Prof. Thomson on the Conductivity of Commercial Copper. 475 



metals, produced by every application and by the altered condition 

 left after the withdrawal of excessive stress*; and I have already 

 made a partial examination of these effects in copper, iron, and pla- 

 tinum wires, and found them to be in all cases so minute, that the 

 present results as to copper wire are only what was to be expected. 



To find whether or not there is any sensible loss of conducting 

 power on the whole due to the spiral forms given to the individual 

 wires when spun into a strand, it would be well worth while to com- 

 pare very carefully the resistances of single wires with those of strands 

 spun from exactly the same stock. This I have not yet had an 

 opportunity of doing ; but the following results show that any defi- 

 ciency which the strand may present when accurately compared with 

 solid wire, is nothing in comparison with the differences presented by 

 different samples chosen at random from various stocks of solid wire 

 and strand in the process of preparation for telegraphic purposes. 



No. 16 Solid "Wire. Pairs of samples in different states of prepara- 

 tion, each 1000 inches long. 



* See the Bakerian Lecture, " On the Electro-dynamic Qualities of Metals," 

 §§ 104, 105 and 150, Philosophical Transactions for 1856. 



t These resistances were measured, by means of a Joule's tangent galvanometer 

 with a coil of 400 turns of fine wire, in terms of the resistance of a standard con- 

 ductor as unity. The resistance of this standard has been determined for me in 

 absolute measure through the kindness of Professor W. Weber, and has been 



found to be 20,055,000 German units ( '"^^''^ ^ , or 6,580,000 British units 



Vseconds^ 



— ). The numbers in the last column, headed " Specific resistances re- 



seconds^' 

 duccd to British measure," express the resistances of condiictors composed of ten 

 different qualities of metal, and each one foot long and weighing one grain. I 

 is impossible to over-estimate the great practical value of tliis system of absolute 

 measurement carried out by Weber into every department of electrical sciencei 

 after its first introduction into the observations of terrestrial magnetism by Gaussj 

 See " Messungen galvanisclien Leitungswiderstiinde nach eiiiem absoluten Maasse,' 

 PoggendorfTs Annalcn, March 1851. See also the author's articles entitled " On 

 the Mechanical Theory of Electrolysis," and "Application of the Principle of 

 Mechanical Effect to the Measurement of Electromotive Force, and of Galvanic 

 Resistances in Absolute Units," Philosoi)hical Magazine, December 1851. 



213 



