FRANK G. BROCKMAN

and the two factors are separated and the truedeclination is found.21

The sensitivity of individuals in the space-

21 This technique with the beads makes it possible tofind both so and r without the need for the declination(geared lens) unit at all. The beads are adjusted first toappear inclined the same as the cross appears inclined (toappear to lie in the plane of the cross), and second toappear vertical. One finds, then, o=-12(6b,+b,) and7 = (8bC- 4b), where bC and abV are the declinations asso-ciated with the first and second adjustments. The difficultylies in the fact that, when the cross appears inclined, thesubject experiences difficulty in judging the orientationof the cross about a vertical axis which is needed to correctthe vertical component of the image size difference.

eikonometer to the various test elements for themeasurement of aniseikonic errors has been dis-cussed in another paper.22

This unit has also proved of value in researchexperiments where controlled studies of stereo-scopic responses to declination changes betweenthe images in the two eyes are to be made, andwhich introduces no empirical factor of depthperception.2 2

22 K. N. Ogle and V. J. Ellerbrock, "Stereoscopic sensi-tivity in the space-eikonometer," Arch. f. Ophthal. atpublishers.

JOURNAL OF THE OPTICAL SOCIETY OF AMERICA VOLUME 36, NUMBER I JANUARY, 1946

Production and Properties of Nickel Bolometers

FRANK G. BROCKMAAN*Research and Development Division, Socony- Vacuum Oil Comipany, Incorporated, Paulsboro, New Jersey

(Received July 13, 1944)

As a part of the development of an alternating-current bolometer, a method for the pro-duction of very thin nickel ribbons (and, incidentally, nickel foil) has been developed. Ribbonfilaments have been prepared which are as thin as 0.1 micron. These filaments appear topossess the properties of bulk nickel. Certain properties of these filaments have been studiedand are described. The method is not restricted to nickel, since bismuth ribbons have beenprepared by the same procedure.

THE high temperature coefficient of re-T sistivity of nickel (0.006)1 makes this metalan attractive one for the production of bolometerfilaments. Bosworth2 prepared nickel filaments byelectroplating nickel upon stainless steel, strippingthe plate, and cutting the ribbons from the thinfoil so produced. In order to avoid the necessityof working with very thin foil, a method wassought whereby the nickel could be depositedupon a backing which subsequently could bedissolved. The method described below resultedfrom this search.

PREPARATION OF NICKEL RIBBON BOLOMETERS

With copper foil about 0.002 inch thick as abase, nickel was deposited by the ordinary

* Now with the Richmond Hill Laboratories of the NorthAmerican Philips Company, Incorporated, Dobbs Ferry,New York.

Smithsonian Physical Tables (1933).2 R. C. L. Bosworth, Trans. Faraday Soc. 30, 549 and 554

(1934).

electroplating method. The copper foil cathodewas supported by folding it flat around a copperplate. This allowed deposition of nickel on onlyone face of the copper foil. The anode was "PureGrade A Nickel." Gentle agitation of the elec-trolyte solution was provided by a small electricstirrer. The electrolyte solution' was

NiSO4 - 6H2 0NH4 ClH3 B03

120 g per liter15 g per liter15 g per liter

Reagent grade chemicals were used. The H ofthe solution was adjusted to 6. Plating wasperformed at room temperature with a currentdensity of 0.001 ampere per square cm (0.100ampere, cathode a flat plate 6.5X8.0 cm). Thetime of plating was varied from 5 to 60 minutesdepending upon the thickness of deposit desired.

I William Blurn and George B. Hogaboom, Principles ofElectroplating and Electroforming (McGraw-Hill BookCompany, Inc., New York, New York, 1924).

32

NICKEL BOLOMETERS

A double-edged shearing punch was used to cutthe copper foil, nickel plated on one side only,into ribbons 0.038 cm by about 5 cm. One of theseribbons was then soldered to a platinum wireframe. This frame (Fig. 1) was so made that theribbon could be soldered at either end of theframe as well as to a platinum support midwaybetween the ends. This permitted two ribbons ofequal length to be prepared simultaneously. Thetwo ends and the midpoint were made electricallyindependent by glass beads.

