236 PHILlPS TECHNICAL REVIEW VOLUME 19

THE FOUCAULT PENDULUM IN THE UNITED NATIONS BUILDINGIN NEW YORK

by J. A. HARINGX and H. van SUCHTELEN. 525.36.002.2

Among the gifts presented by member slates for embellishing the United Nations buildingin New York is a large Foucault pendulum from the Netherlands. The pendulum, which issuspended in the entrance hall leading to the main Assembly Hall, continuously demonstratesby its movement that the earth is in rotation.

The article below briefly describes the suspension and driving systems of this pendulum.The design was based on the requirement that the pendulum should function well fOT a very longtime without supervision and maintenance.

On 7 December 1955, Mr. H. Luns, Dutch ForeignMinister, presented to the Chairman of the UnitedNations General Assembly a Foucault pendulum onbehalf of the people of the Netherlands. The pendu-lum is suspended above the central column of thestaircase in the entrance hall of the United Nationsbuilding in New York (see fig. 1). The pendulumwas specially designed in the Research Laboratoryof N.V. Philips' Gloeilampenfabrieken, the guidingconsideration being that the pendulum shouldfunction uninterruptedly for many years withoutrequiring supervision or maintenance.

A Foucault pendulum is in principle merely aweight (which we shall henceforth refer to as the"bob") suspended by a long wire, swinging in a verti-cal plane. Owing to the manner of suspension theplane of swing is not, however, fixed with respect tothe earth but, in consequence of the rotation of theearth, turns about the vertical through the point of

suspension. In the northern hemisphere the plane ofswing deviates in the clockwise direction,in the south-ern hemisphere in the opposite direction. It can beshown 1) that for an idealized case (very long pendu-lum, very small amplitude) the time T' in which theplane of swing rotates 360° is given by the formula:

TT'=--. ,

sm cp(1)

in which T is the period of revolution of the earthand sp is the geographic latitude. At the poles T' = T,while at the equator T' = 00, i.e, there is no rota-tion ofthc plane. In New York (g; = 40°45'), timeT' = 36 hours 50 minutes. It further appears fromthe theory that the angular velocity with which theplane rotates with respect to the meridian isconstant.

') See e.g. A. Sommerfeld, Vorlesungen über theoretischePhysik , Part I, Dieterich Verl., Wiesbaden 1952.

1957/58, No. 7-8 FOUCAULT PENDULUM

Fig. 1. The entrance hall in the United Nations building in New York. The Foucaultpendulum, a gift from the Netherlands, is mounted above the central column of the staircase.

In practical conditions, the time of rotation isfound from the formula:

approximately 8t sec. The weight of the pendulumbob is about 90 kg.If we try to construct such a pendulum and repeat

the celebrated experiment carried out by Foucaultin 1851, a number of disturbing effects - due, forexample, to imperfect.ions in the rotational symmetryof the wire support - cause the pendulum, aftersome time, to execute an elliptical or even a circularmotion. This had to be avoided with the presentpendulum, which was required to swing continuously

TIf

= Si~ ~ ( 1- ~ ;:) ,

in which a is the amplitude and l the length of thependulum 2). In the case of the pendulum in theUnited Nations building, I is about 17t m and aabout 80 cm. This length gives a period of swing of

(2)

2) Handbuch der Physik, Part V, p. 339, Springer, Berlin 1927.

237

238 PHILIPS TECHNICAL REVIEW VOLUME 19

without supervision. There were two other technicalproblems. In the first place there had to be thecertainty that the wire would not break after sometime of continuous operation, and in the secondplace a driving system was needed that would ~eable to provide the pendulum with sufficient energyto compensate for the loss of energy due to airresistance.A simple method of overcoming the "ellipsing"

problem has been described by Charron 3). At adistance I' below the point of suspension A (seeJig. 2a) he fixed a ring B having an internal diameterslightly larger than the thickness of the pendulumwire, leaving an annular spacing of d. As soon as thedeflection of the bob exceeds the value Id/I', the wiretouches the ring and the point of corrtact thenfunctions as the "point of suspension". The conse-quence is that the minor axis of an elliptical orbitwhich may have been forming is rapidly diminished.With reference to fig. 2b, this can be roughly ex-plained as follows.Assuming that the wire, as long as it does not

