Dielectric loss due to polar molecules in solid paraffin...

Dielectric loss due to polar molecules insolid paraffin wax

By D. R. P elmore

( Communicated by E. V. Appleton, F.R.S.— Received 27- 1939)

Introduction

According to most theories of dielectric loss the maximum rate of change of dielectric constant and the maximum value of the specific energy loss per unit volume occur a t an angular frequency oj( = 2nv) which is the inverse of a quantity r known as the relaxation time of the dielectric.

The relaxation time is the time required for the polarization of the dielectric to revert to 1 fe of its value after the removal of the applied electric field: and this is a quantity which can be determined experimentally.

According to Debye’s theory of polar molecules, part of the dielectric polarization is due to the orientation of the dipoles in line with the applied field and the relaxation time is related closely to the time taken for the molecules to revert to their random positions after removal of the field.

On the assumption that Stokes’ law is applicable to the polar molecules, it can be shown that

( 1 )

where a = molecular radius, y ■= coefficient of viscosity, |= Boltzmann’s constant.

Thus from the experimentally determined values of r a t any temperature we can obtain values for the molecular radii of the rotating molecules in terms of the viscosity. The method has in fact been used for the estimation of molecular weights of substances in solution: thus Bridgeman (1938) finds that the molecular weights of zein, gliadin, haemoglobin, etc., deduced from dielectric measurements on the solutions in benzene, are in agreement with the values obtained from other methods.

Debye’s theory was based on assumptions which are really valid only for gases and dilute solutions of polar substances in non-polar solvents, but the work of recent experimenters has shown that the theory is also able to explain the observed dielectric properties of many solids.

Thus Jackson (19356) showed that the dielectric relaxation time of a chlorinated diphenyl was proportional to the macroscopic viscosity over a

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wide range of temperature and that a change of state from a brittle glass to a mobile liquid did not alter the form of the curves of the dielectric loss as a function of temperature. Jackson (1935a) also found tha t the behaviour of cetyl palmitate (spermaceti) mixed with solid paraffin wax could be accounted for on the assumption tha t the power loss was due to the rotation of the polar molecules (or part of them) and that the system had the characteristics of a polar liquid. These experiments are still less amenable to interpretation by Debye’s analysis, because the paraffin and the ester molecules consist of long, rigid chains of carbon atoms, and it is not permissible to liken such to a sphere. Jackson suggested tha t the polar group might possibly be capable of rotation in the ester chain and that a rotation of this group did not involve a rigid body rotation of the whole ester molecule. According to this supposition he deduced from Debye’s equation (equation 1) th a t the internal viscosity of solid paraffin wax was of the same order as the viscosity of castor oil.

Jackson’s experiments were extended by Sillars (1938) who studied the behaviour of a solution consisting of 5% of various normal esters dissolved in the same wax which Jackson had used. Sillars’ experiments showed th a t the relaxation time in this wax depended enormously on the chain length of the ester, thus showing tha t Jackson’s suggestion of free rotation of the polar group was untenable: for if this group could rotate without carrying the whole molecular chain with it, then the length of the chain would have no effect on the relaxation time. Sillars also showed that the relaxation time appropriate to a given chain length did not depend on the position of the polar group in the chain: thus the relaxation time of butyl palmitate was the same as th a t of ethyl stearate, despite the movement of the polar group from a position in the first at 1 /5th of the chain length to 1/10th in the second. The relaxation time increases as the length of the ester chain is increased, but becomes substantially constant when the ester is longer than the paraffin molecule.

This paper describes experiments which are a logical extension of Sillars’ work. In the main, it examines how the relaxation time is affected by altering the chain length of the wax in which a given ester is dissolved: in contrast to Sillars who changed the length of the esters dissolved in a given wax.

