The Mechanism of the Ignition of Methane-Air Mixture by Electric Detonators (2) Effects of construction materials and characteristics of an electric detonator on the incendivity to Methane-Air mixture;(part 1)(*in Japanese)電気雷管によるメタン−空気混合物の着火に関する研究 (第ニ報) (雷管のメタン着火率に及ぼす雷管製造構成要因の影響その1)