Table of contents

The influence of tension on the Thomson effect in wires has been investigated by Nettleton's method. For pianoforte steel, charcoal iron, constantan and nickel, the Thomson coefficient numerically decreases with tension until the elastic limit is reached, after which it increases. With removal and restoration of tension a new definite cycle is followed. The Thomson effect in tungsten, here measured for the first time, increases with tension and also follows a cycle. No change in the Thomson effect is noticed in brass or German silver. Heterogeneity was not present in the specimens investigated.

The Paper gives an account of investigations on the flashing of discharge tubes having various types of electrodes, and in which the filling gas, a mixture of argon and nitrogen, had a wide pressure range.

It is shown that the observations are in general agreement with those obtained previously on neon and air discharge tubes, minor differences between tubes being discussed, and that at higher pressures the discharge was no longer of the "glow" type, but took place between definite points on the electrodes.

The viscosity of ammonia gas has been determined at three different temperatures by transpiring the gas through a capillary tube which had previously been calibrated with air. Sutherland's constant is found to be roughly 370, and the mean collisional area of the ammonia molecule 0.633 × 10^-15 sq. cm.

The valve maintained tuning fork is now a much used piece of apparatus, but the influence of the conductance of the valve electrodes on the design has not been considered. The present paper discusses the design of valve maintained tuning forks without condensers and shows that the conductance of the valve grid decides the direction in which the electrode coils must be wound; and also that it is advantageous, particularly in the case of low frequency forks, to interpose transformers between the valve electrodes and the fork magnets.

An earlier Paper (Proc. Phys. Soc., Vol. 26, 1914, p. 137) discussed a number of data for the times of occurrence of the "Sudden Commencements" (S.C.s) of magnetic storms which had been published by Dr. L. A. Bauer in "Terrestrial Magnetism," and came to a conclusion adverse to Dr. Bauer's claim that S.C.s were propagated from east to west or west to east, with velocities of from 100 to 200 kilometres per second. Since that date Papers on the subject have been published by Prof. S. Chapman and Fr. Rodés, of the Ebro Observatory, propounding theories differing from one another and from Dr. Bauer's. In a recent Paper Dr. Bauer and Mr. W. J. Peters have re-discussed the subject. They conclude that the motion in longitude is much more rapid than according to Dr. Bauer's original estimate, and suggest that is is really a case of propagation from the magnetic equator towards either magnetic pole.

The Section of Terrestrial Magnetism and Electricity of the International Union of Geodesy and Geophysics has recently approved a scheme aiming at the construction and use of special instruments to find out whether S.C.s have a finite rate of propagation. The present Paper discusses the whole subject, partly with a view to facilitating a decision as to the new apparatus and the stations most suitable for the investigation.

In measuring temperature by thermocouples in unequally heated enclosures it is shown that the recorded temperature depends upon the thickness of the couple, the nature of the walls of the enclosure and the nature of the gas. An explanation is suggested based upon the fact that the amount of radiation received and emitted by a couple depends upon its position in the enclosure, and that the amount of energy transferred to it by molecular impact varies with the nature of the gas.

By means of two photo-electric cells with suitable coloured filters the energy radiated in two distinct spectral bands is compared. By combining the ascertained sensitivities of the two cells and their filters with the radiation law the black-body temperature of the radiator can be determined.

The quantity of ozone present in the atmosphere has been measured by spectroscopic means on about a hundred days. A marked connexion with the pressure distribution at the surface, and particularly with the conditions at about 10 km., has been found.

The absorption coefficients of electrons having from 0.5 to 100 volts velocity were observed in the vapours of mercury, cadmium and zinc. Except for a small irregularity in cadmium, the absorption coefficient decreased uniformly with increasing velocity.

Descriptions are given of a new type of decrement gauge in which the individual oscillations are photographed, and a variation of the Pirani gauge, in which no metal is exposed to the vacuum.

The spectrum of the source is photographed through a neutral wedge placed immediately in front of a photographic plate. The curved boundary of the spectrum varies in height, the variation depending upon the distribution of energy in the source - i.e., upon its temperature. It is assumed that the source radiates as a black body or a grey body, but with certain limitations the method is applicable to bodies which are neither black or grey.

