US2419975A - Increasing speed and contrast of photographic emulsions - Google Patents

Description

DENSITY I I 22 v l,

y 1947. A. P. H. TRlVELLi ETAL 2,419,975

INCREASING SPEED AND CONTRAST OF PHOTOGRAPHIC EMULSIONS I Filed Aug. 2a, 1945 w 4/ F 1.2 H v 285 705' 1725 7,45 176s 778s .aruzs .45 a .85

LOG. E.-

ADRIAN P. H. TRIVELLI WILLIAM F. SMITH 'INVENTORS Patented May 6, 1947 INCREASING SPEED AND-CONTRAST "F rnoToGRArmo-EMULsIoNs Adrian P. H. Trivelli and William F. Smith, Rochester, N. Y., assignors to Eastman Kodak Company, Rochester, N. Y., a 001'1101'317101101? New Jersey implication August 26, 1943, Serial No. scones 16 Claims. (01. 95-6) This invention relates to photographic emul- 510115 and p a y to a method for increasing the speed and contrast of such emulsions.

The invention relates to the use -of hydrazine and related compounds in a photographic process.

Hydrazine and hydrazine derivatives have been used in photographic emulsions or developers for many purposes. One well known use of hydrazine compounds in photographic emulsions is that of a halogen acceptor or halogen absorber. During the exposure of a sen'sitive silver halide emulsion, the silver halide is thought to release halogen such as bromine or chlorine and-the "sensitivity of the material is improved if the halogen thus released is immediately combined with a compound which prevents it from recombining with silver. The use of hydrazines for this purpose is described in Caldwell U. S. Patent "956,567. This efiect of absorbing or accepting halogen is manifested only during exposure of the sensitive material.

-2 material but are used only to fog all of the unexposed silver halide.

It is an object dfthepresent invention'to provide an improved method for sensitizing photographic silver halide emulsions. A further object is to provide a new method for increasing the speed of photographic silver halide emulsions. A still further object is to provide a new method for increasing the contrast of photographic silver halide emulsions. further object is to provide a new method for processing photographic silver halide emulsions "to effect improved properties in the resulting image. "Other objects will appear from the following description of our invention.

These objects are accomplished by developing an undevelopedsilver halide photographic emulsion, after exposuraat a pH of about 9.5 or higher in the presence of a hydrazine, hydrazine salt, or hydrazine derivative of the general formula:

R [R The hydrazine "or other compound used in this way is intended to be used up during the exposure of the sensitive material and it is customary to R R remove the unused compound prior to development.

Hydrazine derivatives have also been used as anti-foggants in sensitive emulsions. This use is describedin Sheppard and Wightman U. S. Patent 1,763,990. When used for this purpose, the hy drazines prevent the formation of emulsion fog at the time of exposure of the sensitive material or shortly before or thereaftert They have no function in the development of the exposed material.

Hydrazines have also been used in photographic emulsions to increase the infra red speed and contrast of the material as described in Trivelli and Carroll U. S. Patent 2,245,236. When used in this way the hydrazines act in the infra-red region of optically sensitized material. It was not known that an increase in speed and contrast in the visible and ultraviolet regions of the spectrum could be obtained by the use of these materials.

Hydrazines have also been used in photographic developers for two purposes. They have been used as the developing agent and have also been used in fogging second developers to render reversal exposure of the material unnecessary. When used in this way'they do not affect the speed or contrast of the material. Since the efiective' speed and contrast are determined by the original development, the hydrazines in the second developer do not aifect these characteristics of the where at least two Rs are hydrogen atoms and the remainder are carbon radicals, more specifically an aliphatic radical, -a heterocyclic radical, or a carbon l group uirectr connected to an aliphatic carbon atom of a carbon radical, a heterocyclic carbon atom of acarbon radical, an oxygen atom of a carbon radical or a nitrogen atom of a carbon radical.

The drawing illustrates typical sensitometric curves showing the improvement obtained by our invention.

We secure an improvement in the sensitivity characteristics, that the speed or contrast, or'

both, of photographic material by developing the exposed material in the presence of a hydrazine or a compound related to hydrazine. It is important that these compounds be present at the time of photographic development since the effect which we obtain is a development effect. Our

compounds may, accordingly, be added to the.

as the developing agent or as the fogging agent 12. in a reversal developer and would not efiect an improvement in sensitometric properties of the photographic material.