When the ribbon was fixed to the platinumframe, the copper was dissolved electrochemically.The e.m.f. series of metals in aqueous solutions ofpotassium cyanide' indicates that copper in con-tact with platinum in potassium cyanide solutionshould dissolve and that nickel should dissolvewith less ease. This is indeed the behavior. Theframe with the nickel-plated copper ribbon wasimmersed in a concentrated solution of potassiumcyanide. The copper dissolves, going into solutionas a complex copper cyanide, while hydrogen gasis evolved at the platinum frame. This is an

MAKE GLASS BEADS (®&®4 AS STRONG ASPOSSIBLE CONSISTENT WITH GIVEN DIMENSIONS.

FLATTENED SURFACES Q,()&) SHOULDBE ABOUT If TO 2 mm WIDE AND 3 mm LONG.ALL SHOULD LIE IN THE SAME PLANE.

@) THIS MUST FREELY ENTER A I.D.TUBE.

a) TO CONSERVE PLATINUM SPOT WELDAT ABOUT THIS POINT TO 6" LENGTH OF #18OR #20 COPPER WIRE EACH LEAD.

( GRIND THESE SURFACES.,

FIG. 1. Bolometer mounting.

4 J. W. Mellor, A Comprehensive Treatise on Inorganic andTheoretical Chemistry (Longmans, Green and Company,Ltd., London, England, 1928), Vol. III, p. 503.

toz0of

Iy

0 10 20 30 40PLATING TIME - MINUTES

b

FIG. 2. Thickness versus plating time.

advantage over the Wollaston method in whichgas is evolved at the surface of the ribbon.Raising the temperature moderately (to about50 0C) accelerates the dissolution of the copper.When the copper is completely dissolved, gassingceases and the nickel ribbon remains. The rate ofsolution of nickel is many times slower than thatof copper. As with the Wollaston method, whenthe ribbons are very thin, removal of the frameand nickel ribbons from the cyanide solutionmust be carried out slowly and with great care toprevent the breaking of the ribbons by thesurface tension of the liquid. The frame andribbons were washed in distilled water and then inacetone.

THICKNESS DETERMINATIONS

With a similar technique, nickel foil was pre-pared in sheets about 5 cm by 5 cm. From themeasured area and the weight of the foil, thethickness was determined, using 8.9 g per cc asthe density of nickel. When the thickness isplotted against the plating time (at the constantplating current density corresponding to 0.10Oampere) used to produce the foils, a linear re--lationship, Fig. 2, is obtained.

In addition to this method, ribbons of knownwidth and length were prepared and the resist-ances determined. By using the bulk resistivityof nickel (7.8X 10-6 ohm cm at 20 0C), the thick-nesses of the ribbons were calculated. Figure 2includes the results of these determinations also.The two methods are in good agreement.

It is apparent that no anomaly appears inFig. 2. This suggests that, within the range ofthicknesses here investigated, the thin nickel

if I ~~I I I 1

* MASS METHOD

o RI'ESISTANCe MTHOD0 1 L J I v

33

bu

\Z11

FRANK G. BROCKMAN

10.0 20

9. 19

9.0 18

8.5

z

7.5

17

15

7.0 14

6.5 1320 30 40 50 60 70 so 90 100

TEMPERATURE - DEGREES C

FIG. 3.

produced by the above process retains theproperties of bulk nickel.

Figure 2 served as a convenient guide for theproduction of ribbon filaments of controlledthickness.

TEMPERATURE COEFFICIENT OF RESISTIVITY OFTHE NICKEL BOLOMETERS

Bosworth 2 found that the temperature coeffi-cient of resistivity of the nickel bolometers pre-pared by him decreased with decreasing thickness.In order to test this observation, three filamentsof different thicknesses were prepared. Theplating was carried out by using the same anodeand electrolyte solution for all three. Theseribbon filaments were mounted in a transformeroil bath and resistance-temperature curves wereobtained. The current through the ribbons was1.5 milliamperes, at which current it was shownthat the resistances measured were practicallyequal to the extrapolated zero-current resistance.The results are recorded as Fig. 3. Over the rangeof thickness from 0.14 to 0.54 micron thecoefficient of resistivity was constant within ourprecision of measurement. The difference be-tween this observed and the reported coefficient(0.006) may be due to traces of impurities in thenickel. This observation is further substantiatedby the data in Table I. These are the thicknessesand temperature coefficients of filaments made atdifferent times. The coefficients were calculatedfrom determinations of resistances at two tem-peratures in an air bath.

The coefficients are not all equal, but they arenot related to thickness. Although every effortwas made to keep the plating bath free of con-tamination, the random variation may be due tovarying concentrations of impurities.

