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rader Wolframdraht die für Wolfram typische selek- tive Strahlung aussendet, ist die Strahlung einer Wolframwendel gemischt, die Außenseite der Wendel strahlt selektiv, die aus dem Inneren kommende Strahlung ist mehr oder weniger geschwärzt, d. h.

lichtökonomisch ungünstiger. Das Mischungsverhält- nis der selektiven und schwarzen Strahlung hängt von dem Windungsabstand der Wendel ab. Demgemäß ist bei gleicher Brenntemperatur die Lichtausbeute des geraden Drahtes größer als die der Wendel, und der Unterschied beträgt bis zu etwa 8%7. Am un- günstigsten verhält sich eine Wendel, wenn der Win- dungsabstand etwa die Hälfte des Drahtdurchmessers beträgt. Mit enger werdendem Abstand nimmt der Anteil der schwärzeren Innenstrahlung ab, mit größe-

7 Vgl. E. Lax u. M. P i r a n i , Hdb. Physik von Geiger u. Scheel XIX, 434 [1928], ferner G. R . F o n d a u. A. A. V e r n o n , J. opt. Soc. America 22, 223 [1932].

rem Abstand kann auch die Innenseite der Wendel zunehmend frei strahlen. Wird der Abstand größer als der Drahtdurchmesser, so nähert sich das Verhal- ten der Wendel dem des geraden Drahtes.

Bei den üblichen Einfachwendeln beträgt, wie oben erwähnt, der Abstand rund einen halben Drahtdurch- messer, die Strahlungsverhältnisse sind demgemäß am ungünstigsten. Bei der Primärwicklung der Dop- pelwendel wird der Abstand von vorneherein schon nahezu gleich dem Drahtdurchmesser gewählt. Durch das zweite Wickeln um einen relativ engen Kern wird die Primärwendel noch stark aufgespreizt, so daß der Wendelabstand das P^-fache des Drahtdurchmessers und mehr beträgt. Die dadurch bewirkte Verbesse- rung der Strahlungsökonomie kann wohl auf etwa 4 % der Lichtausbeute geschätzt werden, wodurch der nach dem Langmuirschen Prinzip nicht geklärte Teil der Qualitätssteigerung seine Deutung findet.

B E R I C H T E

Photographic Sensitivity and Chemical Sensitisation of Emulsions*

B y W . F . B E R G

Besearch Laboratory, Kodak Limited, Harrow, Middlesex (England)

(Z. Naturforschg. 6 a, 408—411 [1951]; eingegangen am 1. Juni 1951)

To Professor John Eggert for his 60th birthday

A

s a result of the war, a considerable amount of mat- serial has been published which would normally be hidden behind the silver curtain1 that guards the emul- sion makers' secrets; this material was discussed recently at a conference held at B r i s t o l2 and it may be useful to attempt to give a coherent picture of our knowledge in this field.

Unfortunately, as mostly in photographic theory, it is necessary to adopt deductive reasoning, the process being too complicated to allow the fundamental mechanism to be derived from experimental experience. This inevitably lends our picture an indirect and unsatisfactory atmo- sphere, and few points are capable of direct and unequi- vocal proof. It might be safer to label any of the mechanisms discussed as working hypotheses rather than as fully fledged theories.

The G u r n e y - M o t t theorv3 has found almost uni- versal acceptance in describing the mechanism of latent- image formation in a dry emulsion layer; it quite

* Based on the Benwick Memorial Lecture of the Boyal Photographic Society, held in Birmingham, England, on 18.4. 1951.

1 With apologies to Dr. H. Baines for this plagiarism.

2 'Fundamental Mechanisms of Photographic Sensiti- vity' Butterworth, London 1951.

deliberately avoids the discussion of the preparation of the sensitive emulsion. It is well known that this theory visualises the latent image as a small speck of silver which is produced during exposure by the separate movement of photo-electrons and silver ions, which condense on a nucleus, such as for example a speck of silver or silver-sulphide, which acts by trapping the electron. It is interesting to speculate upon the fact that E g g e r t4 suggested very much the same mechanism in 1926, but this was not taken seriously at the time, apparently even by its originator, and was soon forgotten.

It seems that the time was simply not ripe for this advance in thought.

T h e d e f i n i t i o n of p h o t o g r a p h i c s e n s i t i v i t y

We are here interested in the number of quanta which an emulsion grain must absorb to become developable under given conditions of exposure and processing;

sensitivity in our sense is defined in terms of this number.

