Part 3 (1/2)
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.]