touch the ring, is straight, it will describe at the levelof the ring an ellipse geometrically similar to theelliptical orbit of the bob reduced in the ratio L'/l.This is represented in the figure by the small brokenellipse; it is traversed, for example, in the directionof the arrow. The circle represents the limit set bythe ring to the deflection of the wire. The momentthe wire touches the ring, point BI functions as thepoint of suspension. Assuming that the velocity ofthe ball in the Y direction is small at that moment,the ball will thereupon proceed to describe theellipse CD'E', the axes of which lie along the linesBIX' and BI Y', We see, then, that the amplitude

E

E'

a b

Fig. 2. a) Suspension of pendulum according to Charron.The point of suspension is at A. The ring B, whose internaldiameter is only slightly larger than the diameter of the wire,prevents the bob C from describing an ellipse.b) Functioning of Charron's ring. When the pendulum describesan ellipse with semi-axes a and b, the wire touches the ringe.g. at point B,. If there is sufficient friction between ringand wire, B, then functions for a while as the effective point ofsuspension and the bob describes the ellipse CD'E' instead ofthe ellipse CDE, which it would describe in the absence of thering. The amplitude in the Y direction is thus diminished in ahalf period by the length EE', which is equal to 2bZ'/1.

3) F. Charron, Bull. Soc. Astr. Fr. 45, 457,1931.

Fig. 3. The actual suspension system.

in the Y direction _at the beginning of the followinghalf swing has been reduced by the distance EE',which is approximately equal to 2bZ' [l, The relativereduction for each full swing amounts then toiJb/b = 41'1l.It has been tacitly assumed in the foregoing that

while the bob describes the orbit CD'E' the wirestays pressed against the ring at point BI' As a rulethe contact friction will not be sufficient for this tooccur. In reality, therefore, LJb/b is smaller thancalculated here, but this does not alter the fact thatthe mounting of the ring is a very effective means ofcombating the elliptical motion.

Charron deduced that the use of this constructionwould slightly shorten the period of rotation andderived the following expression for the period:

T ( 3 a2 4d)

T'" = sin rp 1-812- na .

For the pendulum installed in New York a sus-pension was designed, wich,while equivalent to thatof Charron, differs considerably in its construction.Fig. 3 shows a photograph ofthe suspension and Jig. 4a diagram indicating its construction. The mountingplate A, fixed on two beams, carries via three rodsB a ring C. This ring supports by means of threeflexible rods D a three-armed yoke E fitted with auniversal joint F. The wire is suspended from F.By the bending of the flexible rods the yoke canmove radially over a distance equal to the radial

(3)

1957/58, No. 7-8 FOUCAULT PENDULUM 239

play s between pin G and the periphery of a hole inthe mounting plate. The flexible rods can be movedvertically and their length accurately adjusted; inthis way the system as a whole can he given therequired lateral. stiffness and the pin made to lieexactly in the centre of the hole in the position ofrest.If we compare this arrangement with that of

fig. 2a,we notice that the universa] joint correspondsto the point of the wire which, in the Charron system,is at the height of the ring B. The hole in the mount-ing plate fulfils here the function of the ring, so thatin the new construction the play s is equivalent tothe play d in the old. The use of the universal joint

w

Fig. 4. Schematic diagram of the suspension system of theFoucault pendulum in the United Nations building. Mountingplate A carries three rigid rods 'B to which a ring C is fixed.Three flexible rods D, clamped on the latter, support a yoke E,which is fitted with a universaljointF. Pin Cmounted on E, liescentrally in a hole in the mounting plate, and limits the lateraldisplacement of the yoke to the small distance s. The suspensionwire W is fixed to the joint by clamping in a bifurcated stee]cone. The dimensions of the flexible rods are such as to ensu'réthe required lateral stiffness and to ensure that the bendingstress in these rods is below their fatigue bending strength.

avoids the friction and bending which the wireundergoes in Charron's construction at the level ofthe ring, thus greatly reducing the risk of wear orfatigue failure, The three flexible rods, like Charron'swire length AB, bear the weight G' of the pendulum'bob etc. and, via the yoke, exert on the joint a een-trally directed force which is proportional to theradial displacement. At a displacement u the above-mentioned point of Charron's wire is subjected to aforce of magnitude Gu/l'. By appropriately dimen-sioning the flexible rods in the new construction, the

~ same magnitude of restoring force can be produced.

