ON STANDARDS OF ELECTRICAL RESISTANCE. 357 



ings and those first taken, the observation was rejected and another one 

 taken. 



Four observations were made on each wire at intervals of from twenty to 

 forty minutes. 



Before noting dovm the scale-reading all the connectors were moved, and 

 if no change in resistance was observed the connexions were presumed to be 

 good. 



All results are given in terms of weight and length, as it is impossible to 

 measure the diameter of a small wire with the accuracy with which the 

 weight can be found ; moreover, the cross section of a wire is not generally a 

 circle, and the mean diameter varies shghtly from point to jjoint however 

 carefully it may be drawn. 



A great oversight was made in not observing the specific gravity of each 

 wire, so that the results of the experiments now made could be compared 

 with former ones. This omission was first made because it was thought 

 that the results of former experiments could be xised; but after several 

 measurements had been made it was found that the values of the specific 

 gravity of wires of the same metal, given by dificrent observers, varied so 

 much that it was impossible to find the resistance of a wire of a metal of 

 which the length and sectional area are kno-v\'n, from the resistance of a wire 

 of which the length and weight are known without taking the specific gravity 

 of the wire actually experimented on. 



Three silver wires were compared. 



T. From commercially pure nitrate of silver. 

 II. From French coin. 

 III. From English coin. 



The silver was first dissolved in nitric acid and thed diluted with water and 

 precipitated by hydrochloric acid. The chloride was then weU washed, and 

 afterwards fused with pure carbonate of sodium. The resulting button of 

 silver was fused a second time with borax and a little nitrate of potas- 

 sium ; lastly, before casting, it was fused with a piece of charcoal floating on 

 the top. The mould was about 35 miUimetrcs long by 4| millimetres dia- 

 meter. The drawing of the wire was conducted with the utmost care. The 

 wire was annealed only twice during the process. 



In di'awing aU wires the end first entering the hole was reversed at each 

 successive drawing, after it had been drawn down to about one-half its re- 

 quired diameter. The wires were twice drawn through each of the smallest 

 holes, the ends being reversed as before. 



To measure the harder wires they were straightened by rolling them 

 between two smooth boards, and then passed through a thermometer tube of 

 such a length that the ends just projected fi-om the tube, the long ones being- 

 cut into two or three lengths for the purpo^se. It was found that the wire 

 could be pulled out of the tube and reinserted many times without altering 

 the length by half one-tenth of a millimetre. Some care was necessary in 

 soldering the wires to their connexions. A small lump of hot solder Avas 

 placed in the terminal, and the end of the wire steadily and slowly pushed 

 into it until it set. Thus the boimdary between the wire and solder was weU 

 defined, and the wire could be cut off at exactly the required point. The 

 wires were weighed and measured after the resistance had been taken. 



The care taken in di-awing the silver wires accounts for the close agree- 