RESISTANCE VERSUS CURRENT PROPERTIES

Since the resistance of these filaments is afunction of the current, this relationship wasstudied experimentally at two constant ambienttemperatures. The experimental data have beenintroduced into an equation, derived below,which relates the resistance of the filaments tothe ambient temperature and the current throughthe filaments.

Assuming Newton's law of cooling, the thermalequilibrium condition of a filament carrying cur-rent can be expressed:

RI 2= K(T-T) . (1)where R is the resistance of the filament, I is thecurrent through the filament, T is the tempera-ture of the filament, and Ta is the ambienttemperature.

The resistance R at any temperature T isrelated to the resistance R, at some referencetemperature T 5:

R=R,(1+a(T-T 5)) (2)

in which a is the temperature coefficient ofresistivity.

If the reference temperature be chosen as theambient temperature, then, from Eqs. (1) and (2),

(R-R 5)/RR, =AI2 , (3)

where A =a/K. This also has the form:

1/R= I/R 5-A F. (4

The resistance R, is a reference resistance atzero current and at any one ambient temperature.If the ambient temperature is altered, R 5 will

TABLE I.

Thickness, Temperature coefficientmicrons of resistivity

0.09 0.00490.10 0.00500.12 0.00440.13 0.00430.13 0.00480.14 0.00510.16 0.004 70.17 0.0045

40- RESISTANCE VS. TEMPERATURERIBBON LENGTH 2.0 cm

,3ReS.oN WTrH 0.038 cm /h30-

Pt= n °f =R[i+as(t-ts)]

i3; - / ,< ts -30 t 90 _

DY i/ THICKNESS 0.54F, 0.28F 0.14}l30 PY , t 955 18.48 38.04_

z ~~~~R, 7.38 14.25 29.45< as~~~~~~~I 0.00491 0.00495 0.00485

34

(4)

NICKEL BOLOMETERS 35

vary according to Eq. (2), which is rewrittenwith changed subscripts:

Rs, = Ro, (1 + a (T, - T°)), (2a)

where R, signifies the resistance at zero current ata fixed reference temperature To. The tempera-ture difference TIs- T0 is now replaced by A\T,

ture of the bolometer above the ambient temper-ature to fall to a value such that the differencebetween this value and the ambient temperatureis 1/e times the difference between the initialtemperature and the ambient.

With this definition and assuming Newton'slw of cooling:

Rs=R,(l +aAT). (2b)

When aT<<, Eq. (2b) can be written as anapproximation:

11Rs= 1R, - (aR,,)AT. (5)

By combining Eqs. (4) and (5), we obtain:

1/R= /R 0- (a/R 0)AT-A 2. (6)

Equation (6) has been applied to the experi-mental data obtained on the two filaments nowin use, and the two following equations expressthe conductances of the two filaments with anaverage deviation of less than percent over therange 250 to 35'C in ambient temperature, and0 to 40 milliamperes in current:

C1= 0.04499 - 0.000184AT- 7.1812, (6a)

C2 =0.05015-0.000226AT-7.1612 . (6b)

The reference temperature is 250 C and the cur-rent is in amperes. The equations apply for thefilaments in air at atmospheric pressure with thefront surface blackened with bismuth black.Because of the approximation made to obtainEq. (5), the numerical value of the constant a isdifferent from the temperature coefficient calcu-lated by Eq. (2).

TIME CONSTANT

Let the time constant of the bolometer bedefined as the time, r, required for the tempera-

(T-T 0 )=(Ti-T 0 ) exp -- t ,Mc

(7)

where T is the temperature of the ribbon at anyinstant, To is the ambient temperature, Tj is theinitial temperature of the ribbon, K is a pro-portionality constant, m is the mass of the ribbon,c is the specific heat of the ribbon, and t is thetime, we have

T = Mc/K. (8)

The constant K is included in the constant Ain Eqs. (3) and (6) and can be evaluated from theexperimental Eqs. (6a) and (6b).

Inserting the values for the ribbons now in use(m=6.6X10-6 g, c=0.46 joule/g/degree C, andK=6.0 X 10-4 joule/second/degree), the timeconstant of these ribbons is 5 X 10-3 second.

That this time constant is of the right order ofmagnitude is substantiated by the fact that,when used in an alternating-current bolometersystem fed with 1000-cycle power, the ribbonsact as harmonic generators.

ACKNOWLEDGMENT

Grateful acknowledgment is made of the as-sistance of Mr. John W. Wescott 2nd, now in thearmed forces. Mr. Wescott rendered valuableassistance in developing the practical phases ofthis technique.

35