1. KOSE, V., MELCHERT, F., FACK, H., and HETZEL, W., IEEE-IM 21,314(1972)
2. also PTB-Mitteilungen 82, 230 (1972)
3. KOSE, V., MELCHERT, F., ENGELLAND, W., FACK, H., FUHRMANN, B., GUTMANN, P.,and WARNECKE,P.,IEEE-IM 23, 271 (1974)
4. SULLIVAN, D.B., Rev.Sci.Instrum. 42, 612 (1971)
5. WARNECKE, P. and KOSE, V., Rev.Sci.Instrum. to be published