Methods and apparatus

The loss angle of a condenser containing the material under test was measured at various temperatures and frequencies: the loss angle of the material was then deduced after subtracting the previously determined

Dielectric loss due to polar molecules in solid paraffin wax 503



504 D. R. Pelmore

loss angle inherent to the circuit, when the condenser did not contain the test specimen. The condenser was identical in size and construction with that used by Sillars, and has been described already (Sillars 1938). At radio frequencies the loss angle of the circuit was determined from the resonance curve obtained by varying a small calibrated condenser connected in parallel with the main condenser. (In Sillars’ measurements, the resonance curve was obtained by varying the frequency, the parameters of the test circuit being invariable.)

The loss angle of the material is given by

where C = capacity of test condenser as measured by substitution,Cs = stray capacities in test condenser and leads (i.e. that part of C

which is not due to the solid dielectric),AC\ — width of resonance curve at 1/ /̂2 of its maximum height,AC2 = the same width when the test condenser contains only air.

The value of Cs/C was less than 10 %.Experience shows that repeated heating and cooling sometimes alter the

capacitance by a detectable amount, indicating that the effective value of Cs has increased slightly. For this reason equation (3) must tend to yield a value for F which is less than it ought to be: Sillars suggests (Sillars 1938, p. 69) that uncertainty from this cause may be as much as — 7 %.

A Schering type bridge with Wagner earth was used to treasure the loss angle at frequencies up to 25kc./sec. The methods of preventing condensation and leakage have been described already by Sillars.

These experiments extend Jackson’s work on mixtures of spermaceti with a paraffin wax. For this purpose a number of waxes with different setting point were procured and in all cases the form of the curve relating loss angle and temperature was similar to that found by Jackson, and is described approximately by Debye’s equation

tan 8 = a c 1- a c 2 c a c 1- a c 22 C C - C s ~ ’

R esults and discussion

yJ{e0eoo)A 2 + (1)2T2’(4)



where e0 is the dielectric constant for static fields,Coo is the dielectric constant for high-frequency fields, to is the angular frequency of the alternating field, t is the relaxation time of the polar molecules a t the relevant tem

perature.


I t follows from (4) tha t tan 8is a maximum when tor and in practice the value of A differs very little from unity. When tor< ̂1, the permanent dipoles can rotate freely without appreciable dissipation of energy: when (i/r> 1, the rotation is negligible and hence again the energy loss is very small.

In general tan 8 is measured as a function of temperature for some constant value of'&>. At low temperatures the viscous restraint on the polar molecules is very great, thus making the relaxation time very long: hence tor > 1. At high temperatures this restraint becomes small, and hence tor < 1. I t is only for a narrow range of temperature that 1/r is comparable with to and within which appreciable power loss takes place.

tan 8 has been measured as a function of temperature for several different constant frequencies. Jackson made these measurements on a 57-60° C setting point wax and showed that if a wax of slightly different setting point was used then the curves of tan 8 were shifted along the temperature axis by about the same amount as the change in setting point.

In order to obtain more information about the dependence of relaxation time on the nature of the paraffin wax medium, it was thought desirable to use waxes with a wide range of setting point. The writer wishes to take this opportunity of thanking the Burmah Oil Company for presenting the waxes and to members of their Research Department for discussing the problem with him and for supplying the following information about the samples chosen:

(1) Fraction (1) from “ Match W ax” , s.p. 35-5° C.(2) Fraction (6) from “ Match W ax”, s.p. 40-2° C.(3) Fraction (2) from 130/135° F grade, s.p. 50-0° C.(4) Fraction (5) from 130/135° F grade, s.p. 55° C.(5) Fraction (2) from 145/150° F grade, s.p. 59*3° C.

The above samples were obtained by fractional distillation under reduced pressure of the specified commercial grades. They probably consist of mixtures of straight chain normal paraffins, together with some oily impurity.



506 D. R. Pelmore

All these fractions were recrystallized twice from benzene, which removes the oily impurities: in some cases the setting point was slightly higher after recrystallization. Care was taken to remove the last traces of the solvent by passing carbon dioxide for many hours through the molten wax. (Note: I t is important to avoid heating the wax in the presence of air, because the oxidation products have a deleterious effect on the dielectric properties— see Jackson 1935a, p. 201.)

