The hollow-wire system of gasoline lighting possesses the advantage of simplicity in construction and ease of installation that makes it attractive, particularly for use in small dwellings. The ease with which plants of this character are installed in buildings already constructed and its relatively low cost has made it a popular means of lighting. The same principle as that used in the hollow-wire system is applied to portable gasoline lamps in which a remarkably convenient and brilliant lamp is made to take the place of the customary kerosene lamp. Small portable gasoline lamps are now extensively used for the same purpose as ordinary oil lanterns. These lamps are convenient as a source of light, make a handsome appearance and are relatively inexpensive to operate.
The hollow-wire system as commonly employed is illustrated in Figs. 186 and 187. In the gravity type of the system as illustrated in Fig. 186, the supply of gasoline is stored in the upper part of the house in a tank T and conducted to the burners below, through a system of small copper tubes as indicated by the heavy lines in the drawing. The same tank is used to supply the gasoline for the stove S in the kitchen and the lamps L in the different apartments. The gasoline supply in this case, is obtained entirely by gravity. This type of plant is not approved by the National Board of Underwriters but its use is quite generally permitted. The storage of gasoline in this form should be done with caution as carelessness or accident might lead to serious results. With an arrangement of this kind the force of gravity gives the pressure which supplies the burners below but it would not be possible to use the lamps on the same floor with the tank.
Fig. 186. - Hollow-wire system of gasoline lighting with gravity feed.
Where it is desired to use lamps on both floors, a pressure tank is employed for supplying the gasoline to the lamps, as indicated in Fig. 187. In this plant the pressure tanks S, T in the basement, furnish the pressure which forces the supply of gasoline through the small tubes to the lamps L in the different rooms and also to the stove R in the kitchen.
Fig. 187. - Hollow-wire system of gasoline lighting with pressure-tank feed.
The means of furnishing the pressure for supplying the gasoline to the burners may be a simple tank as that in Fig. 188, or the more elaborate apparatus shown in the double tank of Fig. 189. Either style will give good results but the double tank requires the least attention in operation and is therefore more satisfactory in use.
The tank in Fig. 188 is made of sheet metal of such weight as will safely withstand the pressure necessary in its use. It is arranged with an opening E, for filling with gasoline, a pressure gage for indicating the air pressure to which the gasoline is subjected, and two needle valves; C, for attaching an air pump and D, to which the hollow wire is attached for distributing the gasoline to the places of use. The tank is filled with gasoline to about the line A, and then air pressure is applied with an ordinary air pump to say 20 pounds to the square inch. This pressure will be much more than will be necessary to force the gasoline through the tubes but it is intended to last for a considerable length of time.
Fig. 188. - Simple gasoline pressure-tank.
Fig. 189. - Double-pressure tank for constant pressure service in gasoline lighting systems.
The principle of operation is that known in physics as Boyles law, that "the temperature being constant, the pressure of a confined gas will be inversely as its volume" That is, if the tank is perfectly tight, the pressure above the line A, in the tank, will gradually become less as the gasoline is used and when its level is at the line B, where the volume is twice the original amount, the pressure will be one-half what it was originally, and will still be sufficient to force the gasoline through the tubes to the lamps. It is evident that once the tank is charged and the air pressure applied it will require no further attention until a considerable part of the gasoline is consumed. If at any time the pressure in the tank becomes too low to feed the lamps, a few strokes of the pump will raise it to the required amount.
While the single tank does the required work, its use is not perfect because the pressure is constantly varying. If a lamp is set to burn at a definite pressure, any decrease in the gasoline supply due to falling pressure will change the amount of light given by the lamp; while the variation in the pressure of the single supply tank is not great, a more perfect effect is attained in the double type of tank as that of Fig. 189.
The object attained in the use of two tanks differs with different manufacturers. The tank shown in Fig. 183, being intended to maintain a constant pressure on the gasoline, is quite different from those described in Fig. 197 in use with the central-generator system of lighting, to be described later. In Fig. 189 tank No. 1 is for air supply alone and tank No. 2 is the storage tank for gasoline. Between the two tanks is a pressure-regulating valve 6-7, which keeps a constant pressure on tank No. 2 so long as the air pressure of the tank No. 1 is equal or greater than the other. The gasoline in tank No. 2 will therefore be always under the same pressure and when the lamps are once burning the gasoline supply to each lamp will be a constant amount.
Tank No. 2 is separated by the head 13 into two compartments, marked 18 and 19. The connection between the two compartments is made by the valve 15 and the connection 16. The gasoline supply for the lighting system is taken from the lower chamber at the valve marked 17.
It is possible to refill this tank with gasoline while the system is working. To accomplish this, the air supply is cut off from tank No. 1, by closing valve 9 and the valve 15 is closed to retain the pressure on the lower chamber of tank No. 2. The screw-plug is then taken from the tube 12 and the tank refilled. The screw-plug is then returned to its place, the valves 9 and 15 are again opened and the regulating valve, immediately restores the desired pressure.
The amount of pressure required on the system will depend on the height to which the gasoline is carried within the building. The pressure is generally 1 pound to each foot in height and to do the best work the pressure must be constant.
These plants may serve as a fuel supply for gasoline stove as indicated at R or any other source of domestic heating. The usual gravity supply tank is replaced by the hollow wire through which is the gasoline from the tank in the basement.