3 G u r n e y and M o t t , Proc. Boy. Soc. [London], Ser. A 164, 151 [1938].

4 E g g e r t, Z. Elektrochem. angew. physik. Chem.

32, 491 [1926],

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. A. Á

jj^k 9 ^^^ 10 A *

12

A A

13

1

17

18 Ä„ A

20

Abb. 1. Normaler Brennkonus, etwa stöchiometrisehes Gemisch.

Abb. 2, 3. Wie Abb. 1, Strömungsgeschwindigkeit nahe gleich normaler Verbrennungsgeschwindigkeit (Zündge-

schwindigkeit), fast ebene Flamme.

Abb. 4, 5. Ähnlich den beiden vorhergehenden Abbildungen jedoch teilweise abgelöst.

Abb. 6. Fette (kohlenwasserstcffreichere) Flamme; die ur- sprünglich ebene Brennfläche asymmetrisch deformiert, an

einer Stelle Rußaustritt.

Abb. 7, 8. Fette Flamme mit Leuchtspitze und Beginn von Störungen im Brennkonus.

Abb. 9—11. Leicht überfettete Flammen mit Störungen der Brennfläche und n-zähliger Symmetrieachse (n > 3).

Abb. 12—17. Leicht überfettete Flammen, ähnlich den vorangehenden, jedoch ohne Symmetrieachse und nicht

mehr stationär.

Abb. 18—20. Stärker überfettete Flammen mit asymmetri- schen Störungen und mehr als einer instationären

Rußzunge.

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21 22 23 24 25 26 42 43 44

4

31 35

t '

r

36 Abb. 21—26. Leicht überfettete gestörte Flammen, von oben photographiert, die letzten davon etwa den Aufnah- men 9 und 10 entsprechend. Abb. 27—34. Freischwebende stationäre Flammen, leicht bis stark überfettet.

J

45 Abb. 35—38. „Hüpfende", stark überfettete Flammen. Abb. 39—47. Schattenaufnahmen. Abb. 39. Stöchiometrisches Gemisdi, normaler Brennkonus. Abb. 40. Überfettet, mit Bußaustritt aus der Spitze.

46 47 Abb. 41, 42. Flache Flammen. Abb. 43—45. Gestörte Flammen. Abb. 46, 47. Freischwebende Flammen, entsprechend Abb. 33, 34.

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In practice this can never be determined for an individual grain and we can determine average values only, taken over, for example, the grain-size frequency distribution, the distribution of the quanta over the different grains, as influenced by their position in the emulsion layer, and the distribution of sensitivity amongst the grains. A com- plete analysis of the individual factors is never possible, but a useful discussion of the problem has been given by B u r t o n5. If these limitations are kept in mind, quite useful statements can be made regarding the change in sensitivity of the average individual grain on chemical sensitisation.

T h e n o n - s e n s i t i s e d e m u l s i o n g r a i n A description or definition of a non-sensitised emulsion is necessary so that we may have a starting point from which to study any one process of sensitisation. L o w e , J o n e s and R o b e r t s6 refer all their work to a 'primitive' emulsion which is obtained by excluding carefully all processes of chemical sensitisation known to them, or by subjecting a normal emulsion to bromination followed by a 'normalising' treatment with sodium nitrite or sulphite which removes surplus bromine from the grains without sensitising them. The same result is obtained with both methods, the emulsion giving low speed and contrast when developed in a grain-surface developer, and considerably higher speed when processed in an internal developer which contains a silver halide solvent and thus reacts to latent images situated inside the grains.

They explain the remaining low surface sensitivity by the existence of 'primitive' non-oxidisable traps on which the latent image forms in non-sensitised emulsions. Whilst their methods yield a reproducible starting point of low sensitivity, it is impossible to decide whether their grains are chemically non-sensitised, and work by L o e n i n g7 suggests that a much lower level of sensitisation of silver halide micro-crystals is possible.

S e n s i t i s a t i o n of s i l v e r h a l i d e s o l s He showed that silver bromide sols produce latent images which allow the processes of development, fixation, etc., to be carried out by techniques quite closely related to normal methods even to the extent of demonstrating surface and internal images.

A latent image in a sol, prepared like an emulsion with a surplus of bromide ions, but without gelatin, is produced with a very low efficiency, the time-scale characteristic curve showing low slope, low maximum density and early reversal. The reversal is due to fading of the latent image, which is accompanied by a complete desensitisation described as 'depletion'. This is not due to a deficiency of the photolysis which in a sol be followed into the

s B u r t o n , Ref. i, S. 188.

