The length of the tube required for a given engine is a matter of experiment, as is also the location of the heated portion. High compression naturally forces the mixture farther into the tube than low, therefore the flame should come into contact with the tube at a point nearer the outer end with high compression than with a low compression. Shortening the tube causes advanced ignition, as the mixture reaches the heated portion sooner, or earlier in the stroke, because of the decreased cushioning effect of the residue gases in the tube. The length of tube and location of maximum heat zone should be so proportioned that combustion will take place at the highest compression. Moving flame to outer end of the tube retards ignition. Moving the flame toward the cylinder advances it. While the hot tube is the acme of simplicity in construction, it is not the easiest thing to properly adjust, as the adjustment depends on compression, temperature of the tube, and the quality of the mixture. Any of these variables may cause improper firing. The hot tube is rather an expensive type of ignition with high priced fuel, as the burner consumes a considerable amount of gas, and is burning continuously during the idle strokes as well as during the time of firing. It is practically impossible to obtain satisfactory results from a hot tube on an engine that regulates its speed by varying the mixture or compression, as engines running on a light load will not have sufficient compression to cause the mixture to come into contact with the hot surface, the engine misfiring on light loads. The tubes are made of porcelain, nickel steel alloy, or common gas pipe, and are of various diameters and lengths. All of these materials have their faults. Porcelain being very brittle, is liable to breakage. Gas pipe burns out and corrodes rapidly. Nickel alloy is not liable to breakage, is not so susceptible to corrosion as iron, but is far from being a permanent fixture. Timing valves are a feature of some systems of hot tube ignition, which correct to a certain extent the irregularity of firing of the plain type of tube. The timing valve is introduced in the passage connecting the cylinder and tube, and prevents the gas in the cylinder from coming into contact with the heated surface until ignition is desired. The valve is operated by means of mechanism connecting it with the crank shaft. It is evident that with sufficient compression in the cylinder, the time of ignition can be obtained with certainty. This mechanism is rather complicated, and subject to wear, and the advantage gained by the fixed point of ignition is offset by mechanical complication and consequent trouble.