CHAPTER IV.
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.
[Ill.u.s.tration: 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 a.s.sist 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 bra.s.s 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 sh.e.l.lac varnish applied to the windings will make a better insulation. Sh.e.l.lac varnish is readily made by dissolving one part gum sh.e.l.lac 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 gla.s.s 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.
[Ill.u.s.tration: FIG. 28.]
[Ill.u.s.tration: FIG. 29.]
AUTOMATIC SECTIONAL CUT-OUT.
[Ill.u.s.tration: FIG. 30.]
This cut-out, Fig. 29, is representative of the cla.s.s 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 pa.s.s 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.
[Ill.u.s.tration: 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 gla.s.s bulbs partially filled with a volatile fluid (such as ether), and joined by a gla.s.s tube. In one of these bulbs is a platinum wire which is included in the circuit and heats upon the pa.s.sage 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.
[Ill.u.s.tration: FIG. 32.]
CHAPTER V.
LIGHTING OF LARGE BUILDINGS.
The jump spark system is used where it is desired to light cl.u.s.ters 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-f.l.a.n.g.e 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.
[Ill.u.s.tration: FIG. 33.]
[Ill.u.s.tration: FIG. 35.]
[Ill.u.s.tration: 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.
[Ill.u.s.tration: 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.
[Ill.u.s.tration: 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 pa.s.sed into it and holds against the lower end as far away from the wire as possible.
Gla.s.s tubes should be pa.s.sed 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.
[Ill.u.s.tration: 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.