The following classes of compounds have been found suitable for use according to our invention:

Hydrazines of the general formula:

in which R is hydrogen, an acid ester group or a heterocyolic group. The following compounds illustrate this class.

1. NHzNH2.2H C1 Hydrazlne hydrochloride 15 2. NHzNHzJIz O Hydrazine hydrate 3. NHzNH C O O 02H: 14.

Ethyl hydrazine carbonate C-NHNH:

2-benzothiazolylhydrazine 16.

/ NHNH: so

2-quinolylhydrazine 6. N HNH:

A 16. iew

4-hydraziuoquinaldine NHNHz 17.

2-pyridylhydrazine Alkyl or aryl hydrazines of the general formula:

where R and R, are alkyl or aryl groups.

The following compounds illustrate this group:

H O NHNH2.H Cl

p-Hydroxyphenylhydrazine H01 NNHz.HC1

as Dimethyl hydrazine hydrochloride 2,4-dinitrophenylhydrazine N NH2.HC1

a-Benzyl-a-phenylhydrazlne hydrochloride raw-O-mmmnor p-Nitrophenylhydrazine hydrochloride a-Methyl-a-phenyl hydrazine sulfate Hydrazides (acyl hydrazines) having the general formula:

0 R-PL-NHNH: or R- i-NHNH c-R' where R is an alkyl radical or a chemical bond to a similar radical and R is an aryl radical.

C( CIEhG ONHNH:

l-Acethydrazide pyrldinlum chloride CHzCONHNHCO-O fl-Benzoylacethydrazidopyridinium chloride The product made by adding ethyl chloroacetate to 2-pl1enyl pyrimidazole, followed by treatment with hydrazine. The structure is presumably as given CONHNH:

Malonyl dihydrazide pyridimum bromide o omo ONHNHQ H-C O NHNH:

Aconitlc acid trlhydrazlde Semicarbazides, carbohydrazides and amino biurets having "the formula:

ll lL-c-NHNm where -R i hyd o en and B is hydro n. z which may or may not be substituted, an .aryl radical, or a carboxylic acidiamide radical. s

24. N-HzC-ONHN-Hzfl c1 -Semicarbazidehydrochloride as. NH NfHC-ON-HNHaHGl 12.0

'Carbohydrazidehydrochloride I '25 N ,01 CHIC ONHNHOONHNH:

Acetcarbohydrazide pyridinium chloride .27.

N o ONHNHC ONHNH: Br on CONHNHC ONHNH:

Malonyldicarbohydrazido pyridinium bromide NHZN'HO 0 NH-QCI 4-(p-chlorophenyDsemicarbazide NHQNHC ONH JJHa 4-(o-to1yl)-semicarbazide NHzNHC 0 NH--- 7 0.1

4-(4-ehloro-2-methyl-pheny1)-semicarbazide 31.

\N/ O ONENHQ V om so;C om

N-methyl-Z-carbohydrazyl pyridinium petoluenesulionate 32. NH2.NH.OO.NH.CO.NH.NH2

Diamino biuret as. NHz.O0.NH.C 0.NH,NH:

Amino b u et Hydrazones. These compounds conta n the grouping:

Typical compounds are :iollows:

The increase sensitivity or contrast which We obtain by the introduction of hydrazines to the emulsion or developer, whether as watersoluble or as water-insoluble compounds increases considerably with increase in pH of the developer. The pH of the developer used with .ourmaterials should be about 9.5 or higher. With a developer having a pH of 12.8, the use of hydrazine in a silver chlorobromide emulsion produces a strong .efiect by which the gamma can be increased from 3 to a gamma of 10 or higher. With some emulsions a'gamma as *high as 24 has been obtained. Speed increase without gamma. increase, speed increase with gamma increase and gamma .increase without speed increase have been observed.