6 L o w e , J o n e s and R o b e r t s , Ref. p. 111.

7 L o e n i n g , Ref.1, p. 124.

8 P a u l i , Helv. chim. Acta 32, 795 [1949].

f S h e p p a r d , Photographic J. 65, 380 [1925].

ioa c 1 a r k , Brit. J. Photogr. 74, 227, 243 [1927],

latent image region by direct titration methods. L o e n i n g found it difficult to reconcile this behaviour with the normal picture of the latent image consisting of specks of metallic silver and assumed that it was 'colloidal' silver in the sense of P a u l i8, i. e., silver with complexes formed with the surrounding ions, depletion being due to a breakdown of the complexes to metallic silver. This was thought to be catalytically less active than the colloidal silver.

He found that the low sensitivity of the sol and the instability of the latent image were cured by adding gelatin, independently of the nature or chemical purity of the gelatin. He considered that the well-known stabilising action on colloids of gelatin did not account for its action completely, since other treatments whidi stabilised the latent image and thus inhibited the reversal of the characteristic curve, did not increase the sensitivity appreciably. Conversely, sensitising agents were found, such as sodium nitrite, which did not eliminate the fading.

L o e n i n g therefore suggested that a complex of gelatin with silver ions, which is inevitably formed on the crystal surfaces, acted as a sensitiser, forming traps in the G u r n e y - M o t t sense in the course of exposure, the complex itself having- been demonstrated to be sensitive to light. Whatever the underlying mechanism, however, it is important to realise that gelatin stabilises the latent image and acts as a powerful sensitiser in the most primitive photographic silver halide system studied so far. Other colloids were found to have similar effects;

in making conventional emulsions, this sensitisation occurs and we take it for granted.

R e d u c t i o n s e n s i t i s i n g

The classical picture of chemical sensitisation was that by S h e p p a r d9 and his co-workers; it may be sum- marised by stating that sensitisation is brought about by forming on the grain surfaces minute specks of silver sulphide, produced from sulphur compounds normally present in the gelatin. It has often been suggested in the literature10, but never been proved11 that chemical sensitisation might also be brought about by metallic silver. Sensitisation can certainly be caused by a pre- exposure which forms stable but non-developable (under given processing conditions) 'Sub-image'12 specks, so that silver in similar form produced by chemical means should also act as sensitiser.

L o w e and his co-workers demonstrated that the addition of very small amounts of stannous chloride to an emulsion caused an increase in sensitivity, which was diaracterised by a chemical behaviour quite different from that typical for any other known method of sensi- tisation. The effect did not depend on the nature of the emulsion vehicle; this ruled out any connection with sulphur sensitising, and furthermore, sulphur sensitising was found to be additive to that by stannous ion. They

iob C h i b i s o f f , T i t o v and M i k h a i l o v a , J.

Physic. Chem. (USSR) 21, 643 [1947],

11 S h e p p a r d , Brit. J. Photogr. 75, 207 [1928],

12 Burton and Berg, Photographic J. 86B, 1 [1946];

88B, 84 [1948],

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Property Method of Sensitisation Property

Reduction Sulphur Light (l.i. & sub.i.) surface

sensitivity increased, bleached u

increased,

bleached bleached internal

sensitivity increased,

bleached unchanged,

not bleached not bleached surface fog bleached not bleached bleached internal fog not bleached bleaching not

detectable since surface not bleached

not bleached

Hg hyper-

sensitisation does not

function functions Hg laten- sification Table 1. Chemical behaviour of different methods of sensitisation. The bleaching is carried out by potassium

ferricyanide 5—100 parts per 10e of silver halide.

naturally deduced that stannous chloride acted by reducing the silver halide to* some form of silver.

The differences in chemical behaviour are set out in table 1, which shows that the result of reduction sensitising is essentially more reactive than that due to sulphur sensitising or pre-exposure (fog and sub-image); reduction fog seems to be rather similar to the latent image or sub-image. The work by L o w e et al. may thus be con- sidered to have established the separate entity of reduc- tion sensitisation; this realisation has become important at once in correlating other methods of sensitisation.