In connection with the dimensions of the flexible rods itshould be borne in mind that the lateral stiffnesscis determinednot only by the dimensions and the modulus of elasticity butalso to a large extent by the compressive force t C'. The lengthl' no longer corresponds to any physical quantity such as thelength of the flexible rods, but is equalto C'/3c.The lateral stiffness for this case (parallel-constrained ends)

is given by the formula

P !q11c = -1 1 1'······ (4)

1 taniq 1-iq 1

where q2 = PIEl and P = t C'. Further, 11is the length of theflexible rods, E their modulus of elasticity and I their secondmoment of area in bending.

B

The suspension wire is of hard drawn stainlesssteel with a diameter of 2.5 mm. The wire is clampedat both ends by a truncated cone of hardened steelwith internal teeth. The cone has diametric saw cutsat its narrower end and is drawnfast into a ring bythe weight of the bob.

In the design of the driving mechanism, which isnecessary to keep the pendulum swinging withoutany reduction in' amplitude, account had to betaken of the fact that access to the point of suspen-sion is extremely difficult once the pendulum ismounted. The drive is therefore provided at the bobof the pendulum. The method adopted was to repelthe bob by means of eddy currents generated in itby a coil mounted on the pedestal (jig. 5), the coilbeing energized at suitable moments by alternatingcurrent. If a copper plate is placed above such a coilat right angles to its axis, voltages will be inducedin the plate which are 90° out of phase with the field~and currents which are in their turn almost 90° outof phase with these voltages. The eddy currents aretherefore almost 180° out of phase with the currentin the coil, thus giving rise to repulsion. If a not toolarge, horizontal, round plate is placed with itscentre-point exactly in the axis of the vertical coil,the resulting force acting on the plate is a vertical.one, but if the plate is slightly eccentric to the coilaxis, the force will also have a horizontal componentwhich can be used for driving the pendulum. A plate

' ... ".

92971

240 PHILlPS TECHNICAL REVIEW VOLUME 19

Fig. 5. The pedestal, designed by architect G. Rietveld, on which the drive coil is mounted.The pedestal bears the inscription:

It is a privilege to liveThis day and to-morrow .

.Iuliana

of this kind is contained in the lower half of thependulum bob. In order to concentrate the magneticfield on the plate, 18 Uvshaped yokes of ferroxcubeare fitted around the coil.This method, then, avoids the use of iron in the

ball, and thus prevents the pendulum from beinginfluenced by the magnetic field of the earth or bythe steel structure of the building.

Some remarks may be added on the transfer of energy in thisdriving system. Eddy currents in the copper plate react on thecoil with the result that the self-inductance of the latter issmaller than when the plate is absent. When the plate movesaway this self-inductance increases and the energy transferredto the plate is approximately equal to J2iJL, where iJL is thechange in the self-inductance and I the current. This doesnot take into account that I itself changes slightly, but,partly because of the circuit arrangement (see below), thisrelation holds fairly accurately.

1957/58, ~o. 7-8 FOUCAULT PE~DULUM 241

The moment at which the coil is energized isdetermined by the moment at which the, ):od in theyoke (G, fig. 4) looses contact with the mountingplate 4). For this purpose, rod and hole were design-ed to constitute an electrical contact. A delay circuit,which also controls the duration of energization,'ensures that thè current in the coil is sw{tch~d on; ata specific time after the moment the rod 'leaves themounting plate, viz. at approximately the samemoment at which the centre of the bob passes theaxis of the coil. The anode load of tube I I in thecircuit (see jig. 6) includes the D.e. winding of atransductor T5). T~e A.C. winding Ll of this trans-ductor is connected in series with the drive coil L2

and a capacitor C". In addition, another capacitorC' is connected in parallel with Ll' When the D.C.coil of the transdnctor is not energized, the circuitL1-C' is in resonance and represents such a highimpedance that the current through L2 is only 85mA.