The Paper deals with the diameters of the atoms of the elements, and compares the estimates obtained by various methods - viz., (1) Atomic volumes; (2) X-ray analysis; (3) Viscosity of gases; (4) Van der Waals' equation and its modifications; (5) Compressibility; (6) Viscosity of liquids; (7) Viscosity of solutions; (8) Diffusion of ions; (9) Mobility of ions in solutions; (10) Latent heat of solution; (11) Speculations as to atomic laws of force; (12) Scattering of X-rays; (13) Bohr's theory in the case of the hydrogen atom; (14) Ionisation potentials; (15) Band spectra; (16) Chemical constant; (17) Optical rotation; (18) Molecular films; (19) Langevin's theory of diamagnetism; (20) Impacts of a stream of electrons on gas molecules; (21) Density changes in permutite; (22) Melting points of solids.

The results obtained by these methods, of which an extensive bibliography is given, are fairly accordant, and their mean shows that the addition of an inner electron shell gives an increase in diameter of roughly 0.5Å.

An edge-tone is heard when a stream of air issuing from a slit or hole strikes a sharp edge, or surface bounded by sharp edges. A brief description is given of previous work, and it is shown that the eddies which give rise to edge-tones conform to a simple Kármán vortex-system. On the assumptions (1) that the distance a from the edge to the slit is always equal to l, the distance between two eddies in the same row, and (2) that tone is destroyed when the edge - when moved across the jet towards still air - crosses the line of eddy centres, one can measure h/l, where h is the separation of the eddy-rows, directly from measurements of the boundaries for tone. From the experiments it is found that at low pressures and with wide slits h/l, for air, = 0.276, which may be compared with v. Kármán's prediction of 0.283 for an infinite system in a perfect fluid.

If the edge is moved towards the slit, h, the separation of the eddy-rows, must decrease proportionally. The experiments show that when the eddies are formed very close to the edges of the slit, and therefore in a field of high velocity-gradient at right angles to the direction of motion, they are deflected towards the middle, or principal, plane of the jet. The amount of this deflection increases very rapidly as a approaches a₀, at which distance tone can only be obtained when the edge lies in the principal plane. The experiments show that with wide slits, when a = a₀, then l=a₀; h = 0.67d, where d is the width of the slit. The minimum distance for tone, a₀, varies with the velocity v, and the width of the slit.

The results suggest an equation of dynamic similarity:

B(ν/vd) = (a₀/d)² - 1.50

where B is a number (about 2,000 for air) and v is the coefficient of kinematic viscosity. By considering the acceleration that produces the deflection, an expression of somewhat similar form can be obtained theoretically. The tonal boundaries in the region of simple tone approach two straight-line asymptotes, equally inclined to the principal plane, which converge at a point 0 near the plane of the slit. If a and a₀ denote the distance of the edge and the minimum distance for tone respectively from 0, the total separation, y, of the two boundaries at any distance a is given by the equation:

y = (h/l)ā - (h/l)ā₀ exp[-m²(ā² - ā₀²)]

where 1/m is approximately equal to d, when the slit is very wide. The measurements of the frequency of the tone and the deductions made from them will form the subject of a subsequent communication.

Details are given of a method whereby vertical spectrum tubes (demanded by certain investigations) can be critically examined for vanishingly small quantities of mercury. The tubes have two U-limbs partially immersed in insulated water. The water serves as external electrodes. An end-on view of the glow within the excited tube is obtained by a totally reflecting prism formed upon the summit of one of the limbs. The light is thus deflected horizontally and received in the usual way by a spectrometer.

The author describes and gives a figure of an apparatus used for storing small quantities of rare or highly purified gases in such a way that contaminating air is excluded The apparatus is designed for use in conjunction with a Sprengel pump, which withdraws the gas from any experimental plant and delivers it into the storage apparatus. With the aid of barometric traps, etc., the gas can be readmitted either partially or wholly into the experimental plant.

Sheets of aluminium have been examined by the ionisation spectrometer and by photographic methods, with a view to discriminating between two contradictory results obtained by previous observers with the photographic method as to the final orientation of crystals in rolled material.

The results show that, when a cast specimen of aluminium containing a number of large crystals oriented at random throughout the body of the material is rolled, the crystals break up into a large number of minute crystals. As the thickness of the material is progressively diminished, the small crystals tend to take up a definite orientation, each crystal having a cube diagonal in the direction of rolling and a (211) plane in the plane of rolling. Two sets of crystals exist in the material after rolling, the one set being the optical image of the other in the plane of rolling.

The observations obtained by the ionisation spectrometer indicate that the type of the space lattice remains unaltered, and that the parameter of the material in the severely worked condition does not differ from that of the annealed material by more than 0.5 per cent.