In addition to pH, the pAg or silver ion concentration, of the emulsions to which an active compound is added, is an important factor in controlling the effect which we obtain. Higher silver ion concentrations tend in general to .increase the effect as do high pHs. It is also important not to add carbonyl hardening a ents to the emulsion. If present in equim'olar concentrations such hardeners usually destroy the effect probably due to hydrazone formation. The explanation of this apparent inconsistency is that the hydrazones listed above are believed to decompose to release hydrazine or hydrazine derivatives. The hydrazones formed by reaction of the carbonyl hardening agents with gelatin are not readily decomposed and therefore do not improve the sensitimetric characteristics of the emulsion. Chrome alum may, however, be used as a hardening agent. The pAg of the emulsions which we use lies in the range of 6.0 to 9.5. Emulsions of various types such as chloride, bromide, bromoiodide, and chlorobromide may be employed. The effect has been found to be especially noticeable in the case of high contrast emulsions.

The increase in sensitivity or contrast which we produce is obtained with any type of developing agent such as the phenolic type, such as monomethyl-p-aminophenolsulfate, catechol, phydroquinone and pyrogallol. A developer of the Sodium sulfite (desiccated) grams 9O Hydroquinone "do-.." 45 Sodium hydroxide ..do 37 Potassium bromide do 30 Water to liters 1 A developer of the following composition, such as Kodak Formula D-l9, which his a pH of 10.2, is also suitable:

Monomethyl-p-aminophenol sulfate grams 2.2

Hydroquinone do 9 Sodium sulfite (desiccated) do 96 Sodium carbonate (desiccated) do.... 48 Potassium bromide do 5 Water to liters.. 1

It has been found that the novel effect produced by hydrazine or its derivatives may be extended to developers of normal pH, by regulating the concentration of sulfite in the developer. In developers of pH 10 or higher, such as Kodak Formula D-l9, the concentration of sulfite may be high. With developers of lower pH the concentration of sulfite must be decreased in order to achieve the advantages of the use of hydrazine or its derivatives. For example, a developer of the following composition, which has a pH of 9.7, and contains hydrazine dihydrochloride is suitable for increasing the speed of silver halide emulsions.

Monomethyl-para-aminophenolsulfate grams 2.2 p-Hydroquinone d 8.8 Sodium sulfite (desiccated) do 16.0

Potassium bromide do 5.0 Sodium carbonate do 48.0 Hydrazine dihydrochloride do 0.2 S-Nitrobenzimidazole nitrate do 0.05 Water to ..liters l The improved eifect obtained by controlling the sulfite concentration of the developer is not limited to the particular formula stated above. The formula which may be employed depends upon the type of emulsion and the sensitometric characteristics sought. The anti-foggant employed in this developer is not essential although its use is desirable for controlling fog and development time. The hydrazine compound may also be incorporated in the emulsion and the improved effect obtained by development in a developing solution of low sulfite content. The amount of hydrazine or hydrazine derivative added to the developer differ depending upon the composition of the hydrazine and of the developer employed. Amounts in the range of 0.01 gram to grams per liter of developer in general are satisfactory.

When the compounds are added to the emul sion, they may be added before, during or after second ripening. A necessary requirement is that they be present during the development of the exposed material. It is also essential to our invention that the hydrazine compounds should not function as anti-foggants in the emulsion. The amounts used in the emulsion will vary with the composition of the hydrazine compound and of the emulsion but the amount used should not be sufiicient to cause fog. In general, amounts ranging from 0 to 2 grams per pound of wet emulsion are satisfactory.

The following table shows the improvement in speed and gamma obtained upon addition of hydrazine dihydrochloride in an amount of 0.06 gram to 1 liter of D-8 developer having the composition stated above. A normal speed gelatinosilver bromide emulsion was exposed and de- 5 veloped for 1% minutes in the developer, a comparison being made with the developing solution without the hydrazine. The speed contrast and fog recorded in the two cases were as follows, the speed being measured as ten times the reciprocal 10 of the inertia.

fgfi Gamma Fog Check 9. 55 2.3 .07

+Hydrazine dihydrochloride (0.06 g. per 1.

