456 Scientific Intelligence. 
cipal difficulty arises from the fact that the current generates 
heat, so that it is extremely difficult to keep the temperature con- 
stant with different currents. Since the resistance is the same 
whichever way the current passes, the resistance, if not constant, 
must depend upon even powers of the intensity of the current 
through each element of the conductor. Hence if we cause a cur- 
conductor of smaller section, and if the resistances are equal for 
mall currents they will be no longer equal for large currents, 
The first test was by means of five coils each of thirty ohms resist- 
nce, and two others to complete the bridge. A difference of over 
ieensailiontte in the relative a of any two of these coils 
proved to be measurable. According to Ohm’s law, if either four 
of these were connected two an oe rie the resistance should be 
the other four. The results showed a small pes from the 
law, probably due to irregularity in the conducting power of the 
connections since it was not confirmed in the more faerie tests 
ngtabetoes appliec 
A second method was next adopted in which weak and strong 
path were alternately passed through two wires of nearly 
equal resistance but one short and fine the other coarse and long. 
The currents were changed thirty, and sometimes sixty times a 
pieced: so that the wire could not sen ~~ change in temperature 
in the interval. Since the current has ar greater intensity in the 
fine than in So coarse wire the ‘leviation sho — e — 
acai must arise from a succession af equal and so 
currents those in one direction due to the stout wire, those in the 
ad 
result. Mr. Chieystal: by whom the experiments were perior” 
as put his result in the following form: If a conductor of i 
platinum or German silver of one square centimeter section has 
a resistance of one ohm for infinitely small currents, its pire 
when acted on by an electromotive force of one volt (provided 
mperature is kept the same) is not altered by so mue 
millionth of the millionth part. d to 
It is seldom, if ever, that so searching a test has been applied 
a law which was originally om TRE by experimeut, an me 
= still be considered a purely empirical law, as it has not ee 
rto been deduced from the Scikceeentat irincapiee of dynamics. 
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