(6) Residue from 125/130° F grade, s.p. 64-3° C.(7) Residue from 145/150° F grade, s.p. 70-1° C.Samples (6) and (7) are believed to contain substantial proportions of

“ iso” (branched chain) paraffins.(8) Wax from “ slop wax distillate”, s.p. 70-3° C.This is believed to consist of normal paraffins and required no further

purification.(9) “ Sucker rod w ax” no. 2, s.p. 94-4° C.This wax is the highest setting point paraffin procurable from petroleum.

I t is not quite pure and probably contains a small amount of oil: further purification, however, is troublesome, because of its small solubility in all ordinary solvents.

In order to obtain a wax having a melting point much lower than can be obtained from petroleum, the writer prepared the synthetic hydrocarbon

(10) Hexadecane, m.p. 15*5° C. Made from cetyl alcohol by convertingto the iodide and reducing this by means of zinc dust and acetic acid: the product was fractionated under reduced pressure. ■

The dielectric loss inherent in the hexadecane was measured in the temperature range to be used subsequently in experiments on mixtures of esters with it: it was found that tan was too small to measure a t room temperature but increased to 0-3 % at — 60° C (compare with Jackson’s experience, loc. cit., of 0-005 % for 57-60° C wax). This residual loss is probably due to impurities: it is, however, small compared with the maximum values of tan 8 (2-1 %) which occur when the wax contains 5 % of spermacete or of ethyl stearate. The value of tan 8 appropriate to these two esters was deemed to be the measured value less tha t found for the hexadecane at the relevant temperature.

Two further synthetic paraffins were kindly made by Mr E. L. Simons:(11) Octadecane, C18H38, m.p. 27-28° C. From octadecyl alcohol by the

same method as (10).



(12) Docosane, C22H 46, m.p. 44° C. From erucic acid. The ethyl ester was reduced to ethyl behenate with hydrogen using a palladium catalyst and this was then reduced by the Bouveault Blanc method to the alcohol C22H45OH, which was then converted to the paraffin as before.

The effect of temperature on the relaxation time of spermaceti in all these waxes can be presented conveniently by plotting log 1 // against Tm, where Tm is the temperature at which tan $ is a maximum for frequency / . I t will be remembered th a t a t this maximum ojt — 1 , and hence tha t


log 1 I f = log 27

temperature °C

F ig . 1. The temperatures in the figure denote the setting points of the waxes.A different sign is used to denote the points for each wax.

Both Jackson and Sillars found tha t the relation between log 1/f and Tm was linear over a wide range.

This representation for the waxes (1)-(12) is used in fig. 1. Doubtless it will be noticed tha t each fine in fig. 1 is determined from a few points only: since, however, these lines are all sensibly parallel and moreover parallel to those obtained by Sillars (1938, fig. 6) for various esters in a 57-60° C wax, there seemed no doubt tha t the linear relationship between log l / f and Tm was fulfilled.

The data summarized in fig. 1 enable us to connect (at any constant frequency) and also f m (at any constant temperature) with the setting point of the wax: this is done in figs. 2 and 3 for/ = 105 cycles/sec. and 0° C respectively. (Note: The point in each graph which lies far off the curves



508 D. R. Pelmore

refers to a second peak which occurs in the wax of setting point 41° C—see figs. 6 and 8.)

I t is interesting to compare these curves with the results of Sillars for a series of esters of different chain length in a wax of setting point 57-60° C. He considered that the ester rotated as a whole about its long axis and found that the relaxation time depended mainly on its chain length and not on the position of the polar group: he plotted T m as a function of the number of carbon atoms in the ester chain. I t is therefore interesting to try to relate the relaxation time of spermaceti with the mean chain length of the wax medium.

100 KC/S

o 20 4-0 ou 0 20 4 0 60 80 IOOsetting point of wax °C setting point of wax °C

Fig. 2. Spermaceti in paraffin wax. Tem- Fig. 3. Spermaceti ̂in paraffin wax.perature for maximum tan 8 (Tm) and log10 2m and setting point of wax. setting point of wax.

