AND STRAIN ON THE ACTION OF PHYSICAL FORCES. 61 
formula cr= . — 1 of the hard drawn nickel is of course impossible, as was the case, it 
will be remembered, with the value of a with hard-drawn copper, brass, German-silver, 
and platinum-silver, and the wire therefore furnishes another proof that the formula 
cannot be applied to hard drawn metal. 
The most remarkable feature, however, presented by nickel is that shown in 
Table V., where we learn that the resistance is absolutely decreased up to a certain 
extent of loading and then begins to increase. We see, moreover, that the maxi¬ 
mum decrease becomes less and less as the wire receives more and more permanent 
extension, and that the point of loading vdiere this maximum occurs gradually 
rises with the amount of permanent extension.* As might be supposed, therefore, 
the decreases of resistance obtained with two loads of 3 and 6 kilogs. were with 
the hard drawn metal much less than with the annealed one. Again, if we take 
the average decrease of resistance produced by a load of 1 grm. per square cen¬ 
timetre between the points of zero load and the load producing maximum decrease, 
we find it in the case of the outer curve to be 3216 XlO -13 , and the product 
of this number by e=6'994, whilst the decrease of specific resistance attending unit 
increase of length, or the number corresponding to z in Table I., would amount to 
8'860. All these numbers, especially the last, are very considerably greater than the 
corresponding increases of -resistance obtained with any of the other metals. We 
thus observe that whether we regard the peculiarity of loading up to a certain point 
producing decrease of resistance, and after this point increase, or the comparatively 
enormous temporary variations of' resistance produced by loading, nickel stands by 
itself, and the idea at once suggested itself that this abnormal behaviour of the metal 
might be due to the influence of circular magnetization caused by the current employed 
in balancing the wire and the comparison-wire. Accordingly two experiments were 
made of the following nature:—First, as the alteration of resistance might be only 
apparent and due to the fact that Yillari’s shock-currents were not the same in the 
stretched and unstretched wires,t both the galvanometer circuit and the battery circuit 
were kept closed, and the position of the light on the scale noted with different 
stretching weights: the readings taken in this way gave alterations of resistance which 
were exactly the same as those obtained by the usual method. Secondly, as the altera¬ 
tions might be really those of resistance, but due to the fact that circular magnetization, 
nfight| cause an alteration of resistance in both iron and nickel, and unequally in 
the stretched and unstretched wires, the resistances in the bridge were so adjusted that 
currents of one-half and one-fourth respectively of the current which had previously 
* The numbers 16, 18, 20, 22, 24, on the curves represent the load which had previously been on the 
wire before testing for the temporary effect of loading. 
t I did not think this likely, as I had not been able to detect anything of the kind when iron wire 
was used. 
+ According to Auerbach, but not according to experiments tried by myself. 
