olo 



VII. " On the Measurement of Electric Resistance." By Pro- 

 fessor W. THOMSON, F.R.S. Received June 6, 1861. 



Part I. New Electrodynamic Balance for resistances of short 

 bars or wires. 



In measuring the resistances of short lengths of wire by Wheat- 

 stone's Balance *, I have often experienced considerable difficulty in 

 consequence of the resistances presented by the contacts between the 

 ends of the several connected branches or arcs. This difficulty may 

 generally be overcome by soldering or amalgamating the contacts, 

 when allowable ; but even with soldered connexions there is some 

 uncertainty relating to the dimensions of the solder itself, when the 

 wires tested are very short. When soldering was not admissible, I 

 have avoided being led into error, by repeating the experiment several 

 times with slightly varied connexions; but I have in consequence 

 sometimes altogether failed to obtain results by either Wheatstone's 

 or any other method hitherto practised, as for instance in attempting 

 to measure the electric resistances of a number of metallic bars each 

 C millimetres long and 1 millim. square section, which were put into 

 my hands by Mr. Calvert of Manchester, being those of which he 

 and Mr. Johnstone determined the relative thermal conductivities in 

 their investigation published in the Transactions of the Royal Society 

 for March 1858. I have thus been compelled to plan a new method 

 for measuring electric resistances in which no sensible error can 

 be produced by uncertainty of the connexions, even though made 

 with no extraordinary care. 



Let AB and CD be the standard and the tested conductors respect- 

 ively. Let the actual standard of resistance be the resistance of the 

 portion of AB between marks f S, S' on it, and let it be required to 

 find a portion TT' of CD which has a resistance either equal, or 

 bearing a stated ratio, to that standard. 



Join BC either by direct metallic contact between them, or by 



* I have given this name to the beautiful arrangement first invented by Pro- 

 fessor Wheatstone, and called by himself a " differential resistance measurer." 

 It is frequently called " Wheatstone's Bridge," especially by German writers. 

 It is sometimes also, but most falsely, called " Wheatstone's Parallelogram." 



f On the same principle as the " metre k traits " instead of the " metre a 

 bouts " for a standard of length. 



VOL. XI. 2 A 