As stated previously, there is reason to believe that the waxes 1-5,8 and 9 consisted mainly of mixtures of normal paraffins. The waxes gave sharp X-ray powder photographs which showed that the principal chain length was approximately that of the pure paraffin having the same setting point. Accordingly, in figs. 4 and 5, T m and log 2ttt respectively are plotted against the number of carbon atoms inferred from the setting point. (The values were read from a curve connecting melting point and number of carbon aioms) (Moullin 1938).

Spermaceti consists mainly of cetyl palmitate (C15H31COOC16H33), together with esters of comparable chain length and a small proportion of other high aliphatic compounds. Having regard to Sillars’ demonstration




tha t the relaxation time is insensitive to the length of the ester chain provided this exceeds about 30 carbon atoms, it is considered justifiable, for our purpose, to regard spermaceti as though it were homogeneous and of

o -to

chain length (no. of atoms)Fig. 4. Spermaceti in paraffin wax. Temperature for maximum tan (Tm) and

chain length of wax. The point marked thus -> refers to spermaceti by itself.

chain length (no. of atoms)Fig. 5. Spermaceti in paraffin wax. log10 2nr and chain length of wax.

chain length 33 (that is, 32 carbon atoms plus one unit for the oxygen link in the ester group).

We must now attempt to interpret the information contained in figs. 1- 5, and in particular that the curves in figs. 2-5 all flatten out for the higher paraffins, whereas the relaxation time in the lower paraffins is sensitive to

Vol 172. A 33



510 D. R. Pelmore

the setting point, a behaviour which has an exact parallel in the dependence of the relaxation time of a series of esters in a particular paraffin wax (Sillars 1938).

According to Debye’s theory of dilute solutions of polar molecules in a non-polar liquid solvent, the relaxation time is given by equation (1),

n a m e l y ’ 47Tva*r = ^ L . (ibis)k T

In our experiments, as with those of Jackson and of Sillars, it is not possible to assign a value to tj: however, the value of rja3 deduced from (1) is grotesquely small (even if a? is taken as 100 A3, the volume of H C 02CH3, Jackson found that the value deduced for tj was of the same order as that of castor oil). I t is clear that the Debye theory combined with the strict application of Stokes’ law cannot be used directly.,

I t is clear from the work of Muller and others that in certain circumstances long-chain molecules in the solid state are able to rotate about their long axes. Sillars considers that such rotation might be the cause of power loss in these substances, and the present experiments tend to support this view.

The restriction of this rotation may be of various kinds. Where X-ray examination discloses molecular rotation, the rotation does not occur until the temperature reaches a definite transition point which in general is near the melting point of the solid. Below this transition point the molecules are able to vibrate about one or other of two equivalent equilibrium positions, separated by a potential barrier: the difficulty of surmounting this barrier increases with the length of the molecule. I f our systems of esters and paraffins consist of true solid solutions and the £ster takes its place in the paraffin lattice, it is possible that the ease of rotation is governed by the length of the ester molecule provided this is less than the length of the wax and is governed by the length of the wax molecule when this is less than the ester. This supposition is supported by consideration of figs. 4 and 5 and also by Sillars’ results: this explanation, however, is not necessarily fundamental and moreover it seems unlikely tha t these systems are true solid solutions.

I t is important to know the precise environment of the ester molecules. Jackson assumed that spermaceti formed a solid solution with the wax, because of the similarity in structure. Some subsequent experimental evidence, however, seems to contradict this hypothesis.

The relation between concentration and freezing point was found for spermaceti, butyl stearate and octadecyl palmitate in paraffin wax: the molecular concentrations were plotted against the freezing point and it



was found tha t the points for all three substances lay on the same straight line for concentrations up to 50%. The molecular weights calculated from the gradient of this line and the cryoscopic constant of the wax (deduced from the latent heat) were found to be in good agreement with the known values. This is taken as evidence tha t the systems are eutectics although the complete freezing point curves for the binary mixtures show only a slight indication of a eutectic point.


® / 4 J KC/S• € / 9 KC/S

temperature °CF ig. 6. Spermaceti, m.p. 47° C.

