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.

[Ill.u.s.tration: FIG. 39.]

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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 pa.s.sing 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 pa.s.s 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 pa.s.sage of the high-tension current. The base should always be of rubber or gla.s.s. Sh.e.l.lacked-wood, or such subst.i.tutes, are productive of trouble.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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 b.a.l.l.s (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 pa.s.ses 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.

[Ill.u.s.tration: 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 pa.s.sing 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 sc.r.a.ped 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 pa.s.s 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 gla.s.s 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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: FIG. 48.]

The coil can now be attached to the base by means of screws pa.s.sing 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.

CHAPTER VI.

HOW TO SELECT BATTERIES FOR GAS LIGHTING.