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XLIV. A new Determination of the Ratio of the Electro- 

 magnetic to the Electrostatic Unit of Electric Quantity. By 

 W. E. Ayrton and John Perry *. 

 [Plate XL] 

 Introduction. 



THE fact that metals had a different power of conducting 

 electricity was discovered by Sir Humphry Davy in 

 1821 f , although the idea of resistance as a property of a con- 

 ductor was not introduced until the publication of Ohm's law in 

 1827, in which a resistance was first regarded as a magnitude. 

 Now a magnitude necessarily implies a unit of measurement ; 

 but the earlier writers merely contented themselves with re- 

 ducing by calculation the resistance of all parts of a hetero- 

 geneous circuit into a given length of some given part of that 

 circuit ; so that they generally spoke of the resistance " as the 

 reduced length of the conductor." 



The next step was naturally to refer these " reduced lengths " 

 to the length of some standard wire which might perhaps not 

 be employed in the circuits under test, and to consider the 

 resistance of unit length of this standard wire as the unit re- 

 sistance. Consequently we find the unit which was employed 

 by Lenz, in 1838, to be denned as that of 1 foot of No. 11 

 copper wire, and the unit of Wheatstone, in 1840, as that of 1 

 foot of copper wire weighing 100 grains. Until the year 

 1850 measurements of resistance were confined, with few ex- 

 ceptions, to the laboratory ; but about that time underground 

 wires, followed shortly after by submarine cables, began to 

 be employed ; and when on these new lines it w r as no longer 

 possible to determine the position of a fault by inspection, an 

 intimate knowledge of the laws of electricity, combined with 

 an accurate standard of resistance, became of great practical 

 importance to the telegraph-engineer. The unit of length 

 in the laboratory, "the foot," was replaced in construction by 

 " the mile ;" thus the unit of resistance in England became that 

 of a mile of no. 16 copper wire, and in France that of a kilometre 

 of iron wire 4 millimetres in diameter. Several other units were 

 from time to time proposed, of wdiich two, that of Weber and 

 that of Thomson, differed altogether from the others in their 

 fundamental conception. Excluding the two last, all the units 

 of resistance proposed were based on the obstruction offered 

 to an electric current by a given length of a given material, 



* Communicated by the Authors, having been read before the Society 

 of Telegraph Engineers, February 26. 



t " Report to the Royal Society on the new Unit of Electrical Resist- 

 ance, &c," by Prof. E. Jenkin. 



