jgg     2009 29 (3):  145-147    ISSN: 0412-1961  CN: 21-1139/TG DEVELOPMENT OF DIFFERENCE EXTENSOMETER Lu Chongwu ;and Yang Jiang Institute of Seismology,CEA, Wuhan 430071 Received: 1900-01-01  Revised: 1900-01-01  Online: null Supporting info null