162 Mr. F. Jenkin on the Question of the 



determined by a Subcommittee consisting of Professor Thomson, 

 Professor Maxwell, Mr. Balfour Stewart, and myself. 



The arguments in favour of one or the other of these units 

 should be kept quite distinct from those in favour of the best ma- 

 terials for the reproduction or manufacture of material standards. 

 If mercury be the best material for a permanent standard, any 

 unit can be made of mercury. If mercury be the best material for 

 reproducing a standard, then when the specific resistance of pure 

 mercury in any unit has been determined, mercury can be used 

 for the reproduction of that unit. Conversely, if Dr. Siemens' s 

 be the best definition for a unit of resistance upon some inde- 

 pendent grounds, then the fact that mercury was a bad substance 

 for constructing or reproducing the unit might not even weaken 

 the arguments in favour of his definition. 



Now, apart from the question of manufacture and reproduc- 

 tion, why should a mercury unit be chosen ? I see no arguments 

 in Dr. Siemens' s paper in favour of his own definition, except 

 perhaps the statement that " mercury is the conductor which is 

 without doubt the best suited to serve as unit of conducting-power." 



Dr. Siemens here separates conducting-power from resistance, 

 and seems to think that the specific conducting-powers of mate- 

 rials should be referred to some one standard material, and that 

 the unit of conducting-power may be distinct from the unit of 

 resistance. But conducting-power is naturally defined in calcu- 

 lations as the reciprocal of resistance, the unit of resistance and 

 of conducting-power being the same ; there is therefore no need 

 whatever to select any substance as a unit of conducting-power, 

 which would indeed simply introduce confusion into mathema- 

 tical expressions of electrical relations. The Committee have 

 therefore adopted Professor Thomson's definition of the specific 

 conducting-power, of metals as the conducting-power of the wire 

 of unit length and weight (being the reciprocal of the resistance 

 of the same wire) . In this way the numbers expressing the spe- 

 cific properties are rendered independent of comparison with any 

 material whatever, whereas, on the other plan, we should always 

 have to ask on whose determination of mercury, silver, or gold 

 &c. the values given were based. Moreover Professor Thomson's 

 plan is infinitely more convenient in calculating from the specific 

 values the actual resistance or conducting-power of any given 

 wire. These considerations absolve me from the necessity of 

 even considering whether, if a unit-material must be selected, 

 mercury would be the best — a question which I nevertheless 

 think might not be decided affirmatively. 



Dr. Siemens also claims practical advantages, but does not 

 state their nature. Mercury is little used in telegraphy as com- 

 pared with copper, German silver, or iron. 



