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.
Fig. 33., Fig. 34. & Fig. 35
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.
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.