184 MERCURY UNIT. 



copying magnified the variations by a large percentage. 

 My firm had up to that time taken the resistance of 

 a German mile (4 3 | 4 miles English) of copper wire of 

 1 millimetre diameter as unit, and produced graduated 

 scales of resistance on the basis of this unit. It 

 appeared however that the copper itself with the 

 utmost possible purity had essentially different specific 

 resistance, and changed its resistance in the course of 

 time. To adopt the absolute unit of Weber as funda- 

 mental standard was rendered impossible by the then 

 state of electrical measuring, which at the time made 

 an agreement of the various productions of this unit 

 unattainable. Under these circumstances I resolved 

 to make pure mercury the basis of a reproducible 

 standard of resistance , and proposed to take the 

 resistance of a mercurial prism of 1 square millimetre 

 in cross section and 1 metre in length at the freezing 

 point of water as the unit of resistance. I shall return 

 to this standard of resistance in the description of my 

 papers on this subject , and shall only remark here 

 that the scales of resistance with the mercury unit, 

 regulated according to the weight system, prepared by 

 my firm, proved extremely useful in laying the cable 

 from Suez to Aden, and for the first time made reliable 

 determinations of faults possible. - 



The cable -laying in the Red Sea was also rich 

 in interesting personal experiences for myself. The very 

 day after embarking at Trieste in the beginning of 

 April, I was so fortunate as to witness a splendid 

 zodiacal light in the evening sky. Scientists contended 



