4o8 



NA TURE 



[August 23. 1894 



the minimum current ■ i vt i was 3'6 x lO"' amperes. 

 In order to extend the determinations to higher frequencies, 

 recourse was had to magnetised tuning-forks vibrating with 

 known amplitudes. With a frequency of 512 the mini- 

 mum current was ""o x lO"^ and with a frequency of 

 640 it was 44 :; 10"* amperes. Lord Rayleigh's second paper 

 was on the quantitati\"e theory of the telephone. About this so 

 little is known that even an attempt to determine the order of 

 magnitude of the physical quantities involved is of great value. 

 The method adopted is to consider first the case of an infinitely 

 long thin rod of iron, divided by a transverse gap, and 

 encomp.assed by an infinite coaxial magnetising coil. 

 He finds the force exerted across the gap by a periodic 

 current, and then replaces one-half of the infinite rod 

 by the plate of the telephone, and reduces the coil to 

 the actual dimensions used in practice. The force in 

 dynes exerted at the centre of the telephone plate is calculated 

 to be equal to i 7 x 10'' C, where C is the current in amperes. 

 By actual experiment the force was found to be equal to 06 C. 

 Experiment also showed that the displacement of the plate pro- 

 duced by a current C was C x ooS cm. The amplitude of 

 the motion produced depends largely upon the relation between 

 the frequency of the impressed vibration and those natural 

 to the plate. For the telephone in question, assuming the 

 olate to be clamped all round the edge, the frequency of 

 thegravest symmetrical mode is calculated to be about 991. On 

 making the plate speak on its own account, the frequency found 

 was 896. As it is almost impossible to estimate the amplitude 

 when the frequency of the force is near any of the free 

 frequencies, the viliration number 256 is taken for calculation. 

 \\ this pitch the minimum recorded current is S'3 x 10"" 

 amperes ; and the amplitude corresponding to this is 



bending over. The fall is very rapid so far as the experiments 

 have been tried, shows no sign of becoming asymptotic, but 

 runs straight towards the zero line. Soft iron and hard steel 

 give the same results, the difTerences between them correspond- 

 ing to their differences in the B H curve. The three states of 

 molecular arrangement, which are the essential point of the 

 molecular theory, are exactly reproduced in the hysteresis curve. 

 This first stage of quasi-elastic movement gives a very small 

 hysteresis v.alue. The second stage of irregular molecular 

 groups and magnetic combinations gives a v.alue approximately 

 proportional to the induction at a steep inclination ; this extends 

 to the knee of the B'H curve. The third stage of approaching 

 saturation gives a rapidly diminishing hysteresis when the 

 molecular magnets are ranged in regular order along lines of 

 force, and new combinations and irregular movements are pre- 

 vented. Since the non-appearance of a correspondence between 

 the B/H curve and the hysteresis curve in alternating fields has 

 been urged as an argument against the molecular theory of 

 magnetism, this complete accord and verification of the deduc- 

 tion previously made is important as giving powerful support to 

 Prof. Ewing's theory. 



Prof. S. P. Thompson briefly explained how he had verified 

 the magnetic analogues of well-known propositions respecting 

 optical images in plane mirrors. The experiments were made 

 by placing a magnetic pole in front of a sheet of iron, and in- 

 vestigating the field by an exploring coil connected to a ballistic 

 galvanometer. Prof. .V. M. Mayer showed how beats and 

 beat-tones could be produced by two vibrating bodies whose 

 frequencies of vibration are so great as to surpass the limit of 

 audibility. He has also employed the transverse vibration of 

 bars at various temperatures 10 determine the v,ariation of the 

 modulus of elasticity with change of temperature. 

 6 8 X 10"' cm. Assuming the telephone to be applied closely ^^On Tuesday jnorning there was a joint meeting with Section 

 to the ear, so as to include 20 c.c. of air, it is shown that theW'M, to discuss theories of vision. Prof. Oliver Lodge showed 



~ iri experiments to illustrate .Maxwell's theory of light. Electro- 



condensation (in atmospheres) produced is r4 x 10"'. For 

 higher frequencies llian 512 the actual sensitiveness, in virtue 

 of resonance, is greater than the value calculated by the above 

 method. 



Prof. J. A. Ewing exhibited an apparatus for measuring 

 small strains. The measurement of Young's modulus for con- 

 siderable lengths of wires, as carried out in physical laboratories, 

 is an easy matter ; but engineers have to investigate the be- 

 haviour of short bars, and require an instrument which should 

 be convenient and expeditious in use. In the instrument de- 

 scribed these ends are achieved without any sacrifice of accuracy. 

