May 14, 1885] 
NATURE 
of the Hypodermei and the Gasteromycetes.—Plante Raddeanz 
Monopetalz, by Ferd. von Herder (continued).—Solution of a 
problem of the theory of comets, by N. Joukovski (Russian). 
The geocentric position of a particle of the tail which has left 
the nucleus since a given time under the action of a given re- 
pulsive force, to determine the displacement of the particle for a 
given change in the repulsive force—such is the problem treated. 
—Analyses of salt and mud from a volcano of Trans-Caucasia. 
—An essay on the solution of the geodetical problem, by Th. 
Sloudsky (in French). The already-known formule already give 
the possibility of embodying all anomalies less than 30” in lati- 
tude and less than 15 oscillations of the pendulum in twenty- 
fours against the calculated ones. The author tries, however, to 
give a more theoretical formula, which might at the same time 
embody larger anomalies.—List of the herbaria of the Mos- 
cow University and of the Society of Naturalists, by J. Goro- 
shankin.— Studies on the averages of the relative moistness, by 
Dr. K. Weihrauch (continued ; in German).—Necrology and 
Annual Report. 
Rendiconti del R. Istituto Lombardo, March 26.—History of 
the first century (1783-1883) of the Reale Istituto, by G. B. 
Venturi.—On the persistence of the thymus gland in children 
and adults, by Prof. Giovanni Zoja.—Account of a successful 
operation performed on a young girl for the purpose of closing 
an open sore on the left cheek produced by a severe attack of 
typhoid fever.—Further notes on conformable representations in 
higher mathematical analysis, by Prof. Giulio Ascoli.—Meteoro- 
logical observations made at the Royal Observatory of Brera, 
Milan, during the month of March. 
Rivista Scientifico-Industriale, March 31.—A new explanation 
of the red crepuscular lights that have been attributed to the 
Krakatoa eruption, by Prof. Carlo Marangoni.—Variations in 
the electric resistance of solid and pure metallic wires according 
to the temperature (continued), by Prof. Angelo Emo.—A 
visitation of caterpillars (Zithosta caniola, HI.) in Florence during 
the present season, by P. Bargagli. 
SOCIETIES AND ACADEMIES 
LONDON 
Royal Society, April 23.—‘‘On the Changes produced by 
Magnetisation in the Length of Rods of Iron, Steel, and 
Nickel.” By Shelford Bidwell, M.A., LL.B. 
The earliest systematic experiments on the effects produced 
by magnetisation upon the length of iron and steel bars are 
those of Joule, an account of which is published in the Phzi. 
Mag. of 1847. Joule’s experiments haye many times been re- 
peated, and his general results confirmed. In particular, Prof. 
A. M. Mayer carried out a series of very careful observations 
with apparatus of elaborate construction and great delicacy. 
The conclusions at which he arrived were in accord with those 
of Joule, so far as regards iron; in the case of steel there was 
some apparent discrepancy, which, however, might to a great 
extent be accounted for by differences in the quality of the metal 
used and in the manner of conducting the experiments. In 
1882 Prof. Barrett published in NATURE an account of some 
experiments which he had made, not only on iron but also on 
bars of nickel and cobalt, with the view of ascertaining the 
effect of magnetisation upon their length. 
The knowledge on the subject up to the present time may be 
summarised as follows :— 
(1) Magnetisation causes in iron bars an elongation, the 
amount of which varies up to a certain point as the square of 
the magnetising force. When the saturation-point is approached 
the elongation is less than this law would require. The effect is 
greater in proportion to the softness of the metal. 
(2) When a rod or wire of iron is stretched by a weight, the 
elongating effect of magnetisation is diminished ; and if the 
ratio of the weight to the section of the wire exceeds a certain 
limit, magnetisation causes retraction instead of elongation. 
(3) Soft steel behaves like iron, but the elongation for a given 
magnetising force is smaller (Joule). Hard steel is slightly 
elongated, both when the magnetising current is made and when 
it is interrupted, provided that the strength of the successive 
currents is gradually increased (Joule). The first application of 
the magnetising force causes elongation of a steel bar if it is 
tempered blue, and retraction if it is tempered yellow: subse- 
quent applications of the same external magnetising force cause 
45 
temporary retraction, whether the temper of the steel is blue or 
yellow (Mayer). 
