334 
carried on in them, The three buildings endowed and equipped 
by Sir William C. Macdonald for engineering, chemistry and 
mining, and physics, afford excellent facilities for study and 
research. There are special laboratories and workshops in 
which machinery of full size has been erected, so that all in- 
vestigations can be carried on in all respects under working 
conditions. 
THE tenth report of the Technical Instruction Committee of 
the County Borough of Plymouth has been received. The 
concluding words of the report show that the committee realises 
that fundamental principles rather than technical details should 
be the object of the work in such municipal science, art and 
technical schools as that at Plymouth. The committee re- 
marks :—‘‘ It must not be assumed that the work of the schools 
is intended to embrace what are commonly called technical 
subjects only. Their object is to give such higher education 
and training, combined with manual and technical skill, as may 
enable their students to perform their work in life with greater 
intelligence, ability and success.” 
Since Prof. Perry brought forward the subject of ‘* The 
Teaching of Mathematics” at the meeting of the British Asso- 
ciation last September, several associations of teachers have 
discussed the reforms suggested or appointed committees to 
report upon the matter. A committee of the Assistant Masters’ 
Association has had the subject under consideration, and a pre- 
liminary report has been drawn up, from which it appears that 
masters in secondary schools are in favour of most of the reforms 
advocated by speakers at the British Association meeting. The 
report is as follows :—I. Arithmetic. (1) The method of teaching 
in the early stages should be inductive and concrete. Actual 
measuring and weighing should be introduced as early as pos- 
sible. (2) Decimals should be treated as an extension of the 
ordinary notation, their nature being illustrated by actual metric 
weights and measures. Multiplication and division of a decimal 
by a decimal would, we think, have to follow vulgar fractions. 
(3) The decimalisation of English money and English weights 
and measures should be practised frequently. (4) Approximate 
methods should be gradually introduced after the treatment of 
finite decimals. They should be taught with due regard to 
rigidity of proof. Appreciation of the degree of approximation 
should be continually insisted upon. (5) If ‘* commercial 
arithmetic” is to be taught at all, the subject-matter should 
receive more adequate and correct treatment, and the examples 
should be drawn from transactions as they actually occur.— 
Il. Algebra. (1) The foundation of algebra should be ‘literal 
arithmetic,” z.e. algebra should at first be arithmetic generalised. 
(2) The minus sign should receive its extended meaning from 
copious illustrations ; and illustrations, not rigid proof, should 
also be resorted to for the purpose of the ‘rule of signs.” (3) 
Algebra should often be applied to geometry. (4) Logarithms 
should form an important section of the subject. We believe 
that the graphic method could be very usefully employed in this 
connection. (5) We desire to deprecate the waste of time so 
commonly practised in mere manipulation of symbols.— 
Ill. Geometry. (1) Weare strongly of opinion that the ordinary 
deductive geometry should be preceded and continually supple- 
mented by concrete and inductive work. (2) Whilst ‘‘ mensur- 
ation” might possibly be taught in connection with physics and 
arithmetic, we believe that the value of geometry would be 
enhanced by practical applications of the propositions as they 
occur. (3) We feel very strongly that Euclid’s text is very 
unsuitable for teaching geometry. But we are impressed with 
the difficulty of abolishing its use in the face of external examin- 
ations. In the circumstances, we can only hope that examining 
bodies, even if they insist on Euclid’s sequence, will allow 
greater latitude in methods of proof, and give greater prominence 
to easy ‘riders ” and applications.of geometry. 
SOCIETIES AND ACADEMIES. 
Lonpon. 
Royal Society, June 12.—‘‘ The Dissipation of Energy by 
Electric Currents induced in an Iron Cylinder when rotated in 
a Magnetic Field.” By Ernest Wilson, Professor of Electrical 
Engineering, King’s College, London. 
The effect which induced currents have upon the distribution 
of magnetism in an iron cylinder, when rotated about its longi- 
tudinal axis with uniform angular velocity in a magnetic field, 
has already formed the subject of a communication (Wilson, 
NO. 1709, VOL, 66] 
NATURE 
[JuLy 31, 1902 
Roy. Soc. Proc., vol. Ixix. p. 435, also NATURE, vol. Ixv. p. 502). 