The Paper gives an account of preliminary investigations into the spreading of liquids on mercury. Water and solutions of a number of inorganic substances are tested.

Purest "conductivity" water in the presence of air is found to spread very slowly on a clean mercury surface; the spreading is greatly accelerated by traces of acid in solution, and totally inhibited by traces of alkali.

All the neutral salts tested, even chlorides of mercury and silver, are found to produce rapid spreading. By varying the concentration of the solution the rate of spreading is controlled; the phenomena are described and photographed.

All acid solutions tested spread rapidly, probably reacting chemically with the mercury. One part of hydrochloric acid in ten million of water definitely accelerates spreading. A drop of very dilute acid spreads rapidly to cover a definite area and stops, any further spreading being as slow as that of water. The area covered during the rapid stage depends on the nature of the acid; it is proportional to the number of acid molecules present, and is larger than would be expected if a monomolecular layer of the appropriate salt were formed on the mercury surface.

Drops of certain alkaline solutions, notably ammonia, are found to spread after a period of delay, the spreading being due to carbon dioxide absorbed from the air.

Electrostatic fields up to 4,000 volts/cm. applied perpendicular to the surface produce no visible results. Placing a platinum wire from one terminal of a battery in the mercury and from the other terminal in the drop, and applying small voltages, causes spreading even in alkalis if the mercury be positive, and prevents spreading even in dilute acids if the mercury be negative.

Increasing the field beyond that necessary to prevent spreading (mercury negative) sets up an oscillation, the drop spreading till it breaks contact with the terminal, and then contracting. An E.M.F. of 400 volts was required to produce this effect with a drop of conductivity water.

A method of photographing films on the surface of mercury is described. By the same means a photograph is obtained showing a drop of oil spreading on a water surface. The spreading film is preceded by a well defined ridge, probably that observed by Osborne Reynolds, but apparently by none of the more recent writers.

The Paper concludes with a description of a ridge that may be observed under certain conditions when water is draining from a sheet of perfectly clean glass.

Attention is called to the infinite possibilities of obtaining the Einstein equation for the orbit of a planet by using Newtonian mechanics with an extended potential function. The merits of the other two "crucial phenomena" as such are discussed from this point of view. It is implied that, observational tests being equally satisfied, Einstein's theory is to be preferred on account of its extensive unity and the spontaneity of its results.

The principal recorded determinations of the mechanical equivalent of heat are critically discussed, and an attempt is made to correct them for errors not fully taken into account at the time when they were made. The results are weighted according to the relative importance attached to them by the author, and the weighted mean is given as 4.184 joules per calorie at 20 degrees.

The theoretical and experimental aspects of the specific heats of solids, liquids and gases in the light of present knowledge are discussed. Possible lines of advance have been indicated for solids in view of the progress now being made in X-ray crystallography, and the importance of further experimental work on compressibilities and elastic constants is suggested. Tentativeviews on the specific heat of liquids are given. In gases the quantum theory of rotational heat is shown to fail to account satisfactorily for the behaviour of hydrogen at low temperatures. The chemical constant of monatomic and diatomic substances is discussed, both from a thermodynamic and from a statistical point of view. Current views on the nature of this constant are described. The theory of gaseous degeneracy and its bearing upon specific heats and chemical constants is reviewed.

When a liquid lies between two plates inclined to one another at a small angle its surface has a hyperbolic section, from measurement of which the surface tension can be calculated. The method has long been known, but has recently been improved by Grünmach. Errors arise, however, from the difficulty of determining the horizontal and vertical axes of co-ordinates, and according to the present Paper this difficulty is met by plotting two linearly related functions of the observations, the surface tension being deduced from the co-ordinates of the resulting mean straight line and the angle between the plates. The latter may be measured either directly or by calibration with a liquid of known surface tension.

The X-ray shadow of a tube has previously been used for determining the diameter of the bore. The present investigation was designed to test the suitability of a similar method for capillarity measurements, and observations were made of the rise of mercury in copper and steel tubes and between vertical copper plates. The results were satisfactory.

An attempt has been made to find the changes produced by torsion in the conductivities of single aluminium crystals, and of the annealed and hard aluminium wires from which the crystals are prepared. The hard wire gave a decrease of a few parts in a thousand in the thermal conductivity, but no change greater than one part in 1,000 has been found in crystal wires. The decreases in the electrical conductivities, of the order of a few parts in 10,000, appear to be almost the same for all specimens.

