29 
EXPERIMENTAL RESEARCHES ON DRAW STEEL. 
curves here plotted a similar relation between temperature coefficient and resistivity 
in general holds good, the two curves moving oppositely to each other. The change, 
however, in the magnitude of the temperature coefficient of the repeatedly drawn 
steel is small compared to the change in resistivity. 
The temperature coefficient of this steel made glass hard is only 0'00177, about 
half the average value in the drawn state; this again is an example of Barls’ law, 
for the resistivity when glass hard is, as stated, 70 per cent, greater than when 
hard drawn. 
The ascending order of the temperature coefficient is thus : glass hard ; annealed; 
hard drawn ; the inverse of resistivity. 
Young’s Modulus. 
4. The elastic properties of steel are known to undergo a considerable change with 
drawing, and it seemed desirable to discover if longitudinal elasticity was correlated 
in any way with the electric and magnetic properties of these steel wires, and how it 
was modified by annealing, tempering, and traction. The determination of Young’s 
modulus was eftected by measuring the amount of flexure of the wires when loaded 
at the middle, and with the ends resting on rigid supports. The depression produced 
by loading was observed through a telescope supplied with a micrometer eye-})iece, 
which allowed an exceedingly small interval to be measured with accuracy. After a 
leading had been taken of the fiducial mark, with no load hanging from the rod 
except the pan itsell, weights were applied one by one, great care being exercised so 
that there sliould be a minimum of vibration ; the weights were then removed, one 
at a time, the depression corresponding to these weights at each stage being observed 
through the telescope. Two, and in some cases three, independent sets of experi¬ 
ments of this kind were performed on each wire, and nearly always with different 
distances between the supports, and with difterent increments and amounts of load, 
and the mean of the several experiments was taken as the final result. 
In the formula for the calculation of the modulus the fourth power ol the radius 
appears as one of the factors, and hence an accurate value of the radius is required 
if errors are to be avoided. As already mentioned,^' a considerable number of careful 
measurements of the diameters of the wires were made, and these were confirmed from 
determinations of the density.! The values there used have been adopted here. 
The fifth column of Table IX. gives Young’s modulus for the twelve specimens ot 
steel upon which the magnetic and electric experiments already described had been 
cairied out. Diagram VIII. exhibits this column of figures as a curve with sectional 
* Vide § 2. 
t As the deviation of any single deteimination of the density from its mean never exceeded ^ per cent, 
in any one specimen, the maximum error on this accouzit in the fourth power of the radius will not be 
more than double of this. 