 There is only a slight mechanical magnificitionof the extension, 

 but by means of a microscope forming part of the instrument, 

 readings are made to i/ioo,ooothof an inch, and the re.idingsare 

 calibrated by a simple device which forms part of the instru- 

 ment. If the arms have the same coefficient of expansion as 

 the material of bar, there is automatic compensation for change 

 of temperature. Difference readings were given for the exten- 

 sions produced in a half-inch steel bar by twelve successive loads 

 increasing each time by half a ton : these only varied between 

 104 and 107. The instrument is attached to the bar under 

 examination in such a way as to measure strictly the ni/a/ elon- 

 gation. It is well adapted for the investigation of small strains 

 in parts of structures (e.g. members of railway bridges). 



Mr. F. G. Baily made an important and interesting com- 

 munication on hysteresis in iron and steel in a rotating magnetic 

 field. It has long been known that, up to the limits of ex- 

 periment, the value of hysteresis in an alternating magnetic 

 field increases continuously. But it is a deduction from Prof. 

 Ewing's molecular theory of magnetism that in a rotating 

 magnetic field the hysteresis should diminish at a high in- 

 duction, or at least show a reduction in the rate of increase. 

 The following experiment substantiates this deduction in a very 

 complete manner : — An electromagnet is rotated on bearings 

 concentric with the bore of its own pole-pieces, which were 

 bored out cylindrically. Id the polar cavity a finely-laminated 

 armature is suspended between centres, and held by a 

 tpiral upring attached to the axle and to a fixed support. 

 if the armature i» indicated by a beam of light re- 

 1 mirror on it. On rotating the magnet, the arma- 

 • rotate with it by reason of hysteresis. The motion 

 . the spring, ami the consequent deflection is pro- 

 [ '. the instantaneous value of the hysteresis per 



revoluLiun. The curve of hysteresis and induction obtained 

 commences like that in an allernaling field, rising very slowly 

 at &1I, then more rapidly, but finally reaching a maximum and 



NO. 1295, VOL. 50] 



experiments to illustrate .Maxwell's theory of light, 

 magnetic waves produced by a sm.all vibrator were allowed to 

 fall upon a detector placed inside a large copper " h.at." The 

 detector consisted of a glass tube containing iron borings form- 

 ing part of a circuit with a galvanometer. On account of its 



I mode of action, this detector is called by Prof. Lodge a 

 "coherer." Under the action of the waves its resistance 



/ diminishes and the galviinoineter current increases. The 

 coherer was used to demonstrate the reflection, refraction, and 

 polarisation of electromagnetic waves. The audience, which 



) filled every part of the large museum lecture-room, repe.itedly 

 showed its warm appreciation of Prof. Lodge's beautiful experi- 

 ments. His electrical theory of vision m.ay be briefly described 

 as a suggestion that light-waves do not directly produce the 

 sensation of vision, but that their action (like th.at of the 

 electromagnetic waves in these experiments) is a kind of 

 \ " trigger " .action. 



In the subsequent Section-meeting, Principal V'iriamu Jones 

 gave the results of further determinations of resistance in 

 absolute measure by the Lorenz method. The apparatus had 

 previously been used to determine the absolute resistance of 

 mercury, and has now (with modifications ensuring still greater 

 accur.acy) been employed to measure certain coils whose re- 

 sistance in terms of the Cambridge Standards is known. He 

 also exhibited a new form of standard coil of low resistance. 



In the absence of Prof. J. J. Thomson, his paper on the 

 velocity of the cathode rays was read by Prof. Fitzgerald. The 

 phosphorescence shown by glass in the neighbourhood of the 

 cathode was ascribed by Crookes to thp impact of charged mole- 

 cules driven off from the negative electrode. The remarkably in- 

 teresting experiments of Hertz and Lenard, which show that 

 thin films of metal interposed between the cathode and the 

 walls of the tube do not entirely stop the phosphorescence, have 

 led some physicists to doubt whether Crookes' explanation Is 

 the true one, and to regard the phosphorescence as being due 

 to a kind of ultra-violet light. The view to which Lenard has 

 been led by his experiments — lh.at the cathode rays arc ethereal 

 waves — demands the most careful consideration ; for if it is 

 admitted, it follows that the ether must have a structure cither 

 in time or space. .\ m;ignel produces no effect upon ultra- 

 violet light unless this Is passing through a refracting substance. 

 Now these cathode rays are deflected by a magnet, so that on 

 the above view it must follow that In the ether in a magnetic 

 field there must either be some length with which the wave- 

 length of the cathode rays is comparable, or else some lime 

 comparable with the period of vibration of these rays. Prof. 



