RULE 15. HEATING VALUE.--Each utility furnishing manufactured gas shall supply gas which at any point at least 1 mile from the plant, and tested in the place where it is consumed, shall have a monthly average total heating value of not less than 565 B.t.u. per cubic foot, and at no time shall the total heating value of the gas at such point be less than 530 B.t.u. per cubic foot. To arrive at the monthly average total heating value, the results of all tests made on any one day shall be averaged and the average of all such daily averages shall be taken as the monthly average. RULE 8. RAILROAD COMMISSION OF WISCONSIN.--Each utility furnishing gas service must supply gas giving a monthly average of not less than 600 B.t.u. total heating value per cubic foot, as referred to standard conditions of temperature and pressure. The minimum heating value shall never fall below 550. The tests to determine the heating value of the gas shall be made anywhere within a 1-mile radius of the center of distribution. =Gas Ranges.=--Gas ranges and all other heaters using gas as a fuel are constructed to utilize the principle of the Bunsen burner. Fig. 180 illustrates the type of burner used in the Jewel gas range. This represents the form adapted to the top burners for all direct-contact cooking or heating. The burners are of different sizes and arranged as they appear in Fig. 181. This picture shows the top of the range as seen from above, looking directly downward. The gas supply pipe and individual valves for each burner are in position as they appear in front of the range. [Illustration: FIG. 180.--Detroit Jewel one-piece, star-shaped burner.] [Illustration: FIG. 181. FIG. 182. FIG. 181.--Showing top burners and valve attachment of a gas stove. FIG. 182.--Section showing arrangement of oven burners and lighter of a gas oven.] In operation, the nozzles of the gas valves stand directly in front of the opening _G_, in Fig. 180. The stream of gas in passing into the burner induces a flow of air through the opening _A_. The mixture of gas and air is such as will burn with the characteristic Bunsen flame without smoke. The oven burners are different in form but the individual flames are the same as those of the top burners. They extend across the oven as shown in Fig. 182. In this the top of the oven is removed and burners as seen are viewed from above. The top burners are lighted by direct application of a burning match but the oven burners must be lighted by first igniting a special torch or “pilot lighter.” The middle gas valve of Fig. 182 is turned and the torch lighted, then the other valves are opened and the jets are instantly ignited. As soon as they are burning the pilot lighter is extinguished by turning its valve.