M e r c u r y s e n s i t i s a t i o n

The method of hyper-sensitising dry photographic emulsion layers by mercury vapour discovered by Bank- 1 o h12a, was considered by D e r s c h and D ü r r1 3 to be due to a kind of amalgamation of silver specks, a view which was supported by the somewhat evanescent nature of the result, and by the fact that an existing sub-image could be intensified by mercury vapour. Since the effect of latent image intensification, by mercury vapour was quite stable, however, S h i b e r s t o f f1 4 suggested the hyper- sensitisation as being an example of reduction sensitising, according to the reaction Ag + + Hg = Ag + Hg+. This suggestion was supported bv L o w e et al., since they

12a B a n k 1 o h , Z. wiss. Photogr. Photophvsik Photo- chem. 25, 233 [1928],

13 D e r s c h and Dürr, J. Soc. Motion Picture Engr.

28, 178 [1937].

14 S h i b e r s t o f f , J. techn. Physics [USSR] 16, 395.

No. 4 [1946],

15 M u e l l e r , J. opt. Soc. America 39, 494 [1949], is K r o p f f , Photographische Ind. 1925, 1145.

17 Koslowski and Mueller, Agfa Film Plant, Wol- fen, Germany, Reports Sept.—Oct. 1936. B i b l i o g r a p h y of S c i e n t i f i c a n d I n d u s t r i a l R e p o r t s (US.

Dept. of Commerce, Washington 10, 873, PB. 70053, 831—50.

found that mercury hypersensitisation was not additive to reduction sensitising by stannous ion.

G o l d s e n s i t i s a t i o n .

Yet another method of chemical sensitising has been described recently by various authors, which, according to M u e l l e r1 5. . . ' is being used in modern emulsion manufacture. Yet it may not have been realised from some of these disclosures that it was the use of certain complex gold 'sensitisers' which led to one of the greatest advances in the manufacture of emulsions in this country (U.S.A.) about a decade ago. At this time the use of this technique made it possible to raise film speeds by a factor of 4 or to produce fine-grain emulsions with a sensitivity which seemed previously impossible' . . . References to gold sensitisation go back to 19251°, but the first thorough investigation was stated 17 to have been carried out by Koslowski in the Agfa Laboratories in Wolf en and was reported in conjunction with M u e l l e r i7 in the course of industrial publications which appeared as the result of the last war.

M u e l l e r has stated that gold salts are effective only if used with thiocyanate, but not with cyanate ion*. Gold forms aurous complexes with both these ions, and both are solvents for silver halide. Cyanate, however, also attacks metallic silver, thiocyanate does not. M u e l l e r has therefore formulated a picture of gold sensitisation which assumes that reduction sensitising occurs during emulsion making, producing small silver specks inside the emulsion grains. These are laid bare by the thio- cyanate and the silver replaced by gold by a plating-out process. M u e l l e r and H o e r 1 i n 17 recently reported that with certain fine-grain emulsions quite spectacular increases in sensitivity, up to a factor of 5, can be obtained towards exposure to X-rays and y-rays. Thus it is clear that gold sensitising can effect quite important increases in practical sensitivity and the underlying mechanism is an interesting subject of study. The mecha- nism is elucidated a little by the findingthat gold sensitisation is most effective at short times of exposure, and a parallel may be drawn between this and X-ray exposures, since X-rays presumably act by means of secondary electrons whose passage through a single grain takes but a fraction of a micro-second. Intensification by a physical developer depositing gold has also been reported 19 to be most effective for high-intensity ex- posures.

Now H o e r 1 i n and M u e l l e r consider that the effect of gold treatment is to produce more effective electron traps on the grain surface. This, however, is not in agreement with our knowledge of the processes whidi are responsible for the loss of photographic sensitivity at high intensities of light. We haye seen above that ex-

* M u e l l e r ' s reference to cyanate is presumably a misprint for cyanide, since cyanate does not attack metallic silver.

18 M u e l l e r and H o e r 1 i n , J. opt. Soc. America 40, 246 [1950],

19 J a m e s , V a n s e l o w and Q u i r k , PSA [Phot.

Soc. America] Journal 14. 349 [1945],

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posure at high intensities forms the latent-image sub- stance in a highly disperse form12. There is in fact no difficulty for any electron trap present to function under high-intensity conditions; the difficulty occurs in the second stage of building up the sub-image to a full image, when the growing speck cannot cope with the inrush of photo- lytic silver. Thus a sub-image is formed quite readily under high-intensity conditions and can be shown up by means of a special method of development19 (when it functions as an ordinary latent image) or by means of the various latensification treatments described in the litera- ture. An ordinary developer cannot develop this sub- image without undue fog, but it would appear that development can occur freely if gold is present. The function of gold sensitising would thus appear to be that of facilitating development for an exposure which in emulsions not so sensitised does not produce a latent image speck of sufficient size to induce development.