An attempt has been made to measure the thermionic and photo-electric work functions for the same specimens of tungsten and platinum. It is found that in all cases the results depend greatly on the previous heat treatment of the material. In the case of platinum the curves showing the dependence of photo-electric sensibility on the wavelength of the irradiation are of four different types, while as regards thermionic properties the specimens can take up either a "large-emission" or a "small-emission" state, according to their treatment. The photo-electric work function of platinum appears to be greater than the thermionic, but no definite results were obtained for tungsten. The photo-electric curves obtained for both metals did not end abruptly so as to indicate a maximum wavelength for emission, but extended asymptotically towards zero emission in the direction of increasing wavelength. These irregularities in behaviour are attributed by the author to the state of the surface of the specimens.

The extent of the analogy between cylindrical sound waves and waves on the surface of a liquid is studied. It is established for inviscid media that as regards velocity potential the differential equations are similar provided the disturbance is small and is harmonic in type. It is then shown that the velocity potential in a field of inviscid surface waves may be used to represent that in a field of cylindrical sound waves, provided the size of any obstacle bears the correct relation to the wavelength of the disturbance. In the case of a liquid having a depth greater than half the wavelength, it further follows that the height of a surface wave is a measure of the condensation of air in the corresponding acoustical condition.

The effect of moderate viscosity is of the same type for sound waves and for surface waves; it is most marked for short wavelengths, and it decreases amplitudes without appreciably altering the wavelength. The decrease in amplitude depends upon the viscosity, and upon the time which has elapsed since the commencement of free oscillations, or, for maintained oscillations, since the disturbance concerned left the source. For surface waves further damping occurs if the depth is less than half a wavelength, and the apparent viscosity may be appreciably increased by contamination of the surface by means of a film of oil of excessive thinness. Viscous effects are generally negligible in the case of sound, but they are appreciable for the water waves that would usually be used in a small ripple tank not greater than, say, 10 ft. in size. Mercury waves are much less affected by viscous damping than water waves.

Experiments conducted with model obstacles in a ripple tank showed that the effects of the meniscus around the obstacle and of the amplitude of the source were not important. When the water becomes stale on exposure - and presumably contaminated - the relative distribution of ripples around obstacles is appreciably modified, but not to an extent which would correspond to any very marked change in the loudness of a sound.

Experiments have also been conducted, outside the limits of strict mathematical analogy, using an impulsive disturbance instead of a maintained train of waves. When it is recognised that a sound pulse travels out singly, whereas with ripples subsidiary wavelets accompany the main pulse, the correspondence between ripple photographs and sound pulse photographs is most striking.

Illustrations are given of the use of a ripple tank in connexion with architectural acoustics, and of its general value where wave phenomena are concerned.

In this Paper the evaporative losses of metal Dewar vessels are discussed, and particulars given of experiments made by the author in Oxygen Laboratory of the Air Ministry. The separation of neck and radiation losses is described, and also a method of testing adsorbents under working conditions. Experimental details are given with graphs and tables.

Formulæ are given for obliquity corrections in radium estimation applicable to sources of the shapes most frequently occurring in practice. The effect of scattered radiation is considered and some experimental results are shown graphically and compared with theory. Calculated values for a particular case are given by way of example.

The Paper describes a method of determining the coefficient of viscosity of water by measuring the rate of convective flow in a long capillary tube, the driving head being obtained by the difference in density due to a difference of temperature between two vertical columns of water.

The results are in agreement with the conclusion reached in a previous Paper by Dr. Constance H. Griffiths and one of us - viz., that the viscosity of water at low rates of shear in glass tubes is apparently no different from that at high rates.

The novel features of the Paper appear to be (1) the method of obtaining the driving head; (2) the new method of introducing a coloured index in a closed capillary circuit, and an improved method of reading its position; (3) the almost complete elimination of what may be called the thermometric effect as distinct from the convective effect; (4) the elimination of the small residual thermometric effect from the final calculations.

It is well known that when a mutual inductance carries alternating current, the value of the effective mutual inductance varies with the frequency, and further that the secondary P.D. is not exactly in quadrature with the primary current. Thus certain frequency corrections are introduced into all alternating-current bridges in which a mutual inductance standard is used. An experimental investigation has been made of the actual magnitude of such corrections for mutual inductance standards of three types: -

1. A stranded wire fixed standard, with spaced winding to minimise capacity effects.

2. The Campbell variable mutual inductance standard.

3. The Tinsley variable mutual inductance standard made in accordance with Butterworth's recommendations.

Values of the corrections are given for all these instruments under various conditions. It is shown that whereas the variation of mutual inductance with frequency is mainly due to capacity effects, dielectric losses in the insulation, or alternating current conductance, play an important part in the determination of the phase defect, the impurity being found to be proportional to a power of the frequency higher than the second, over the telephonic range of frequency.