D-8 developer) 25. 0 9. 4 06 The following tables show the change in speed and contrast obtained when various compounds according to our invention were added to a normal speed chlorobromide emulsion:

Concentration, Compound Gnasfbpcr IO/i Gamma Fog 1 Emulsion Semicarbazide HCl 8 if g; ;3g 0 10 4.15 .10 D 1.0 18 4.40 .00 2.0 14 0.15 .10 3.0 17 7. s0 .00 0 21.5 3.01 .08 4.0 10.0 7.4 .00 0.0 18.0 11.0 .07 D 8.0 22.0 11.0 .07 10. 0 1s. 0 14. 0 .00 12.0 17.0 9.0 .07 14.0 22.0 10.0 .07 10.0 22.5 17.0 .00 Hydrazine dihydrochloride 2 3:; 0 10 4.5 .00 1.0 10.5 0.0 .00 Ethyl hydrazine carbonate 1.5 20.5 6.8 .00 2.0 22.5 7.0 .00 4.0 20.0 12.0 .00 0 15.0 0.0 .00 4(p-cl11or0phenyl)semicarbazide g: g g g: 5 :8? 10.0 15.0 0.0 .04 0 10.0 4.5 .00 0.0 10.0 0.0 .07 4-(o-tolyl)-semicarbazid0 5. 0 l9. 5 9. 4 07 7.0 24.5 13.2 .05 10.0 23.5 9.0 .00 N-chloro-N-pyridinoacetic acid jl 0 16.0 4.5 .06 carbohydrazide 4. 0 23. 5 6. 4 l6 l-(N-chloro-N-pyridine acetic 0 16.0 4.5 .06

acid)-4-carbohydrazone 4.0 22.5 6.1 .14 2-py y ydrazi e 2 5%? as-Dimethyl hydrazine H01.

. 1 2,4-D1n1trophenyl hydrazine. 4. 2 m 5 5' 1 13 a-Benzyl-a-phenyl hydrazine 0 13.5 5.0 .26

- l p-Nlttophenyl hydrazme H01... m 5 5 3 27 The emulsions developed according to our in- .50 vention may be exposed by any kind of actinic radiation, but in certain cases we prefer to employ light of Wave-length shorter than infra-red, that is, visible light, ultra-violet light, X-rays or gamma rays. Charged particles such as u-rays,

p-rays and cosmic rays may also be used for the exposure.

The improvement secured by our invention will be more readily apparent by reference to the accompanying drawing which shows typical sensi- 7 tometric curves of exposed and developed photographic material. These curves were made by plotting density against the logarithm of the exposure in the well known manner. Curve A shows the result of exposing and developing a 7 normal speed chlorobromide emulsion for IV;

m nnow-5 minutes-1m developer at 65 Ff Curve B was obtained" fronrat similar emulsion: exposed: anddevel'opedunden the sameconditions i as theemulsionresulting incurve A but with anca'de dition of"-.625*gram per liter-of emulsionflot ortho i to'lyl semicarbazide. Curve-C wasiobtained withz an emulsion similar2to"th-atof"Curve- A; but with the addi-tion of0.9375 gram per: litenof emulsion: of orthotolyl semicarbazide; It" will be noted that in curves B and G'the speed-of' tlie v emulsion has been increased and the contrast-'- has also been increased? No :increase infog is evident irr these-curves. The untreated emulsion showed a 107:" speed of 859; a contrast: of 3.9; and a; fog:

density 01?.04; The--emulsion-from:which curve 5;

B-was=obtained showed a speedof '9i4and a-gamma of 633-; Theemulsionirom which" curve C was obtained showed a speed of 96-an-da gamma of 8:3; The fog density in all threeemulsions It will 1 be understood that the examples and I modifications included herein are illustrative only and that our'invention is to=be=taken as limited only by the scope-cf the appended claims;-

1. The method of" improving the sensitivitycharacteristicsof a' silverhalide photographic layer*withrespectto radiation consi'sting 'of light of wave length-shorter than red, which=comprises"- exposing an undeveloped silverhalide emulsiorr layer-to said ra'diation and developing-rit with aphenolic developing agent at a' pH of" at least approximately 9:5" in a developing solutionselected from the class of developing solutions containing an appreciable amount of sulfite upto 16 grainsper-liter'whenithe'pH is between 9.5 and 9.7, and-containing morethan 16 grams per liter of sulfite .when the,pH is. higher than 9.7, in the presence of "a compound selected from the group consisting; of hydrazine compoundssof the 4c general formula:

actinic radiation of a silverhalide photographic layer; which 'comprises--exposing arrundeveloped,

silver-halide" emulsion layer-to aetinic radiation and developing-it: withi a? phenolic developing agent at a pH of at least-approximately 9:5;in;

a developing solution selected from the class-of-' developing solutions containing an appreciable amount of sulfite up to 16 grams per liter when the pH is between 9.5 and 9.7, and containing more than 16 grams per liter of sulfite when the pH is higher than 9.7 in the presence of an aliphatic acyl hydrazine.