If the mixtures consist of simple solid suspensions then we should expect the effect of the wax would be small and tha t the relaxation times would differ little from those obtaining in undiluted spermaceti. The dielectric loss characteristics of spermaceti itself were measured and the results are shown in fig. 6. Reference to this will show that the absorption curves are similar to those obtained by Jackson for 5 % of spermaceti in paraffin wax: moreover the magnitude of the absorption shows there is no great departure from the linear relation between concentration and absorption (Jackson 1935a, fig. 5). Fig. 1 includes a line for undiluted spermaceti and thus indicates the manner in which the wax matrix modifies the relaxation time. I t will be noticed that the absorption curve relevant to a frequency of 3460 kc./sec. in fig. 6 is not symmetrical and that tan falls rapidly when the temperature is but a few degrees below the setting point.



512 D. R. Pelmore

These curves for spermaceti in sucker-rod wax (s.p. 94° C) are also abnormal in this respect and are therefore shown in fig. 7. I t is significant that the temperature a t which tan 8 falls rapidly to zero is the same in both figures in spite of the high setting point in the latter case; this suggests that the spermaceti is present as a suspension in the sucker-rod wax and not in true solution.

® //77• se e sc/s* 3645 SC/S

temperature °CF ig. 7. 8-28 % spermaceti in sucker rod wax, s.p. 94° C.

.© 47e .• 3580 sc/s

-60 -40 -20 O 20 40 60

. temperature °CF ig. 8. 4-68 % spermaceti in paraffin wax, s.p. 41° C.

An abnormal absorption curve is shown in fig. 8 and relates to the behaviour of spermaceti in a wax of setting point 41° C: here the curve has two maxima disclosing two possible relaxation times, which probably correspond to two different environments of the polar molecules. (Note: Compare Sillars’ experience of two relaxation times for an impure sample of butyl stearate—see Sillars 1938, figs. 4, 5 and 8.) I t can be seen from fig. 2 that the



lower value of Tm is the one appropriate to a setting point of 41° C, whereas the higher value of Tm is appropriate to a setting point of 70° C.

The behaviour of spermaceti in the wax of s.p. 35*5° C was normal in respect of the shape of the absorption curves but the relaxation times (fig. 1) are longer than in any other waxes and even longer than for spermaceti itself. When the temperature reaches 22° C the relaxation time changes very rapidly with temperature, although the mixture is still solid, a behaviour which seems to be entirely analogous with the observation in a number of aliphatic compounds in which molecular rotation suddenly becomes possible a t temperatures a little below the m.p. (for example, see Muller 1937).

The behaviour in the synthetic paraffins octadecane (m.p. 27° C) and docosane (m.p. 44° C) was similar to that in the natural wax of s.p. 35*5° C: it can be seen from fig. 1 that the relaxation times in these paraffins are not greatly different from those in the spermaceti alone.

The absorption curves for the mixture with docosane show a sharp cut off at about 36° C corresponding closely with the transition point of the wax.

The mixtures with the wax samples (6) and (7), which are believed to contain isoparaffins, gave normal absorption curves and the relaxation times correspond closely with the setting points. Thus the line marked 64*3° C in fig. 1 is due to sample (6) and the point marked * on the fine marked 70-3° C is due to sample (7) which also sets a t 70° C but is believed to contain branched chains: the extent of chain branching in these waxes is not known, and it is not possible to infer the principal chain length from the setting point.

All the mixtures not specifically mentioned gave absorption curves exactly similar to those found by Jackson and the points in fig. 1 give the regions of temperature and frequency for which the loss angle is a maximum.

I t has been suggested earlier that the dielectric relaxation times of paraffin wax media might depend upon the chain length of the polar and non-polar molecules, whichever is the shorter. Against this hypothesis it has been pointed out that the paraffin wax ester systems appear to be simple solid suspensions and not true solid solutions. That the points for spermaceti itself in figs. 4-5 do not lie on the curves drawn through the other points shows that the chain length is certainly not the only factor: in spermaceti itself the relaxation time is longer than in the wax-ester systems at the same temperature, showing that there is more resistance to molecular rotation.