The present paper deals with the energy dissipated by these elec- 
tric currents, and a comparison is made between the results of 
experiment and theory. In connection with the theory of the 
subject a contribution by Mr. J. B. Dale is made use of. The 
cylinder experimented upon has diameter and length each 10 
inches (25°4 cm,), and is rotated between the poles of a magnet 
weighing some tons. It is supplied with exploring coils, 
threaded through holes drilled in a plane containing its longi- 
tudinal axis, by the aid of which the electromotive forces due to 
rotation in a magnetic field have been observed. The results of 
experiment have been obtained graphically by a process of 
double integration. The distribution which has to be assumed 
in connection with the experiments is that the induced currents 
distribute themselves on the surfaces of cylinders similar to and 
concentric with the cylinder experimented upon. Two other 
distributions are also discussed, namely, the distribution 
assumed by Baily (P/z/. Trans. Roy. Soc., A, vol. clxxxvii., 
1896, pp. 715-746), that in any section the electric currents 
flow in rectangular paths similar to the boundary of the section, 
and the distribution in which the current density in any path is 
constant throughout the path. 
Dealing with the distribution assumed in connection with the 
experiments, both graphical treatment and theory agree in 
giving the formula 3°95B*/7/*/10'"p for the watts dissipated per 
cubic centimetre, where B is the intensity of magnetic induction 
assumed constant, / is the frequency, / is the length of the 
cylinder assumed equal to its diameter, and p is its specific 
resistance. In the experiments the frequency was varied from 
1/45 to 1/360, and for each the average intensity of induction 
was varied from 1000 to 20,000. In each case the watts per 
cubic centimetre are less than would be dictated by the above 
formula, The ratio of the results is 1°3 at frequency 1/360, and 
is substantially constant for all values of the induction density. 
At frequency 1/45 this ratio varies from 1°4 to 1°7 for high and 
low values of the induction density, but it is 3°1 for an inter- 
mediate value. A similar, though less marked, effect is 
observed at frequency 1/90. The explanation given is that with 
these intermediate forces at these frequencies very great crowding 
of the induction to the surface occurs ; and, moreover, since the 
wave-form of the electromotive force near the surface of the 
cylinder in all the experiments is more rectangular, the dissipa- 
tion of energy per cubic centimetre is less than the formula 
above would give, since ¢ieve the wave-form is assumed to be a 
sine-curve. On the assumption that the electromotive force at 
the surface is truly rectangular, the formula obtained by graphical 
treatment is 2°08B*/*:*/101%p. 
Having reconciled the results of experiment with those of 
theory, the author compares the dissipation of energy in 
rotating and alternating magnetic fields. It is pointed out that 
in the case of circular plates in which the diameter is very great 
as compared with the thickness, and in which the lines of force 
are uniformly distributed in the plane of the plate, the rotating 
field would dissipate about 1°7 times as much energy as an alter- 
nating magnetic field in the same time. The results are, how- 
ever, greatly influenced by variation in wave-form, and even 
when thelines of force are confined to the plane of the plate, a con- 
dition not always met with in practice, the rate of dissipation of 
energy for a given average induction density may be consider- 
ably reduced if the distribution of magnetic induction is such as 
to give a more rectangular wave-form to the induced electro- 
motive force. 
‘© Note on a Magnetic Detector of Electric Waves, which 
can be Employed as a Receiver for Space Telegraphy.” By 
G. Marconi, M.I.E.E. Communicated by Dr. J. A. Fleming, 
F.R.S. 
The detector is based, in the author’s opinion, on the decrease 
of magnetic hysteresis which takes place in iron when, under 
certain conditions, it is exposed to the effects of Hertzian 
waves. Ona core of thin iron wires is wound a coil consisting 
of one or two layers of insulated copper wire, and over this and 
separated from it by insulating material is wound a second 
longer coil. The ends of the inner coil are connected to earth 
and the aérial conductor, and the ends of the-outer coil to a 
telephone. The iron core is magnetised by a permanent magnet 
at one end, which is rotated by clockwork so as to cause a 
continual slow change in the magnetisation. The magnetisation, 
however, lags behind the magnetic force owing to the hysteresis 
of the iron, but when a high-frequency current passes through 
the inner winding there is a decrease in the hysteresis, due 