3. The method of increasing the speed of a silver halide photographic layer with respect to radiation consisting of light of wave length shorter than red, which comprises exposing an undevelopedi silver halide emulsion layer to said radiation: and developing. it; with a phenolic-de veloping agent at apH of at least approximately" 9L-5 iir a developing: solution selected from: the class of developing. solutions containingan ap-- preciable amount of sulfite up: to lfigramsper liter when the pH is between. 9.5-and 9.7; andcontaining more than l6 grams per liter: of'sulfite when-thepH-is 'high'ertthan 9.7 intthe presence" of a.- semicarbazide;

4; The method: of increasing the speedvof a; silver. halide photographic .layer with respect: to: radiation consisting of." light of wavelength v shorter: than red,: which comprises exposing an undeveloped silver halide" emulsion layer to said'. radiation and developing it. with a phenolic d'ee veloping agent at-ia pfii of atleast approximately 9.5 in: a. developing. solution. selected from the: class". of 1 developing solutions containing an? ape preciab'le. amount of sulfite' up. to 16 gramslp'er" liter-when. thepH isbetween 9.5 and 9.7, and? containing more than: 16 grams: per literiof: sul fite when the pH is higher thanx9'fl intheiprese ence of3hydrazine.dihydrochloride. V

5; The. method of increasing: the speedof a silver halide photographic layer, with respect ito radiationconsisting'of light of wave length shorter r than-' red; which comprises exposing; an unde velopedisilver: halide emulsion layer: to said radia'-- tion: and developing itflwith at phenolic develope ingragent. at a pH-of at least approximately: in a developing solution selectedfrom the'class. of developing solutions containing anapprecialo'lei amount. of: sulfite upto 16 grams: per liter: when theapI-I is between 9.'5 -and 9.7, andcontaining z more than 16 grams per liter of'sulfite when the pI-I-is higher than 9.79m the presence'zofisemicarbazideshydrochloridez fiuiThe method: of: increasing: the: speed; to: actinic radiation-pf: a silverrhalide photographits layer=,'which comprises incorporatingan-aliphatic: acyl :hydrazine :in a =gelatino-silver halide: emul; sion; coating the: emulsion onza support; exposing the undeveloped? layer to 1 actinic radiation antl developing: it with 1 a --phenolicdevelopingagentz ata pH of atleast approximately 915 in a develop ing-solutien selected' from' the class of developing solutions containing an appreciable amount of sulfite up to 16 grams per liter-when the pH is between .95 and: 9.7," and containing more than 16- grams perliter of *"sulfi'te when the pH-is h'iglier than 937 in the presence of said hydrazine:

'7; The method of increasing" the speed ofa' silver:halide photographic layer with respect to radiation consisting of light' of wave length shorter than red; which comprises 'incorporating a semi-carbazide -ina gelatino silver emulsioncoatingthe-emulsion on a support; exposing i the undeveloped layer to said radiationanddeveloping; it with phenolic developing agent at 1 a pH of at least approximately- 9.5 'inadevelopin'g solu tion containing an.appreciable'amount"of sulfite up to lo-grams per'liter when the pI-I'is'between 9-.'5'--'a-nd 9.7', and containing-morethan 16 grams per liter of-sulfite when-the pH is higher'thanQfl in the presence of said semi-carbazide.

8. The method of increasing the speed of a silver halide photographic layer with respect to radiation consisting of light of wave length shorter than red, which comprises incorporating hydrazine dihydrochloride in a gelatino-silver halide emulsion, coating the emulsion on a support, exposing the undeveloped layer to actinic radiation and developing it with a phenolic developing agent at a pH of at least approximately 11 9.5'in a developing solution selected from the class of developing solutions containing an appreciable amount of sulfite up to 16 grams per liter when the pH is between 9.5 and 9.7, and containin more than 16 grams per liter of sulfite when the pH is higher than 9.7 in the presence of said hydrazine dihydrochloride.