To summarise what is known about chemical sensiti- sation, we shall have to distinguish at least four separately identifiable processes. Of these, the sensitisation of bare silver halide crystals in the form of a sol by means of the colloid, such as gelatin, seems quantitatively the most important; although the other methods of sensitisation have not been studied on sols so far, it is clear that this sensitisation must necessarily occur during emulsion making. The emulsion maker is then likely to employ any or all of the three other methods, sulphur sensitisation, reduction sensitising and sensitising by heavy metals such as gold. The resulting sensitisations are clearly distin-

guishable from one another by chemical methods or by the unmistakable identity of the sensitiser.

There is no reliable information in the literature as to whether or not reduction sensitising plays an important part during the last heat treatment (digestion) which an emulsion receives so as to increase its sensitivity, since the two papers on the subject6,10 b rather contradict one another on that point. Without distinguishing between reduction and sulphur sensitisation, it would appear that the function of digestion is to provide electron traps on the grain surfaces, since without them the speed for grain-surface development is low. It is known that during the normal process of digestion, the sensitivity of the emulsion increases more towards exposures at low inten- sities than at high20; thus the processes which are thought to build traps are those which improve low-intensity reciprocity failure. In the case of sulphur sensitisation at least, the sensitisation has been shown by W e s t and C a r r o l l2 1 to be accompanied by increased trapping of the photo-electrons released in the emulsion grains.

The function of gold sensitisation, on the other hand, seems to be to improve the developability of a small latent-image speck as against the developability of the fogged grain. Thus gold sensitisation removes essentially one of the causes of high-intensity reciprocity failure.

20 C h i b i s o f f , Kino-photochem. Ind. [USSR] 2, 96 [1934]; de L a n g h e , Z. wiss. Photogr., Photophysik Photochem. 36, 162 [1939].

21 W e s t u. C a r r o 11, J. chem. Physics 15, 529 [1947], see also W e s t , Ref. i, p. 98.

Einige Bemerkungen zu den ¿-Bestimmungen der letzten Jahre

V o n A . K A R O L U S *

(Z. Naturforschg. 6 a, 411—416 [1951]; eingegangen am 15. Mai 1951)

John Eggert zum 60. Geburtstage gewidmet

I

n den zusammenfassenden Berichten3 von S t i l l e , Du M o n d und C o h e n und von B i r g e wird für die auf Vakuum korrigierte Lichtgeschwindigkeit c0 an- gegeben:

Cq = 299,776 ± 4 km/sec.

Dieser Wert ist der Mittelwert aus 5 c-Bestimmungen, die alle auf optischem Wege, also unter Verwendung von sichtbarem Licht, während der Jahre 1928—1941 durch- geführt worden sind. Mit Ausnahme der Arbeit Nr. 2 (s. Tab. 1) verwenden die übrigen 4 Messungen die vom Autor 1925 vorgeschlagene Methode4, bei welcher die Lichtmodulation durch den Kerr-Effekt erfolgt. Die c-Bestimmung Nr. 2 bildet den Abschluß der Messungen

* Zollikon-Zürich, Höhestr. 52.

1 U. S t i 11 e , Z. Physik 125, 177 [1948],

2 J. W. M. D u M o n d u. E. R. C o h e n , Rev. mod.

Physics 20, 82 [1948].

3 R. B i r g e , Rev. mod. Physics 13, 233 [1941],

von M i c h e l s o n , P e a s e und P e a r s o n5, deren Methode, wie bekannt, in einer sehr schönen Kombination der klassischen Verfahren von F i z e a u und F o u c a u 11 besteht.

In der Tab. 1 sind in zeitlicher Reihenfolge 12 seit dem Jahre 1928 durchgeführte Messungen der Lichtgeschwin- digkeit zusammengestellt. Die Tabelle enthält alle dem Verfasser bekannt gewordenen c-Bestimmungen; nicht aufgenommen wurden Demonstrationsversuche und eine Messung, deren Fehlergrenze mit ± 70 km/sec angegeben ist6. Außer der Methode und dem Resultat sind in der Tabelle die von dem betreffenden Autor selbst veröffent- lichten Fehler aufgeführt.

4 A. K a r o l u s , Ber. Verh. sächs. Akad. Wiss. Leipzig, math.-physische Kl., Dez. 1925.

5 A. A. M i c h e l s o n , F. G. P e a s e u. F. P e a r - s o n , Astrophysik. J. 82, 26 [1935],

6 D. W. R. M c K i n 1 e y , J. Roy. Astron. Soc. Canada 44, 89 [1950],

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