AND STRAIN ON THE ACTION OF PHYSICAL FORCES. 
161 
coincident in direction with a line drawn from end to end of the strip and bisecting 
the two ends. On passing a current through the strip the resistance of the wire was 
altered in such a manner as would make it appear that circular magnetization decreased 
the resistance. I cannot, however, place reliance on these experiments, which at the 
time seemed conclusive, as I have in my recent attempts not been able to verify their 
results when sufficient precaution was taken to avoid change of resistance from change 
of temperature. The following are two experiments made with the above-mentioned 
object:— 
Experiment LXX V. 
An annealed iron wire, 4 feet in length and '085 centim. in diameter, was firmly 
bound by tape for a distance of 2 feet with a caoutchouc-covered copper wire -j^-th 
of an inch in thickness. The two parts of the wire were arranged as usual to form 
two sides of a “Wheatstone’s bridge”; and whilst the B.C. was varied in diffe¬ 
rent trials to very different extents and the current through the copper wire was 
increased from almost 0° to 50° of the tangent galvanometer, no trace whatever could 
in any case be detected of alteration of the resistance of the wire by passing a current 
through the copper wire, though the arrangement was sufficiently delicate at times to 
show an alteration of one in one million. At one time, indeed, it was suspected that 
there was an alteration, but this was afterwards traced to a slight direct action of the 
M.C. on the galvanometer. 
Experiment LXX VI. 
A strip of annealed iron foil, 8 inches in length 2 inches broad and 317 th of an 
inch thick, was placed upon a copper strip 12 inches long 3 inches broad and 117 th 
of an inch thick, the two being separated from each other by two folds of a silk 
handkerchief. Another strip of iron of similar dimensions served as a comparison 
piece, and the two iron strips were arranged as usual in the “ bridge.” Though the 
currents through the strips of both iron and copper were altered to the same extent 
as with the iron wire, there was still no trace of alteration of resistance caused by the 
current in the copper strip. It is hardly necessary to say that such arrangements as 
these are not favourable for bringing out the effect sought, inasmuch as the distances 
of the copper wire and copper strip conveying the current were both greater than the 
thickness of the iron itself;* but, on the other hand, if we make the iron thicker we 
diminish the sensitiveness of the arrangement, and we certainly cannot well diminish 
the distance of the copper from the iron without laying ourselves open to error from 
changes of resistance caused by heating. 
Several attempts were now made to ascertain whether variation of the B.C. itself 
* With the wire twice as gTeat and with the strip nearly equal. 
MDCCCLXXXIII. 
Y 