(4) The lengh of a nickel bar is diminished by magnetisation, 
the maximum retraction being twice as great as the maximum 
elongation of iron (Barrett). 
In order that the results of Joule ard Mayer might be com- 
parable with those obtained by the author, he made an attempt 
to estimate the magnetising forces with which they worked. 
From data contained in their paper, it was calculated that the 
strongest magnetising force used by Joule was about 126 units, 
while the stiongest used by Mayer did not on the highest 
probable estimate exceed 118 units. In the author’s experiments 
the magnetising force was carried up to about 312 units. The 
metal rods, too, were much smaller than any which had been 
before used for the purpose, ranging in diameter from 1°40 to 
625 mm. Their length was in every case 100 mm., and the 
apparatus was capable of measuring with tolerable certainty an 
elongation or retraction equal to a ten-millionth part of this 
length. 
By using thinner iron rods and greater magnetising forces 
than those previously employed, the following curious and inter- 
esting fact was established. If the magnetisation be carried 
beyond a certain critical point, the consequent elongation, instead 
of remaining stationary at a maximum, becomes diminished, the 
diminution increasing with the magnetising force. If the force 
is sufficiently increased, a point is arrived at where the original 
length of the rod is totally unaffected by magnetisation ; and if 
the magnetisation be carried still further, the original length of 
the rod will be reduced. It also appeared that the position of 
the critical point in steel depended in a very remarkable manner 
upon the hardness or temper of the metal ; considerable light is 
thus thrown on the apparently anomalous results obtained by 
Joule and by Mayer. Further experiments disclosed strong 
reason for believing that the value of the critical magnetising 
force in a thin iron rod was greatly reduced by stretching ; this 
would explain the fact that Joule obtained opposite effects with 
stretched and unstretched wires. 
By ascertaining the relative values of the temporary moments 
induced by gradually increasing external magnetising forces, an 
attempt was made to connect the point of maximum elongation 
with a definite phase of the magnetisation of the several rods in 
which the elongation had been observed. 
Though more experiments must be made before it is possible 
to generalise from them with perfect safety, the results so far 
obtained by the author indicate the laws given below. The 
elongations and magnetisations referred to are temporary only ; 
before the beginning of an experiment the rod was permanently 
magnetised by passing through the magnetising coil a current 
equal to the strongest subsequently used. In iron the greatest 
elongation due to permanent magnetisation was generally found 
to be about one-third of the total elongation, while in nickel the 
permanent retraction amounted only to about one-twenty-fifth 
part of the whole. 
I. IRon 
(1) The length of an iron rod is increased by magnetisation 
up to a certain critical value of the magnetising force, when a 
maximum elongation is reached. 
(2) If the critical value of the magnetising force is exceeded, 
the elongation is diminished until with a sufficiently powerful 
magnetising force the original length of the rod is unaffected, 
and, if the force is still further increased, the rod undergoes 
retraction. Shortly after the critical point is passed, the elonga- 
tion diminishes in proportion as the magnetising force incre: ses. 
The greatest actual retraction hitherto observed was equal to 
about half the maximum elongation, but there was no in ‘cation 
of a limit, and a stronger magnetising force would have p oduced 
further retraction. 
(3) The value of the external magnetising force corresponding 
to maximum elongation is for a given rod approximately eq tal 
to twice its value at the ‘‘ turning point.” 
Definition.—The turning point in the magnetisation of an iron 
bar is reached when the temporary moment begins to increase 
less rapidly than the external magnetising force. 
(4) The external force corresponding to the point of maximum 
elongation increases (when the quality of the iron is the same) 
with the diameter of the rod. So also does its value at the 
turning point. 
(5) The amount of the maximum elongation appears to vary 
inversely as the square root of the diameter of the rod, when the 
quality of the iron is the same. 
