Fig. 83 shows a well-known type of this character which is a combination spray and surface carbureter. A U-shaped tube A, with the air inlet at B, and discharge at C, has a butterfly valve D in its latter end. Below the U-shaped bend, is a reservoir E to contain a float F, vertically-movable around a central stem G which is part of and projects down from the U-shaped tube. [Illustration: Fig. 83. Surface Carbureter.] Through this stem G is a duct H, the lower end of which communicates with the gasoline reservoir, or float chamber, and the upper end has a small orifice leading to the U-shaped tube. A valve stem I is adapted to regulate the inflow of gasoline through the duct. THE GASOLINE INLET.--At one side of the reservoir is an extension J, within which is a vertically disposed needle valve K, seated in the duct I, by way of which gasoline is admitted. A lever M, pivoted at N, has one end attached to the float F, and the other end is in engagement with the needle valve K. The float is so arranged as to permit the gasoline to flow up into the U-shaped tube A, and form a small pool of the fuel before it closes the needle valve K. SECURING SURFACE FOR AIR CONTACT.--Directly above the oil inlet duct H, the U-shaped tube is contracted by a downwardly-projecting wall P, the object being to compel all the passing air to intimately come into contact with the gasoline pool, and thus take up as much vapor as possible. In this arrangement the suction of the engine does not draw up the gasoline from the reservoir, but all the energy is expended in moving air through the tube, and past the contracted throat. In starting the engine the float is momentarily depressed by the pin Q, and a drain duct R is provided to prevent flooding of the tube A. CHAPTER XII IGNITION SYSTEMS The universal use of electricity as a means of igniting the fuel in gasoline motors, makes it necessary that the novice should know something of the fundamentals of the science. SEEING THE EFFECT OF ELECTRICITY.--While it is impossible to see a current, there are certain mechanical devices which enables it to be seen by the effects produced on them. One of these devices is the armature which, if placed across the poles of a horseshoe magnet, will adhere to the magnet, by means of its magnetic pull. Another exhibition is the spark caused by separating the contact point of a conductor through which a current is flowing, causing a spark. ACTION OF A CURRENT.--The current flowing over a wire acts substantially the same as water flowing through a pipe, that is, the quantity is dependent on the size of the wire, just as in water where the diameter of the pipe determines the flow. AMPERES AND VOLTS.--Water may flow sluggishly through a pipe, or be forced through with great violence. So with an electric current. Pressure, therefore, expresses the second similarity in the two mediums.