28 
ME. J. REGINALD ASHWORTH; 
as two prominent peaks ; either of these processes increases the resistivity by about 
12 per cent, upon the initial state. Hard drawing after annealing between stages 
(2) and (3) brings the wire down to nearly the original condition, and again, after 
tempering, cold drawing through two or three holes decidedly reduces resistivity, but 
an unmistakable increase sets in at the 11th and 12th stages. From the last jDoint 
the curve has been extended by a broken line to include two more points belonging 
to still finer wires. Nos. 13a and 14a, which are not, however, of identically the same 
material as the others. Nevertheless, the broken line emphasizes the fact that the 
effect of extreme drawing is prejudicial to conductivity. These contrary effects 
of drawing, it will he remembered, were also in evidence in the curve of magnetic 
temperature coefficient, and in both curves the change occurs near to the 8th and 
9 th stages. A length of the steel wire upon which the experiments just described 
were carried out was subsequently made glass hard, and the resistivity in that 
state Avas 2760 X 10~®, or about 70 per cent, higher than the Avire in its state 
of least resistance. It is Avorth notice that minimum resistivity occurs in the 
hard drawn and hot rolled conditions, and hence the order of resistiAuty in an 
ascending scale is : hard draAvn or hot rolled ; annealed or tempered ; glass hard. 
Temperature Coefficient of Resistivity. 
3. A simple modification of the apparatus described in the la^st section alloAved 
the temperature coefficient of resistivity to be obtained. 
The same Avires were used as before. Each Avas fixed in a trough surrounded by a 
Avater-bath, Avhich could he raised in temperature by the apjAlication of gas jets from 
about 16° to 90° C. Readings AA^ere made at intervals during the process of heating 
and cooling, and the usual precautions Avere taken for the elimination of the effects 
of thermo-currents. 
Two independent sets of observations were taken for nearly all the specimens, and 
the mean results, Avhich ai-e given in the fourth column of Table IX., have been 
})lotted on the same diagram as the curve of resistivity, so that the tAvo curves may 
be conveniently compared. 
It will be noticed that the smallest value of the temperature coefficient, namely, 
0‘00294, occurs when the steel is annealed, and the highest Amlue, 0‘00466, AAdien hard 
draAvn, and these least and greatest values coincide respectively Avith the largest and 
smallest values of the resistivity. This confirms and extends a laAv Avhich Barus 
has sliown to he true for the iron carburets, accoixling to Avhich the temjjerature 
coefficient of resistivity is approximately inversely as the resistivity and in the 
* The relation given by Baiius is p {in + a) = Avliere p is the resistivity, a the temperature coefficient, 
and m and « are constants. ‘Bulletin 11.8. Geol. Survey,’ No. 14, 1885. 
