDARY COIL.
The secondary coil consists of about 2-1/2 pounds No. 36 B. & S. gauge
silk or cotton-covered magnet wire, wound evenly in layers in the
sections on the primary coil. Before any wire goes into a section, it
must be seen that the division fits tight to the primary coil. It will
be best to pour around the coil some melted paraffin wax so as to form
an insulating ring, and prevent any possibility of the spark creeping
under the section division into the next. The actual operation of
winding presents no difficulty other than those of keeping the wire
from damage and getting as even layers as possible. If each layer is
separated from its neighbor by a strip of paraffined paper, it makes
even winding easier, and better insulation. As to the insulating of
the secondary coil, it can be done in a variety of ways. The coil can
be soaked in molten paraffin until saturated, or the wire can be made
to pass through a dish of molten paraffin while on its way from the
wire reel to the coil. In the latter case the wire must be guided by
means of glass rollers, as the wax would harden rapidly if touched by
the fingers. In connecting up the sections, the similar ends are to be
joined; that is, the inside ends to inside ends, and outside ends to
outside ends, as per diagram (Fig. 45). This will bring two outside
ends free for attachment to binding posts. Fig. 46 shows direction of
winding and connecting the two middle coils, _A C_ being the inside
layers next to primary and _B D_ the outside layers.
[Illustration: FIG. 45.]
An outside coat of paraffin wax is now given to the coil and a
wrapping of waxed paper laid on. Then, if desired, a cover of
sheet-rubber or a layer of cloth can be put on over all, to finish the
job.
BASE, KEY, AND CONDENSER.
[Illustration: FIG. 46.]
The base for a Ruhmkorff coil generally resembles an oblong shallow
box. The coil is mounted on the lid, and the condenser inside the box,
the connections being made on the lower side of the lid. It is
preferable, except for appearance’ sake, to make all connections
outside the box, but this is left to the worker’s choice.
[Illustration: FIG. 47.]
_The Condenser_ is made up of 70 sheets of tinfoil each about 4 inches
by 7-1/2, and 80 sheets of clean white paper 5 by 8 inches placed
alternately, and saturated with paraffin wax. The tinfoil sheets are
laid so that about 1/2 inch projects out of the paper sheets at each
end, the alternate sheets coming out at the same end, and the
projecting pieces being bent together gives the effect of a pair of
tinfoil sheets insulated from each other, aggregating the sum of all
the small ones.
[Illustration: FIG. 48.]
The coil can now be attached to the base by means of screws passing
through the lid into the coil ends. If a vibrating contact breaker be
desired, reference to Fig. 44 will show method of connection. Fig. 47
shows details of a contact breaker of similar design. _R_ is hammer
head of soft iron, _S_ a spring about thickness of clock spring and
3/8 inch wide or more. _B_ is contact point, both spring and
adjustment screw _A_ being fitted with platinum contacts. _C_ is a
check nut, to hold _A_ from turning. _I_ is an adjustment to tighten
or loosen spring _S_, by means of a lug which it carries on its
shaft. It is well insulated from pillar carrying _A_, by the hard
rubber bushing, _I_.
The condenser is laid in the box under the coil and attached as in
Fig. 44; that is, one set of sheets to the contact pillar, and the
other set to the adjustment screw.
For gas-lighting work, it is generally preferable to use a contact or
strap key (Fig. 48), instead of a vibrator. The key can be mounted on
coil base, in which case the condenser will be attached in same manner
as for the vibrator.