T+---r--------~--~~L~/~

C'127V50c/s

Fig. 6. Control and drive circuit for driving the pendulum. Thecontrol circuit (left) causes the required alternating drivecurrent from the mains to be supplied to the drive coil-L, onlyafter a certain delay after the opening of contact a; the circuitsustains the current for a certain fixed time, even if contact ais again closed in the meantime. Contact a is formed by rod G(fig. 4) and the hole in the mounting plate A. IIi the quiescentstate, tube I is conducting and tube 11cut off.When a opens,the potentialof gl falls and the potentialof al rises: at a certainpoint the circnit triggers, tube 11 now becoming conducting(viaC2) and tube I being cut off.The delay between theopeningof a and the triggering of the circuit depends on the productClrl' The anode current of tube 11 flowsthrough the D.C.winding of transductor T and reduces the value of Ll such thatthe drive circuit (right) comes into series resonance. After aninterval, which depends on the products C2r2 and Carl' thecontrol circuit returns to the quiescent state and Ll rises again.The value of C' is such that resonance now"occurs in theparallel circuit Ll-C'. The latter then represents such a highimpedance in the circuit that the current is reduced to anineffective value.

4) R. Stuart Mackay, Amer. J. Phys. 21, 180, 1953, describesa pendulum driven by eddy currents, the synchronizationbeing derived from the pendulum bob.

5) A transductor (saturable reactor) is a choke with an ironcore, the self-inductance of which can be varied by changingthe magnetization of the iron corewith the aid of a D.C.winding.

0.050

Joule

t, ./..

". . , .

Yr· I~v jJII / "

I ./

0.04()

0.030

0.020

0.010

oo92973

50 80_Amplitude

lOOem20 4()

Fig.7. The energy supplied per period to the pendulum(curve a) and that lost by air resistance (curve b) as a functionof amplitude of swing. At an amplitude of approx. 80 cm thecurves intersect. At smaller amplitudes the energy supplied isgreater than that lost, i.e. the point of intersection representsa stable state of equilibrium.

Q3Q60

When the transductor is energizeq: by the anodecurrent of tube 11, the self-inductance of Ll isreduced by about a half. The capacitance of Gil issuch that the whole circuit now comes into seriesresonance, whereupon the current through L2 risesto 240 mA. The ratio between operating andquiescent currents does not seem particularly large,but it must be remembered that the energy trans-ferred is proportional to the square of the current.In this way about 0.035 joule is supplied to thebob in each period, which is sufficient to provide anamplitude of swing of the required value, viz.approximately 80 cm (see jig. 7).

Summary. A description is given of the suspension and drivingsystems for the Foucault pendulum suspended in the hall of theUnited Nations building in New York. The pendulum waspresented on 7th December 1955 to the Chairman of theGeneral Assembly by the Dutch Foreign Minister H. Lunson behalf of the people of the Netherlands. The main problemof preventing the pendulum bob from describing an ellipse in-stead of swinging in a flat plane was solved by utilizing theprinciple describedby Charron. The construction is so designedas to reduce to a minimum the risk of the wire breaking owingto wear or fatigue. The drive, which is necessary to compensatefor the energy losses, caused mainly by air resistance, iseffected by means of a magnetic coil with ferroxcube coreplaced centrally under the pendulum. This coil is energizedby alternating current and induces eddy currents in a copperplate çontained inside the lower half of the bob. The energizingcurrent is controlled by an electronic relay, which ensures thatthe current is switched on some time after the pendulum itselfhas broken an electrical contact in the suspension system, andkeeps it switched on for a certain period. The coil is mountedon a pedestal designed by the architect G. Rietveld.

242 PHILIPS TECHNICAL REVIEW VOLUME 19

ABSTRACTS OF RECENT SCIENTIFIC PUBLICATIONS BY THE STAFF OFN.V. PHILIPS' GLOEILAMPENFABRIEKEN

Reprints of these papers not marked with an asterisk * can be obtained free of chargeupon application to the Philips Research Laboratory, Eindhoven, Netherlands.

