The Review of ScientificInstrutnents Vol.3MARCH,1932No.3 THEWENNERPOTENTIOMETER By LEOBEHR [RESEARCHDEPARTMENT,LEEDS&NORTHRUPCOMPANY,PHILADELPillA,PA. RECEIVEDNOVEMBER27,1931] ABSTRACT A two-range, five-dial potentiometer is described which incorporates a decade construction suggested by Wenner.The upper range is1.9111volts in steps of10microvolts and the lower range isonetenth ofthis.The effect ofcontact resistance,thermal emf's and oferrors in ad-justment is considered in detail. The limit of error forthe instrument is 0.005 percent or 2 microvolts for the high range and 0.01percent or 0.2microvolt forthe low range. Dr.Frank Wennerhasdescribedapotentiometer decadedesign*in whichthecontactsaresodisposedinthecircuitthattheinevitable electromotive forcesand resistance variations at the contacts influence themeasuredresultonly veryslightly.Hisdescriptionisbrief enough topermitreproductionhere in full. This communication relates to a device for the lower decades of a potentiometer to be used in the measurement ofsmall electromotive forces.From the figure(which showsthe arrange-ment ofone decade)it will be seen that a part ofthe circuit including the potential point Mis shuntedby acomparativelyhighresistance.By meansofadoubledialswitchbothbranch points between the shunt and the main circuit may be shifted in steps of equal resistance so as tointroducealargerorsmallerresistancebetweenTandNwhilekeepingtheresistance shunted constant. The shifting of the switch then produces a change in the potential difference between Mand Nequal tothe change in the resistance between T and Ntimes the current in theshunt. The deviceworksout wellin the designofalowrangetwocircuit potentiometer having a low resistance between potential terminals. T At FIG.1. Thisdecadedesignhasbeenused 'intheconstructionofadouble range,five-dialpotentiometer.The upper range is1.9111volts in steps * Phys.Rev.31,94;1910. 109 Downloaded 10 May 2010 to 146.164.53.135. Redistribution subject to AIP license or copyright; see http://rsi.aip.org/rsi/copyright.jsp110LEOBERR[R.S.l.,3 of 10 microvolts and the lower range is one-tenth of this. The resistance in the galvanometer circuit isapproximately constant at 174 ohms and if the high dial is not in use this resistance can be reduced to 40 ohms by settingthedialat apointprovidedforthatpurpose.Thetwoupper dialsareoftheusualconstruction,withcontactsinthegalvanometer circuit, and the three lower dials are of the Wenner design.A split cir-cuitisusedandprovisionismadeforconvenientlycheckingandad-justingthezeroofthepotentiometerandtheratioofthecurrents through the upper dial and through the remaining dials. FIG.2. This constructionhas certain distinctive advantages: (1)The lowresistancemakesit possibletoattainsufficientgalvanometersensitivityto realize the precision made availableby the potentiometer. (2)Contact resistance in any ofthe dials has an entirely inappreciable effect onthe meas-urements. (3)Electromotive forcesthat may originate during manipulation ofthethree lowerdials have a negligible influence on the measured voltage. (4)The lowresistanceoftheimportant coilsand themannerin whichchangesinthese coils affect the measurements, are both factors conducive to inherently good stability. A low range Wenner potentiometer has been built which also has two ranges, 0.11111 volt in steps of1.0microvolt and 0.011111volt in steps of 0.1microvolt. This instrument will be the subject of a later paper. Downloaded 10 May 2010 to 146.164.53.135. Redistribution subject to AIP license or copyright; see http://rsi.aip.org/rsi/copyright.jspMarch,1932]THEWENNERPOTENTIOMETER111 GENERALDESCRIPTION AphotographofthehighrangepotentiometerisshowninFig.2. The dials Aand B are set to correspond to the standard cell voltage and u ~ c ~ " ~. ~ N g "' ~ ~ , -=-g .., N ~~ Downloaded 10 May 2010 to 146.164.53.135. Redistribution subject to AIP license or copyright; see http://rsi.aip.org/rsi/copyright.jsp112LEOBEHR[R.S.L,3 areadjustable insteps of10microvolts from1.01800to1.01960volts. The smalldialintheupper rightcorner istherange-changingswitch. When it is set on xl, the potentiometer is direct reading, at the x.lset-ting the readings aretobe divided by10.The quick-changeswitch for checking the current against the standard cell is at the lower left corner and the rheostats are at the right side. Fig.3isacomplete wiringdiagram andFig.4isan outline diagram fromwhichsuchdetailsasthebattery rheostat,galvanometershunts andstandardcellswitch havebeenomitted.Thenormalbatterycur-rent is 0.022 ampere and when the range changing switch is at xl., 0.002 amperecirculatesthroughtheresistance(a+ 9a)and0.02ampere flowsto the point P, Fig. 4.At Pthe current divides equally, the resist-ance betweenPand Q by either path being500ohms.The current to the right of Pdivides among the parallel paths asindicated.The main andcompensatingresistancesofdecadesIII,IVandVareequaland the brushes are mechanically connected sothat the current distribution shownismaintained forallsettingsofthedecades. Theresistancesare ofsuch valuethat,withallthe dialsset at zero, the potential drop from E+ to Q is equal to the rise from Q to E- and thenet potential differenceatthe emf posts iszero.DialIconsistsof 18resistances of10 ohms each and dial II has 102-ohm coils.With cur-rents of 0.01and 0.005ampere respectively, the potential difference per step of dial lis 0.1voltand of dial II is 0.01 volt.For the remaining dials it isclearfromDr.Wenner'sdescription,that thepotential difference per step of each dial is equal to the resistanceper step multiplied by the currentthroughthebrush,andthatdialsIII,IVand vhavepotential steps of 0.001,0.0001and 0.00001volt respectively. Thecurrent throughthe potentiometer isstandardized intheusual manner by comparing the potential drop across a known resistance with theemfofastandardcell.Theprincipalpartofthepotentialdrop 1 volt, is obtained acrossthe firsttencoilsofdial I,anddials Aand B make possiblethe useofstandard cellshaving valuesbetween1.01800 and1.01960volts.Theswitchforcheckingthecurrentwithoutdis-turbing the potentiometer setting, is not shown in Fig. 4 but is included inFig.3. Referring to Fig. 4, when the range changing switch is set at x.l, 0.02 ampereflowsthroughtheresistancea,and0.002amperethrough9a and the potentiometer.The potentiometer resistanceand resistancesa and 9aare symmetrically arranged in the figurein order to makeclear how the range-changing switch altersthe currentthroughthepotenti-Downloaded 10 May 2010 to 146.164.53.135. Redistribution subject to AIP license or copyright; see http://rsi.aip.org/rsi/copyright.jspMarch,1932] THEWENNERPOTENTIOMETER 113 2 ~ ~ ~ ~ ~ "1:/ :;;-. 0 2 :. 2 < .; < . ~ 0 -/il ~ ! ! /il .q; ~ ~ e "" 11 N ~ 1 ~ N "