Following the previous experiments of one of the authors on the nature of the green line λ5350 of thallium, a detailed examination of λ4722 of bismuth was made with a quartz Lummer plate, and it has been found that there is not even the slightest trace of absorption in the main component or in the satellites, even at 1,000° C. The fluorescence of the vapour under optical stimulation was studied, and it was found that when excited by radiation of wavelength from λ6000 to 3500 the fluorescence was a banded spectrum in the orange-yellow, while when excited by ultra-violet radiation from a bismuth arc, the fluorescence was bluish, and consisted of λλ4722 and 3068, thereby showing that λ3068 represents the minimum excitation energy and corresponds to transition to the normal orbit.

In studying the intensity distribution in a spectral line emitted by positive rays, a line structure was found which is not familiar in ordinary discharge tube spectra. This structure would not appear under the low resolving powers necessitated by the extreme faintness of the light emitted in most investigations of Stark and Doppler effects in positive rays. The technique required to make possible the study of structure in such faint lines is discussed in detail. The explanation of the structure in terms of Doppler and Stark components is compared with the theory of broadened lines in ordinary discharge tube spectra.

The object of this Paper is to give a brief account of the principles of Einstein's General Theory of Relativity as applied to those problems in which they have received their most striking confirmation. To this end we have limited our investigations to the consideration of dynamical manifolds which are static and isotropic in character. It has then proved possible to abandon the notation and theory of the Absolute Differential Calculus, and to substitute in its place some classical theorems of Lord Kelvin and M. J. Liouville. Although the scope of these inquiries is not extensive, within the limits of our investigations we have explained the method of the construction and solution of Einstein's field equations, and have given the applications of these results to the problems of planetary motion and of the deviation of light rays in the solar gravitational field. At the same time we have endeavoured to make explicit the various assumptions involved in this theory.

The diffraction of light inside the shadow, thrown by a small source of light, of a sphere and a circular disc of the same diameter, was studied, with special reference to the relative intensities of the central bright spots. With the source at about 2 metres from the obstacles, with a quarter-inch polished steel ball, the bright spot could be detected visually up to 3 cm. behind the obstacle, while with a steel disc of the same diameter, with the edges perfectly sharp, smooth and circular, the spot could be traced up to 2 cm.

The relative intensities of the two spots were studied at different distances behind the obstacles, qualitatively by photography and quantitatively by visual photometry. At small distances behind the obstacles, the spot inside the shadow of the sphere is much feebler than the disc-spot, however approximating to the latter as we reach farther back from the obstacles, but even at 100 cm. remaining appreciably feebler.

A general explanation is suggested.

The Paper gives an account of the experiments on the absorption spectrum of nickel by the under-water spark from λ6000-λ2000. In the region λ3800 to λ2100, 180 wavelengths were obtained in absorption. The majority of these lines were classified by Bechert and Sommer. Intensity values of the absorbed lines showed that the intensity rule and the selection rule for inner quantum numbers were accurately fulfilled. The results confirm in a striking manner the recent classifications of Bechert and Sommer in the arc spectrum of nickel.

This Paper gives the results of electro-endosmotic determinations with certain aqueous solutions of electrolytes in glass diaphragms, and values are obtained for the interfacial potential of the Helmholtz electrical double layer. The dependence of the conductivity on the electro-endosmotic flow has been investigated by measurements of the current flowing through the diaphragm during each determination, and the data used to trace the variations in the thickness and the charge of the electrical double layer by the application of a formula derived by Smoluchowski.

The latent heats of a number of the commoner metals have been measured by determining the total heat of liquid and solid from a series of initial high temperatures.

The calorimetry was by the method of mixtures, introducing several refinements, of which the chief were the use of fairly large charges of metal (of the order of 2 kilograms), and a device by which the hot charge was not allowed in contact with the water of the calorimeter until the latter was completely closed; this eliminates error due to production and escape of steam, with a consequent loss of heat. The device referred to consisted in the provision of a sheet metal vessel suspended by threads from the main lid of the calorimeter. The aperture through which the charge was introduced was closed by a rotating lid, in the main lid, and the crucible being introduced, was submerged after the closing of this smaller lid by means of a wire passing through an eyelet in the base of the calorimeter, and out at the top.

