319 



it is impossible to secure that the different parts of each testing-con- 

 ductor shall be accurately at the same temperature ; (2) because the 

 resistances at the ends of the fine stretched wire or wires are always 

 sensible in comparison with the smallest measured differences pro- 

 duced by the slide ; (3) because the stretched wire itself is never of 

 absolutely equal gauge throughout, and, even if sensibly so when first 

 put into the instrument, soon ceases to be so in consequence of the 

 friction of the sliding contact which it experiences in use* ; (4) be- 

 cause, in even the hastiest experiments, provided a rationally 

 planned galvanometer is used, a far higher proportional degree of 

 accuracy is easily attained in measuring electrical resistances against 

 a standard of resistance than can be at all attained, without very 

 extraordinary precautions and the assistance of a microscope, in 

 measuring lengths under a yard or two against a standard of 

 length. 



"When the highest accuracy is required, I always use for primary 

 testing-conductor the bisected conductor which I described to the 

 British Association at its Glasgow meeting in 1855. This consists 

 of a fine, very perfectly insulated wire, doubled on itself and wound 

 on a bobbin, with very stout terminals soldered to its ends, and an 

 electrode soldered to its middle, for joining to the galvanometer elec- 

 trode. The two terminal and the middle electrodes thus attached 

 to the testing-conductor, I have generally hitherto made flexible, 

 either of thick wire, or strand of wires like the conductor of a sub- 

 marine cable ; but, for many applications, it is more convenient 

 to make them solid metal blocks, with binding screws, insulated 

 rigidly upon the bobbin which bears the conductor. The two 

 halves into which the conductor is doubled must be very accurately 

 equalized as to electric resistance when they are wound on the bobbin, 

 and before the terminals are finally attached. This I find can be 

 done with great accuracy ; and when, after the terminals are soldered 

 on, the electric bisection is once found perfect, it seems to remain so, 

 without sensible change, for years. The close juxtaposition of the 

 two branches of the testing-conductor on this plan ensures an almost 

 absolute equality of temperature between them in all circumstances, 



* This . defect I have remedied by frequently putting in a new wire for 

 testing-conductor in working with a sliding-scale Whealstone's balance. 



