PRIMARY COILS AND SAFETY DEVICES.
To construct a primary coil such as used with pendant or automatic
burners presents no difficulty. The most convenient sizes are those 8
to 10 inches in length and about 3 inches in diameter. It is quite
common to speak of these coils as _8 or 10 inch coils_; to the
writer’s knowledge this has been taken to mean a Ruhmkorff or
double-wound induction coil, giving a free 8 or 10 inch spark.
[Illustration: FIG. 27.]
To make such a coil (Fig. 27), proceed as follows: Prepare a spool by
gluing a paper or fibre tube 3/4 inch in outside diameter by about
1-16 inch thick into square or round spool ends three inches square,
one-half inch thick, and having each a centre hole just large enough
to admit of the tube being held tightly. These ends should be firmly
fixed on the tube; a pin or two driven through tube into end will
assist in strengthening the joint. Now wind on the tube about 3 pounds
No. 12 B. & S. cotton-covered magnet wire. This will give about six
layers of 80 turns each, nearly 500 turns in all, a total length of,
say, 150 feet, measuring .25 ohm. The ends of the wire are to be
brought out through holes drilled in the spool ends, and can be fixed
to brass binding posts on those ends.
Into the paper tube push as many iron wires 8 inches long by No. 22 B.
W. gauge as will fill it. These iron wires can be tightened finally by
driving in at each end, a stout wire nail.
Although not absolutely necessary, a coat or two of shellac varnish
applied to the windings will make a better insulation. Shellac varnish
is readily made by dissolving one part gum shellac in four parts of
alcohol. For coils which are likely to be in damp places, a good
saturation with insulating compound, such as P. & B. paint, will
render them waterproof. The need for good insulation in these primary
coils is not so urgent as in Ruhmkorff coils, owing to the lower
potential of the current.
A smaller coil can be made with No. 14 B. & S. wire where the battery
is of higher resistance (or gives less than ten amperes on short
circuit). The remarks on battery selection on another page will be
found to meet application here.
AUTOMATIC CUT-OUTS.
Where there are a number of burners to be installed in different parts
of a house, it becomes desirable to wire in a number of circuits. As
one end of the circuit is already grounded, a second ground will cause
material injury to the battery if not detected in time. It becomes,
therefore, necessary to be able to open a grounded circuit without
affecting all the lights in a house. This is possible with the
multiple circuit arrangement by using a switch, either automatic or
operated by hand.
The simplest form of danger signal is the relay electric bell
attachment, which device is mounted on the end of the gas-lighting
coil. It consists of an armature which closes a circuit when
attracted by the coil core, in which circuit are included a battery
and electric bell.
Now when an ordinary pendant or ratchet burner is pulled, it only
sends a momentary current through the coil, enough to magnetize the
core, but not enough to attract the armature sufficiently long for the
bell to ring. But if a short circuit or ground should occur, the
armature is held against the contact long enough to cause the bell to
ring and give warning. In some cases a constant ringing attachment is
added, in which case the bell rings until some one stops it.
THE SYRACUSE CUT-OUT.
This is a most ingenious device for opening a short circuit, depending
on its action upon the sluggish movement of glycerine (Fig. 28).
A sealed glass tube pivoted near its centre contains a portion of
glycerine sufficient to considerably overbalance it and keep one end
down. A soft iron armature is attached to this tube in such manner
that each time a current flows through a pair of electro-magnets, the
attraction of the armature causes the tube to tilt and the glycerine
flows along to the other end. Now it will be readily seen that if the
tube is only tilted for a second or so, the slow-moving glycerine will
not have flowed sufficiently to the end to overbalance it, but it
requires an attraction of the armature for a considerable period. This
electro-magnet is in circuit with the gas-lighting wires, the tube
being provided with contacts in such manner that, when fully tilted,
the circuit is broken. The momentary jerks imparted to the armature by
the operation of a pendant or automatic burner will not be enough to
permanently tilt the tube and break contact, but a short circuit will
hold the armature tight down, until the increasing weight of glycerine
causes the tube to open the circuit.
[Illustration: FIG. 28.]
[Illustration: FIG. 29.]
AUTOMATIC SECTIONAL CUT-OUT.
[Illustration: FIG. 30.]
This cut-out, Fig. 29, is representative of the class which use
clockwork, and is both simple and reliable. The house circuit is in
series with an electro-magnet which controls a clockwork having a long
pinion shaft. This clockwork starts and runs while the house circuit
is closed, as on operating a burner, but stops when the circuit is
opened and flow of current ceases. The wires leading to different
circuits in the building run through a number of contact springs
mounted on sliding rods, which have teeth cut on the under side (Fig.
30). These rods have soft iron armatures on the opposite ends from the
contact springs, which rest over electro-magnets, also connected to
the house circuits. When the clockwork starts, the pinion shaft
revolves, but does not engage in any of the sliding rods, as they
just clear it. Should a heavy or continuous current pass through one
of the electro-magnets, it attracts the armature on the corresponding
rod (Fig. 31), and the turning pinion engages in the teeth, drawing up
the rod and breaking contact.
[Illustration: FIG. 31.]
Fig. 32 is a form of battery protector which works on the gravity
principle. Here each section is governed by a rocking contact,
operated by two glass bulbs partially filled with a volatile fluid
(such as ether), and joined by a glass tube. In one of these bulbs is
a platinum wire which is included in the circuit and heats upon the
passage of a strong or continuous current. If the circuit is closed
too long, the heating of the platinum wire causes the fluid to flow
into the upper bulb, and, as the bulbs are pivoted, the increased
weight of the upper bulb now overbalances the rocker and breaks the
circuit on that section.
[Illustration: FIG. 32.]