Fune 18, 1885 | 
NATURE 
167 
Tn the following table is shown the ratio of the resistances of 
a specimen of platinoid wire at different temperatures to its 
resistance at zero, The wire used was the same as that specified as 
No. 20 in the table of resistances. The length of the wire ex- 
perimented on was about four-fifths of a metre. The only 
trouble in the experiment was the keeping the oil-bath, which 
was filled with linseed oil, thoroughly stirred, and of uniform 
temperature throughout. 
Resistance. The Res. at 0° C. 
Temperature. being = 1. 
fe) CAC 6 10 
Io 10024 
20 ap 10044 
30 1'0075 
40 10066 
50 10097 
60 10126 
70 a be mt PLNOLS4: 
80 coe a oon een) PXTOTOO 
90 wise ies Ree ILOLSS: 
100 oe ses op ss. 1'0209 
This gives for the average percentage variation of resistance 
per 1° C., between the temperatures 0° C. and roo” C., the 
number 002087. A second wire tested very carefully ina similar 
way gave for this average percentage variation between o° and 
100°, 0'022 per degree, with a steadily increasing rate of varia- 
tion from the beginning. 
To compare this increase in resistance due to increase of 
temperature with that observed in other metals and alloys, we 
find that the percentage increase of resistance for 1° C. at 20° C. 
for copper is 0°388, platinum-silver alloy 0’031, gold silver alloy 
0'065, and for German silver 0'°044. These numbers were ob- 
tained by Matthiessen in the course of his experiments for 
finding a suitable metal or alloy for the purpose of constructing 
the British Association standards of electric resistance. It appears 
that the variation of resistance of platinoid with temperature is 
very much smaller than the smallest observed for any of the 
metals and alloys then examined. 
The modulus of rigidity, the Youag’s modulus (or modulus 
for elastic longitudinal extension), and the breaking weight for 
platinoid wire were also determined. The wire used was a 
portion of that marked 4 in the foregoing table. This wire is a 
little larger than No. 24 of the Board of Trade standard wire 
gauge, and has a diameter of 0°0595 cm. 
The rigidity modulus was found to be 4751°8 x 10° grammes 
weight per square centimetre. The Young’s modulus is 
1222'4 x 10° grammes weight per square centimetre. 
The breaking weight is about 6'029 x 10° grammes weight 
per square centimetre. 
The specific gravity of platinoid wire has also been found by 
the author to be 8°78 compared with water at 20° C.  Platinoid 
when drawn hard is softened, like copper, by heating and sudden 
cooling. 
Physical Society, May 23.—Prof. Guthrie, President, in 
the chair.—Dr. A. H. Fison was elected a Member of the 
Society.—The following communications were read :—Experi- 
ments showing the variations caused by magnetisation in the 
length of iron, steel, and nickel rods, by Mr. Shelford Bidwell. 
The subject of the extension and retraction of bars of iron and 
nickel under the action of magnetic force has been investi- 
gated by Drs. Joule and A. M. Mayer, and by Mr. Barrett. In 
the present experiments the magnetising force has been increased, 
with the result of bringing out some striking and novel charac- 
teristics. The apparatus employed consisted of a vertical mag- 
netising helix considerably longer than the experimental rod, the 
latter forming the central portion of a compound rod, the two 
ends being of brass. The lower end of this rod is plane, and 
stands on a firm support ; the upper end is a knife-edge, which 
bears against a brass lever 18 cm. in length, about 1 cm. from 
the fulcrum ; the portion of the rod to be examined is in the 
central portion of the helix. The above lever is furnished with 
another knife-edge at the end, which acts in a similar manner on 
a second lever, at the extremity of which is a small mirror, A 
lamp and vertical scale being placed at a distance of 470 cm., 
the slightest motion of the mirror could be read with great accu- 
racy, an elongation of the bar, amounting to I-100,oooth mm., 
being easily detected. A few of the more important results are 
as follow :—In the case of soft iron the bar continually increased 
in length till nearly saturated, up to which point Mr. Joule had 
traced it, but then it reached a maximum, decreased, and con. 
tinued decreasing to the limit of the experiments, at which point 
the retraction was about double of what the extension had been. 
