316 



Intelligence and Miscellaneous Articles. 



the melting-point, following the ordinates of a curve the abscissae 

 of which represent the corresponding temperatures, and which 

 generally differs Aery little from a right line. Comparing the re- 

 sistances to the resistance at zero, they can be expressed by a for- 

 mula of the form 



'R t =R (l + at + bt 2 ). 



The constants a and b were calculated by the method of least 

 squares, which makes all the observations cooperate for the deter- 

 mination of the most probable values of the unknown quantities. 



The increment varies from one metal to another. In steel and 

 in iron the initial resistance is doubled at about 170°; in silver, 

 copper, and gold, at about 255° ; in platinum, at about 455°. In 

 alloys the increment is in general less : in Grerman silver, for ex- 

 ample, at 860° the resistance has increased by only 03 of its value 

 at zero. The numbers in this Table express the variation of the 

 specific resistance — i. e. of the resistance reduced in each case to the 

 unit of length and section. If we wish to use them to calculate 

 the resistance at t° of a given wire the resistance of which at zero 

 is known, account must be taken of the changes of dimension of 

 the wire ; in other words, the resistance obtained must be multi- 

 plied by -, & being the coefficient of dilatation, [this correction 



cannot be neglected when the temperature exceeds certain limits. 



Specific Resistances of Metals at Zero. 



Resistance of 1 metre 



length and 1 square 

 millimetre section. Conductivities 



-^ ,, compared with 



Siemens unit. silver. 



0'0161 100 



0-0179 90 



0-0201 80 



0-0227 71 



0-0324 49-7 



0-0443 36-4 



0-0591 27-5 



0-0621 25-9 



0-0716 22-5 



0-0723 22-3 



0-1149 14-0 



0-1214 13-3 



0-1243 13-0 



0-1272 12-7 



0-1447 11-1 



0-1647 9-77 



0-1914 8-41 



0-2075 7-76 



0-2755 ' 5-80 



1-0000 1-61 



ohm. 



Pure silver, annealed 0-0154 



Copper, annealed 0-0171 



Silver ( T VW)> annealed 0-0193 



Pure gold, annealed 0-0217 



Aluminium, annealed 0*0309 



Magnesium, cold-beaten .... 0-0423 

 Pure zinc, annealed at 350° 0*0565 



Pure zinc, cold-beaten 0-0594 



Pure cadmium, cold-beaten . 0-0685 



Brass, annealed 0-0691 



Steel, tempered 0-1099 



Pure tin 0-1161 



Aluminium bronze 0*1189 



Iron, tempered 0-1216 



Palladium, tempered 0*1384 



Platinum, tempered 0*1575 



Thallium 0*1831 



Pure lead 0*1985 



Grerman silver 0*2654 



Pure mercury 0*9564 