2482: C. A. de Bock, J. Brug and J. N. Walop:Antiviral activity of glyoxals (Nature 179,706-707, April 6, 1957).

In screening compounds for antiviral actrvrtyagainst influenza virus, a number of a-keto-alde-hydes appeared to he active. The test compoundswere injected in the allantoic cavity of 11 dayembryonated hen's eggs, followed after 1hr by thevirus. After incubation for 48 hours the hremagglu-tination (H.A.) titre of the allantoic fluid was esti-mated. A compound was considered active if thedifference between the logarithm of this H.A.titre and that of a control was >0.6. Active com-pounds and the difference in log H.A. titre wereCHaCOCOH (2.0), p-OH-C6H4COCOH (2.0),p-OH.m-N02-C6HaCOCOH (2.3), p-Br-C6H4

COCOH (0.7), m-N02-C6H4COCOH (2.1). The virusloses infective power when incubated with low con-centrations (0.002M) of glyoxals. At higher coneen-trations the glyoxals destroy the enzymatic activityof the virus. The virucidal action of the glyoxals isstrong enough to explain the activity in the test.

2483*: H. de Lange Dzn.: Attenuation characteris-tics and phase-shift characteristics of thehuman fovea-cortex systems in relation toflicker-fusion phenomena (Thesis Delft,June 5, 1957).

The well-known frequency-response technique ofsystems analysis, in which the ratio of output am-plitude to sinusoidal input amplitude is plottedagainst frequency in so-called attenuation charac-teristics, can successfully be applied to investigatethe dynamic nature of the human visual organ fromthe retina up to the brain, using sinusoidally modu-lated light. This is possible because Talbot's lawshows that, at flicker-fusion, the brightness-systemworks linearly. The internal threshold-value forflicker-fusion is supposed to be invariable withfrequency at constant mean luminance and actsas a constant output value; the ripple-ratio T =(amplitude of sinusoidal light variation)f(meanIuminance] acts as a variable input of the bright-ness-system, This manner of investigation, previous-ly applied in earlier papers with white light, isextended over nearly the whole range of cone-visionand is continued with coloured light. The existing

theories on flicker-fusion provide no explanationfor the shape of the attenuation characteristicsobtained from the experiments and calculated frominvestigations by other authors. The attenuationcharcteristics show at high luminance a pseudo-resonance effect, the bandwidth is greater and theslopes are such that for T> 2% the ratio (ampli-tude of fundamental)f(mean luminance) is decisivefor flicker-fusion of any shape of interruption ormodulation.

The well-known residual brightness-flicker justabove the colour-flicker limit with heterochromeflicker-photometry with two anti-phase 100% sinus-oidally modulated light beams can he reduced tozero by introducing an external phase correctionLI '1{J, which is found to he a function of luminance,colour difference and frequency. Subtracting log-arithmically the attenuation characteristic of thecolour-system from that of the brightness-system, itis found that the extra delay in colour-perception isidentical to the filter-action of one integration pro-cess at high luminance; at low luminance a tripleintegration process occurs with the same time con-stant of 120 msec.

Using electrical analogues it is shown that thecurves obtained are real attenuation characteristicsof the brightness-system. In accordance with thepseudo-resonance peak in the attenuation charac-teristic for high luminance, an overshoot in thebrightness perception occurs at about 1cfs withperodic rectangular light impulses.

2484: A. Venema: The measurement of the pressurein the determination of pump speed (Vacuum4,272-283,1954, No. 3, published Febr. 1957).

There is no commonly agreed method of deter-mining the speed of a pump. One noteworthymethod was proposed by Dayton in 1948 but it hasnot been accepted generally. Contributions to theproblem made by other workers are briefly reviewed,followed by a close study of the basic elements ofthe definition of pressure. Special attention is givento the case where the number-density of the mole-cules varies along the mean free path. Such varia-tion may occur at low pressures and exists, in par-ticular, in the region of the pump mouth. The actualmethod of measuring pressure needs more consid-eration and it is shown that a gauge connected by a .