The effect depended upon the thickness of the bar, an increase 
of diameter diminishing the maximum extension, and increas- 
ing the critical magnetising force, or that force which pro- 
duced the maximum extension; the results seemed to show 
that this extension varied inversely as the square root of the 
diameter of the bar. The general behaviour of steel was the 
same as that of soft iron, but the critical point varied with 
the hardness and temper of the metal, appearing to be a 
minimum for steel of yellow temper. The results of experiments 
upon nickel coincided with those obtained by Prof. Barrett, the 
effect of magnetisation being to cause a continuous retraction 
greater than that obtained with soft iron. In answer to Prof, 
Hughes, who believed that the effect of the coil was always to 
produce 7e¢raction of the bar, the exfenston at first being due to 
the molecular arrangement ef the particles during magnetisation, 
Mr. Bidwell further described an experiment showing that the 
action of the coil was to produce the exvenston of a magnet. Two 
thin strips of soft iron fastened together at the ends, their central 
portions being about 2cm. apart, were placed in the coil. On 
making the current the ends were drawn out, the sides coming 
together. Prof. Forbes suggested that the effect of thickness 
was really owing to the irregularity of magnetisation produced 
by the ends, and that in future experiments the middle of the 
bar only should be examined.—On the spectral image produced 
by a slowly rotating vacuum-tube, by Mr. Shelford Bidwell.— 
Note on the action of light in diminishing the resistance of 
selenium, by Mr. Shelford Bidwell. As the result of the in- 
vestigation upon the behaviour of selenium, Messrs. Adams and 
Day arrived at the conclusion that it conducted electrolytically. 
Since this would necessitate the assumption that selenium is not 
an element according to accepted theories, caution must be exer- 
cised in accepting this. It seemed possible, however, that since 
theselenium in the cells had always undergone a prolonged cvuoking 
in contact with the metal terminals, selenides of these metals 
might exist in the selenium, forming a kind of network, and thus 
affording conduction through the mass, which, without the 
cooking, is non-conducting. It had not been possible to test 
this directly, but a somewhat analogous case had been tried. 
Some precipitated silver had been heated for some hours with 
sulphur, and the clear liquid poured off. A cell was then made 
by coiling two silver wires side by side upon a strip of mica, the 
spaces between the wires being filled with the prepared sulphur, 
which would contain a small quantity of sulphide of silver. It 
was found necessary to reduce the resistance of the cell by 
placing a small strip of silver leaf over the sulphur and cooking 
again. ‘The cell thus prepared was very sensitive to light: by 
burning a piece of magnesium near, the resistance was reduced 
to one-third. Mr. Clark said that Mr. Bidwell’s cells probably 
contained sulphides of copper or silver, substances which the 
researches of Faraday had shown conducted electrolytically 
in the solid condition. On the other hand, Cu,Se and Ag,Se 
conducted like metals and were probably often present in 
the ordinary selenium light cells. Mr. Clark thought that Mr. 
Bidwell’s paper raised this question: What influence had light 
upon the electrolytic conduction of Cu,S and Ag,S and upon the 
metallic conduction of Cu,Se and Ag,Se?—On certain cases of 
electrolytic decomposition, by Mr. J. W. Clark.—The first part 
of this paper consisted of a critical examination of the behaviour 
of those substances which have been described as exceptions to 
Faraday’s laws, with the object of generalising as to the condi- 
tion of internal or molecular structure corresponding to their 
electrical properties. The second part described an experimental 
investigation into the nature of the conduction of fused mercuric 
iodide and mercuric chloride, both of which were stated to 
undergo electrolytic conduction. Decomposition and re-com- 
bination of the products of electrolytic action may, however, 
follow s» closely as to simulate metallic conduction. The first 
product of electrolytic decomposition of mercuric iodide was 
stated to be iodine and mercuroso-mercuric-iodide (Hg,I,), 
which latter, under the continued action of the current, yields 
free mercury. Similarly it was found that fused mercuric 
chloride, when electrolysed between graphite terminals, split up 
into chlorine and mercurous chloride. Metallic conduction, ze. 
conduction without decomposition, in fused compound solids, 
therefore appears to be unknown.—Note on electrical symbols, 
by Mr. J. Munro. 
Mathematical Society, June 11.—J. W. L. Glaisher, 
F.R.S., President, in the chair.—Prof, J. Larmor was admitted 
