78 THE SCIENTIFIC PAPERS OF 



intricate, but would confine himself to giving an outline which 

 would sufficiently show the relative advantages of the system, 

 referring those who might feel more interested in the subject to a 

 paper by his brother and himself read in 18GO before the British 

 Association at Oxford.* The old system was to test the insulation 

 by the galvanometer, and to judge the condition of the line by the 

 angle of deflection and the battery power employed. This was 

 unsatisfactory, for the angle of deflection of an instrument was 

 never the same for two consecutive days, nor could an instrument 

 be constructed with a constant amount of deflection for the same 

 current ; there was, therefore, no means of comparing results. If 

 a mile of cable was measured by one instrument, and several miles 

 by another, or by the same instrument the next day, no useful 

 comparison could be made. But Messrs. Siemens adopted the 

 method of expressing the conductivity of the insulating coating as 

 well as of the conductor by certain units of resistance. The unit 

 adopted by the author, and which might suffice for the particular 

 case mentioned in the paper, was the mile of No. 16 copper wire. 

 It resulted, however, from the investigations of Dr. Matthiesen 

 that the copper of commerce varied in its conductivity, between 

 the limits of 100 and 7 ; in speaking therefore of the resistance 

 of a mile of copper wire, no distinct estimate of its value could be 

 formed. The unit developed by his brother, and which had since 

 been adopted by them in their operations, was the resistance of a 

 column of pure mercury of one metre in length, and one millimetre 

 in sectional area ; this unit possessed, over others, the advantages 

 of being invariable and of being easily reproduced. 



Coils of resistance were next formed of German silver wire, 

 representing respectively, units, tens, hundreds, thousands, and 

 tens df thousands of units of resistance. By introducing these 

 variable resistances into the three sides of a Wheatstone's bridge, 

 or electric balance, the resistance of the fourth side, which was the 

 gutta-percha or copper conductor of the cable under examination, 

 could be ascertained with the utmost certainty, the limit of error 



* Vide "Outline ef the Principles and Practice involved in dealing with the 

 Electrical Condition of Submarine Electric Telegraphs," by M. Werner and C. W. 

 Siemens, in the Report of the Joint Committee appointed to Inquire into the Con- 

 struction of Submarine Telegraph Cables. Folio. London, 1861, p. 455, and pp. 

 47-65, ante. 