The points relating to mixtures of spermaceti with octadecane, docosane and the natural wax of s.p. 35-5° C have not been included in figs. 2-5




514 D. R. Pelmore

because it is clear from fig. 1 th a t these mixtures behaved very differently to all others. Fig. 1 may be interpreted as showing tha t in general there are two possibilities for the relaxation time in a wax-spermaceti system: either it will be substantially independent of the wax and the same as in undiluted spermaceti or else it will have a value given by curves 2-5.

The samples of octadecane and docosane were made in the expectation of obtaining relaxation times intermediate between those of spermaceti in hexadecane and those in the wax of s.p. 41° C: as can be seen from figs. 1-5, this expectation was not fulfilled.

A further attem pt to bridge the gap between hexadecane and the wax of s.p. 41° C was made by mixing spermaceti with a mixture of hexadecane and docosane (50 % of each) (s.p. of mixture, 32° C). In this mixture of spermaceti with two synthetic paraffins there were two relaxation times corresponding exactly with those in the same paraffins separately.

Some recent experiments of Muller (1938) show th a t an increase of external pressure not only raises the melting point of long-chain ketones but also raises the transition point above which molecular rotation can take place. I t is possible tha t the increase of relaxation time with increasing setting point of the wax is due to the increased hardness of the latter, which increases the relaxation time in the particles of spermaceti in the same way as the external pressure would increase the difficulty of molecular rotation.

The chain-length theory has, however, been taken as a working hypothesis and used to suggest further experiments.

Comparison of the behaviour of ethyl stearate with spermaceti in hexadecane j

I f the chain length of the ester is without effect on the relaxation time, provided tha t it is longer than the chain length of the molecules of the paraffin matrix then ethyl stearate (C17H 33C02C2H5) and cetyl palmitate (C15H31C02C16H33) should have the same relaxation time in hexadecane. Experiment showed that this expectation was substantially fulfilled, as may be seen by reference to fig. 1.

I t is interesting to contrast this result with those of Sillars for the same two esters in a paraffin wax medium of setting point 57-60° C (chain length about 26 carbon atoms):

Temp, for max. loss at Relaxation time in sec. at1 me./sec. — 20° C

ParaffinEster wax, 57-60° Hexadecane

Cetyl palmitate 13-5 — 40-5° CEthyl stearate — 31 — 43-2° C

Paraffinwax

1-56 x IO- 5 3-19 x 10- 8

Hexadecane 6-34 x 10- 9 4 x 10-9

*



I t was found, however, that the absorption in hexadecane was greater than in the harder wax media, being about twice as much for ethyl stearate and about four times as much for cetyl palmitate: it is hoped that this phenomenon will be discussed in a later paper.


Experiments with esters which contain two polar groups

Experiments have also been made with two esters of dibasic acids in a paraffin wax of setting point 57-60° C: the esters chosen were dioctyl sebacate,

C8H 17OOC(CH2)8COOC8H 17and dioctyl azelate,

C8H 1700C(CH2)7G00C8H 17.

The second ester (3*373 % concentration) gave a maximum tan of 0*33 %, whereas with the first ester (2*4 % concentration) no loss in excess of tan 8 —0*025 % was detected in the temperature range — 25 to + 60° C, and frequency range 224 to 3400 kc./sec. For the second ester T m —10° C for f — 224 kc./sec. and T m = — 2° C for / = 507 kc./sec. If these values are compared with the corresponding relaxation times obtained by Sillars for single esters, it will be seen they are sensibly the same as for a single ester of the same total length of chain, suggesting once more that the chain length is a predominant factor.

The sebacate and the azelate differ only in the number of carbon atoms separating the polar groups (the number being 8 and 7 respectively). If the zigzag chain of carbon atoms is straight and rigid in the solid state then it follows tha t the sense of the components of the dipole moments resolved perpendicular to the long axis are opposite in the sebacate and like in the azelate. Hence these experiments support the view that absorption of power is due to molecules of ester which rotate as a rigid whole (compare Wyman 1938) about their long axes, the magnitude of the absorption depending on the component of the dipole moment perpendicular to the long axis.