1957/58, No. 7-8 ABSTRACTS OF RECENT SCIENTIFIC PUBLICATIONS 243

tube to the system records the number of moleculesper unit area arriving at the gauge-end of the tube.The conditions at the pump mouth have been in-vestigated and an interpretation is given of theresults of some measurements with regard to thedistribution of the incident molecules. In ,conclusion,a method of measuring the speed of a pump isproposed which differs from Dayton's method. Thenew method is based on well-known concepts inthe physics of gas flow at very low pressures.It has been put forward before, but simply as apostulate.

2485: M. H. de Lange: Heat transfer in glass fur-naces from a theoretical and practical pointof view (Travaux IVe Congrès int. du Verre,Paris, July 2-7, 1956, pp. 148-152; Chaix,Paris 1957).

An introductory survey is given of the variousequations relevant to the calculation of the contri-bution of radiation to the transmission of heatthrough glass; certain equations relating to station-ary conditions are also established. These equationsare applied to the calculation of the vertical tempera-ture distribution in a vat of glass. The equationsare further applied to give an approximation to theeffect of external cooling on the temperature of theinside wall of the furnace. An attempt is made toexpress the effect of radiated heat in non-stationaryconditions, notably during the heating and thecooling of the glass.

2486: Y. Haven and J. M. Stevels: Note on themechanism of ionic transport in glass (Tra-vaux IVe Congrès int. du Verre, Paris, July2-7, 1956, pp. 343-347; Chaix, Paris 1957).

Note drawing attention to information that canbe derived from a comparison of diffusion and con-ductivity data, and to suggest that the mechanismof transport of Na + ions in glasses uses interstitial-cies in certain cases (similar to the mechanism sug-gested by McCombie and Lidiard for Ag+ in AgCl),whereas in other cases it is possible that vacanciesare involved. The potential minima between thesilicon-oxygen network available for the Na+ ionsare divided into sites -" intersites and vacancies.Some examples are discussed which give some ideaof the utility of the refinement of the theory givenin this paper.

2487: A. Kats: The interaction of U.V. and X-raysradiation with silicate glasses and fusedsilica (Travaux IVe Congrès int. du Verre,

Paris, July 2-7, 1956, pp. 400-411; Chaix,Paris 1957).

The formation of imperfections in silicate glassesand in fused silica under the action of X-rays hasbeen studied. Optical absorption bands are observedin the region 2000-10000 Á; they are attributableto centres which have captured electrons or holes.A number of alkali silicate glasses were irradiatedat low temperature and measured at low tempera-ture (80 °K).1t is shown experimentally that the finalresult of the irradiation (either by u.v, of sufficientenergy or by X-rays) is the loss of an electron bysome of the oxygen ions or a displacement of thelatter. It is still unknown via which intermediatetransition states the centres are formed. Paramag-netic resonance measurements show the existenceof electrons and holes attributable to the samecentres as found optically. Imperfections in fusedquartz after irradiation have also been investigated.It is shown that these may be due to centres origi-nating from aluminium impurities.

'2488: G. Diemer and W. Hoogenstraaten: Ambi-polar and exciton diffusion in CdS crystals(Phys. Chem. Solids 2, 119-130, 1957, No. 2).

The diffusion of photoconductivity in non-illu-minated parts of CdS single crystals has been stu-died both on unactivated samples and on samplesactivated with Ag. Diffusion lengths ranging froma fraction of a micron to several hundreds of mi-crons were observed. Measurements of the spectralresponse, the temperature dependence of the dif-fusion length and the P.E.M. (photo-electro-mag-netic) voltage make it probable that the large valuesof the diffusion length are due to exciton diffusion.An activation energy of about 0.1 eV was foundfor thermal ionization of the excitons. At room tem-perature, in most of the Ag-activated crystals thethermal diffusion of the conductivity is a combina-tion of ambipolar and exciton diffusion, a theoryof which is given for a one-dimensional case.

2489: O. Reifenschweiler: Ionenquellen für kern-physikalische Untersuchungen (Elektrotech-nik und Maschinenbau 74, 96-103, 1957,No. 5). (Ion sources for nucl~ar research; inGerman.) .

Survey of the more important ion sources used innuclear research. The various types of sources aredescribed, against the background of their commonworking princple. Some results of work on H.F.ion sources delivering currents of the order of 10mA are given.

