Fig. 83
15. Electric Gas Lighter ([Fig. 83]).—These usually have two or three small, dry battery cells in the handle. By pushing a button in the handle connection is made between this battery and a short piece of resistance wire in the tip. This wire gets red hot and lights the gas. It is a surprise to many that we can light illuminating gas without bringing a flame to it, and it is equally surprising that some flames, or at least sparks, may not be able to light the gas. The fact is that it is wholly a matter of temperature and kind of gas. Iron heated to dull red will not light the illuminating gas now being furnished in New York City, while iron at a bright red heat will do so. Iron may be hot enough to light illuminating gas but too cool to light gasolene vapour, which requires a dazzling white heat. Iron which is just under the temperature at which it gives any light may set fire to wood and paper. After it has cooled a good deal below that, it will set fire to sulphur, and when it has cooled so that one may hold it in the hand, it is still hot enough to set fire to phosphorus. The glowing end of a lighted cigar, the spark made by striking flint, or the spark from a spark coil with a feeble battery, all fail to set fire to gasolene vapour, simply because they are not hot enough.
Fresh battery cells must occasionally be put in the handle of the electric gas lighter.
Four facts regarding the resistance of wires it is well to remember:
1. The longer the wire the more resistance it offers to the electric current.
2. The smaller the diameter of the wire the more resistance it offers.
3. Some materials offer more resistance than others, for example, iron about six times as much as copper and German silver about twelve times as much as copper.
4. The common metals offer more resistance when hot than when cold, about double the resistance when heated to five hundred degrees. It is the reverse with carbon, which offers more resistance when cold than when hot. The carbon filament lamp offers about double the resistance when cold as when lighted to full brilliancy.