A further conclusion may be drawn which may prove to have considerable industrial significance. The results obtained clearly suggest that in seeking new low-loss dielectrics among natural or synthetic materials which contain compounds whose molecules are in the form of chains containing polar groups, the best results will be achieved if those molecules contain an even number of polar groups so arranged that one half of the dipole components transverse to the chain have opposite sense to the remaining half.



516 D. R. Pelmore

The experimental work was carried out in the Engineering Laboratory, Oxford, under the direction of Mr E. B. Moullin. The author is indebted to Mr Moullin for help with the electrical side of the work and criticism of the manuscript; to Dr J. C. Smith for continual help and invaluable advice with the chemical side of the work; to the Department of Scientific and Industrial Research for a grant which enabled the research to be performed.

Summary

This paper describes an extension of the work of Jackson and Sillars who investigated the causes of dielectric loss in systems consisting of paraffin wax and an aliphatic ester.

I t confirms their conclusions that the loss in these systems is due to the rotation of the polar molecules and can be interpreted in terms of the Debye theory.

The effect of different paraffin waxes has been studied and a suggestion is made that the dielectric relaxation time of these systems depends mainly upon the lengths of the carbon chains of the molecules of the shorter component, a conclusion borne out very strongly by the comparison of the behaviour of ethyl stearate and spermaceti in hexadecane as compared with their behaviour in a natural wax whose mean chain length was approximately 26.

Finally experiments were made with esters containing two ester groups in each molecule and in agreement with expectations it was found that the dielectric loss was positive or negligible according to the relative sense of the two polar groups: when these were arranged so that the' dipole components transverse to the chain had opposite sense, no dielectric loss was observed. On the other hand, when the dipole components had the same sense the dielectric loss had a maximum value (at any given temperature) for a particular frequency and the relaxation time corresponded closely with that anticipated from the chain length.

This appears to confirm the conclusion that the loss is due to the rigid rotation of the carbon chains about their long axes, and also to indicate tha t chain compounds containing polar groups should be most effective as components of low-loss dielectrics prepared for practical use, when the molecules contain an even number of polar groups so arranged that one-half of the dipole components transverse to the chain have opposite sense to the remaining half.




R eferences

Bridgeman 1938 J. Amer. Chem. 60, 530. Jackson 1935a Proc. Roy. Soc. A, 150, 197.— I935& Proc. Roy. Soc. A, 153, 158.

Moullin 1938 Proc. Canib. Phil. Soc. 34, 459. Muller 1937 Proc. Roy. Soc. A, 158, 403.

— 1938 Proc. Roy. Soc. A, 166, 316.Sillars 1938 Proc. Roy. Soc. A, 169, 66. Wyman 1938 J. Amer. Chem. Soc. 60, 328.

Absorption of penetrating cosmic ray particles in gold

B y J. G. W ilson, P h .D.

{Communicated by P. M. S. Blackett, F.R.8.— Received 15 May 1939)

1. Introduction

A considerable amount of experimental data on the energy loss of cosmic- ray particles in metal plates is now available. Much of this, however, represents work carried out before the separat3 nature of the hard and soft components was fully understood, so that in many cases unsuitable conditions make the interpretation of the results difficult. The soft component is known to consist of electrons, and these predominate in the cosmic-ray energy spectrum for energies less than 2 x 108e-volts. I t has been verified for these electrons tha t the energy loss by ionization (Corson and Brode 1938) and by radiative collisions (Blackett 1938) is in close agreement with the theoretical predictions. At energies greater than 2 x 108 e-volts, very few electrons are found at sea level, and for all higher energies the majority of the particles are now considered to be mesotrons. These, together with an uncertain, but small, number of protons form the hard component of the cosmic rays.

Absorption measurements for the hard component are more difficult than for the electrons, since the particles are, in general, of higher energy and the loss of energy in an absorbing plate is very much smaller. The early observations (Blackett and Wilson 1937; Crussard and Leprince Ringuet 1937; Wilson 1938 a) lead to the conclusion that a t an electron energy



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