,,--------------------~----_._--_._- ....,..--..-

244 PHILIPS TECHNICAL REVIEW

2490: N. W. H. Addink: Excitation energies in linespectra (Spectrochimica ,Acta 9, 158-159,1957, No. 2).

The calculation (according to Boltzmann's law)of the number of excited ions may not be carriedout on the basis of simply adding the energies ofionization and excitation. The percentage of ionsmust first be determined (following Saha) andBoltzmann's distribution law then applied to thenumber of ions so found.

2491: S. Woldring: Continue onbloedige bloeddruk-meting bij de mens (Ned. T. Geneesk. 101,949-952, 1957, No. 20). (Continuous non-bloody blood-pressure measurement in man;in Dutch.)

Blood pressure in the arteries of the hand ismeasured on the principle of the "relaxed arterialwall". The pressure gradient established by theelastic tension of the arterial wall is overcomeby compression of the surrounding tissues in aplethysmograph-like system, connected to a low-compliance manometer. Sample records of bloodpressure under varying circumstances are given.

2492: J. H. N. van Vucht: Beitrage zur Kenritnisdes Systems Cer-Aluminium (Z. Metallk. 48,253-258, 1957, No. 5). (Contribution to thestudy of the system cerium-aluminium; inGerman.)

The system Ce-Al was investigated by metallo-graphic, thermoanalytical and X-ray diffractionmethods for cerium concentrations above 50 at. %.Neither the compound Ce2AI, nor the compoundCeaAl2' could be affirmed. Instead we discoveredthe existence of two modifications of a compoundCeaAl. Below 230°C, CeaAIhas a hexagonal NiaSnstructure, above that temperature it is cubic withCuaAu structure. The compound CeAI was indexedas an orthorhombic one. A table with observedspacings and intensities is given.

2493: H. T. Schaap: De waterstofziekte van koper(Metalen 12, 204-208, 1957, No. 11). (Hy-drogen embrittlement of copper; in Dutch.)

A survey, with some results of the author's ex-periments, is given of the well-known effects -ofhydrogen embrittlement of copper. Copper contain-ing more than 0.01 weight % oxygen becomes brittle

yOLUME 19

when heated in hydrogen above 400°C. Hydrogendiffusing into copper containing inclusions of Cu20reduces this oxide to metallic copper and watervapour. High pressures of water vapour are builtup in this rednetion causing local rupture alonggrain boundaries and resulting in embrittlement ofthe metal as a consequence of the fact that the rateof diffusion of hydrogen in copper is much higherthan that of water vapour. In addition to thesephenomena hydrogen embrittlement shows severalother aspects, e.g. (a) cracks on the surface, (b)change of dimensions, (c) blisters on the surface,(d) series of holes along the grain boundaries,(e) holes in the interior of the grains. The conditionsunder which hydrogen embrittlement is encounteredare described. Oxygen must be present as Cu20or as oxides from foreign metals whose affinity foroxygen is not much greater than that of copper'and which show a' meas~able solubility in thatmetal. Our own experiments make it doubtfulwhether oxygen in solid solution may cause em-brittlement. Small quantities of water vapour in thegas atmosphere can only be harmful in the presenceof a material dissociating the water vapour, forinstance chromium.

Now available

F. M. Penning: Electrical discharges in gases,Philips Technical Library, pp. viii-l- 75, 29 figures.

This book is a translation of the Dutch original,which appeared in 1955, two years after the author'sdeath. Although in recent years several new bookshave appeared on the subject 'of gas discharges, it isfelt that the present book nevertheless fulfils a need,offering as it does a concise synopsis which canprofitably be used by students as a basis for furtherstudy,The contents are as follows: 1. Gas discharges,

natural 'and man-made; 2. The conduction of elec-tricity in metals and gases; 3. Non-self-sustainingdischarges; 4. The movement of electrons and ionsthrough a gas; 5. The' non-self-sustaining are dis-charge; 6. The. Townsend discharge and breakdown;7. Sparks and lightning; 8. The glow discharge; 9.The self-sustaining are discharge; 10. The positivecolumn. A bibliography and an index complete thebook.