The results for the latent heat are given below: -

Metal.	Melting point, °C.	Latent Heat. (Calories per gm.)
Aluminium	657	92.4
Antimony	630	24.3
Bismuth	269	13.0
Lead	327	6.26
Magnesium	644	46.5
Tin	232	14.6
Zinc	420	26.6

The Paper also contains values for the specific heats up to the melting point, obtained by differentiation of the temperature-total heat curves.

The quartz piezo-electric resonator is examined experimentally and theoretically with special regard to an equivalent electrical system which can represent it.

It is shown that, as theoretically predicted by Butterworth, such a resonator can be represented by an inductance, a resistance and a capacity all in series. These are pictured as in parallel with another small condenser and the whole is in series with a third condenser, the additional condensers representing air-gaps. The equations for the current in an oscillatory circuit, to which the resonator is attached, are developed and it is found that almost perfect agreement exists between the forms of current curve obtained theoretically and experimentally. This agreement is found to hold for longitudinal resonators of as low a frequency as 44,000 and for transverse resonators of as high a frequency as 15,000,000 periods per second.

It is next shown how the logarithmic decrement of the resonator may be obtained from a rectified line plotted from observation on the current in the oscillatory circuit as a function of frequency width across the crevasse which pierces the summit of the resonance curve.

The methods of analysis of the equivalent mesh into its components are next developed and it is shown that this analysis can be effected by carrying out a series of observations of the current at resonance when the air-gaps are varied by known amounts, or when the effective resistance of the oscillatory electrical circuit is given different known values.

The theoretical and experimental results are found to be consistent. The value of the shunting condenser is somewhat smaller than that measured at a neighbouring frequency outside the region of resonance. A possible explanation is offered.

The effects on frequency of response of variation of air-gap are studied and the difference between prediction and observation is discussed. This difference is only a few parts in a hundred thousand. In the case of transverse resonators remarkable effects occur when the air-gap is varied through the regions where its length is an integral member of half-wavelengths of the supersonic air waves produced by the vibration of the quartz.

The temperature coefficient of frequency of a considerable variety of resonators is examined over a range of temperatures up to 40°C. It is found that very diverse results are obtained and probable explanations are offered.

The effects of displacement of the resonator from the position of centrality are examined. These are shown to be small, but not quite negligible.

The current taken by the quartz mesh is then examined in some detail theoretically, and one or two experimental curves are given, together with a graphical method of deducing the curve of current from the constants of the quartz. A selection of theoretical curves for the conditions under which the experiments of Cady were made have been calculated. The curves are in all respects of the shape and form experimentally found by him.

A number of conclusions and suggestions terminate the Paper.

Experiments are described in which a large electrostatic machine is used as a source of potential for the discharge through (a) a Coolidge X-ray tube, and (b) a non-inductive resistance. The results are represented in the form of characteristic curves. In the former case it is shown that above a certain critical voltage the discharge is intermittent, while in the latter case the discharge is continuous for all voltages.

The cause of the intermittence of the current through the Coolidge tube is assigned to the operation of the Pearson-Anson effect, and this implies the conclusion that the effect of the residual gas in such tubes is not completely insensible above a certain voltage.

The Paper deals with certain advantages and disadvantages which accompany the use of invar steel for the beam of a precision balance.

It was found that such a beam exhibits but little in the way of fatigue effects; and also that its sensitivity is nearly the same for all loads.

Experiments prove that the resting-point of the balance notwithstanding the small coefficient of expansion of invar steel, may be appreciably affected by changes in temperature. This is attributed to slight relative movements amongst the several knife-edges and not to a differential lengthening of the beam itself.

The instrument is affected by magnetic storms and also by the diurnal variations in the earth's magnetism. Magnetic effects may, however, be nullified by weighing according to the method of Gauss or by a method of difference.

The conclusion reached is that provided due precautions are observed, invar steel is eminently suitable for the beams of high-grade precision balances.

The present Paper describes the results of the quantitative measurement of the intensity of X-ray reflexion from different planes of Fe₃O₄. The side of the unit cell=8.400 Å. The parameter, fixing the position of the oxygen atoms, has been determined as x=0.379±0.001. F-curves, representing the dependance of the scattering power of oxygen- and iron-ions on the glancing angle, have been obtained. These F-curves (uncorrected for heat-motion) decline much more strongly than the calculated curves for Stoner's distribution of electrons amongst atomic levels.