9. The method of increasing the speed of a silver halide photographic layer with respect to radiation consisting of light of wave length shorte than red, which comprises incorporating semicarbazide hydrochloride in a gelatino-silver halide emulsion, coating the emulsion on a support, exposing the undeveloped layer to said radiation and developing it with a phenolic developing agent at a pH of at least approximately 9.5 in a developing solution elected from the class of developing solutions containing an appreciable amount of sulfite up to 16 grams per liter when the pH is between 9.5 and 9.7, and containing more than 16 grams per liter of sulfite when the pH is higher than 9.7 in the presence of said semicarbazide hydrochloride.

10. The method of increasing the speed to actinic radiation of a silver halide photographic layer which comprises exposing an undeveloped silver halide emulsion layer to actinic radiation and developing it with a phenolic developing agent in a developing solution having a pH of at least approximately 9.5 and containing an aliphatic acyl hydrazine and being selected from the class of developing solutions containing an appreciable amount of sulfite up to 16 grams per liter when the pH is between 9.5 and 9.7, and containing more than 16 grams per liter of sulfite when the pH is higher than 9.7.

11. The method of increasing the speed to actinic radiation of silver halide photographic layer which comprises exposing an undeveloped silver halide emulsion layer to actinic radiation and developing it with a phenolic developing agent in a developing solution having a pH of at least approximately 9.5 and containing a semicarbazide and being selected from the class of developing solutions containing an appreciable amount of sulfite up to 16 grams per liter when the pH is between 9.5 and 9.7, and containing more than 16 grams per liter of sulfite when the pH is higher than 9.7.

12. The method of increasing the speed to actinic radiation of a silver halide photographic layer which comprises exposing an undeveloped silver halide emulsion layer to actinic radiation and developing it with a, phenolic developing agent in a developing solution containing hydrazine di- 7 hydrochloride at a pH of at least 9.5, said developing solution being selected from the class of developing solutions containing an appreciable amount of sulfite up to 16 grams per liter when the pH is between 9.5 and 9.7, and containing more than 16 grams per liter of sulfite when the pH is higher than 9.7.

13. The method of increasing the speed to actinic radiation of silver halide photographic layer which comprises exposing an undeveloped silver halide emulsion layer to actinic radiation and developing it with a phenolic developing agent in a developing solution having a pH of at least approximately 9.5 and containing a semicarbazide hydrochloride and being selected from the class of developing solutions containing an appreciable amount of sulfite up to 16 grams per liter when the pH is between 9.5 and 9.7, and con taining more than 16 grams per liter of sulfite when the pH is higher than 9.7.

14. A gelatino-silver halide emulsion capable of exhibiting improved speed and contrast characteristics when exposed and developed in a developing solution having a pH of at least approximately 9.5, comprising a gelatino-silver halide emulsion having therein an appreciable amount, not greater than 2 grams per pound of wet emulsion, or a semi-carbazide.

15. A gelatino-silver halide emulsion capable of exhibiting improved speed and contrast characteristics when exposed and developed in a developing solution having a pH of at least approximately 9.5, comprising a gelatino-silver halide emulsion having therein an appreciable amount, not greater than 2 grams per pound of wet emulsion, of an aliphatic acyl hydrazine.

16. A gelatino-silver halide emulsion capable of exhibiting improved speed and contrast characteristics when exposed and developed in a developing solution having a pH of at least approximately 9.5, comprising a gelatino-silver halide emulsion layer having uniformly dispersed therein an appreciable amount, not greater than 2 grams per pound of Wet emulsion, of an aryl semi-carbazide.

ADRIAN P. H. TRIVELLI. WILLIAM F. SMITH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 956,567 Caldwell May 3, 1910 1,082,622 Caldwell Dec. 30, 1913 1,560,240 Jelley Nov. 3, 1925 1,763,990 Sheppard et al June 17, 1930 2,127,573 Sheppard et al Aug. 23, 1938 2,220,929 Kirby Nov. 12, 1940 2,245,236 Trivelli et al June 10, 1941 2,311,098 Swan et al Feb. 16, 1943 2,196,734 Marriage Apr. 9, 1940 2,266,442 Schinzel Dec. 16, 1941 2,286,662 Weyde et al June 16, 1942 2,309,492 Albers et al Jan. 26, 1943 2,339,213 Woodward et al. Oct. 16, 1939 OTHER REFERENCES Carroll, Sensitization of Photographic Emulsions by Colloidal Materials, Bureau of Standards Journal of Research, vol. I, October, 1928, pp. 565-588, p. 584 cited. (Photostat Copy in Division 7

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