A S d L Y l ' I C A L EDITIOS
394
( 2 ) . This machine consists of a rotating disk inside of an off-center ring. I n the rotating disk are cut radial slots in which specially cured test samples are inserted. As the disk rotates the free end of each test strip is bent down so that its plane is at approximately a 90-degree angle to the plane of that part of the strip which is secured in the radial slot. In comparing the two machines, several desirable features are evident in the Firestone machine. With this machine it is possible to evaluate the flex-checking resistance of any stock from which a dumb-bell strip may be cut. This makes the machine valuable t o compounders who are working with stocks that are air-cured. The conditions of test niay be varied so as to simulate any stressing condition desired. Furthermore, all types of stocks may be tested, as the resist-
Yol. 2 ,
so. 4
ance to abrasion of the stock does not enter into the test a t all. The machine flexes the strips to a definite elongation and therefore the rigidity of the test strip in no wise affects the working elongation. Also, this machine can be mounted in the direct sunlight and run under conditions which are very comparable t o actual road test, a feature which recent experience has shown to be highly desirable. Furthermore, the niachine is small enough so that it can be encased and flexing determinations made a t higher or lower than normal temperatures. Literature Cited (1) International Critical Tables, Vol. 11, p. 269. (2) Torrance and Peterson, India Rubber World, 8 , 6 2 (1929)
Separation of Alkyl and Aryl Halogen' A Modification of the Stepanow Method, with Particular Reference to the Analysis of Certain Insecticides Quick Landis? and H. J. Wichmann FOOD,D R C G ,A N D INSECTICIDE ADMIXISTRATION, SAN FRASCISCO, CALIF
The Stepanow method for organic halogen is modiset-up is required. Tlie Asfied (Method 1) by refluxing with a ten to fifteen fold tance and increase in sociation of Official dgriculexcess of sodium in kerosene or xylene and a few cubic the use of insecticides tural Chemists ( I C ) has decentimeters of amyl alcohol. Benzenoid halogen is containing chlorinated hydroveloped a method, applied reported t o be completely decomposed by this procarbons bring with them the principally to carbon tetracedure. Aliphatic halogen in the presence of aryl is desirability of d eve1 o p i n g c h l o r i d e and t r i c h l o r o determined (Method 2) by decomposition in kerosene methane, in which the comsimplified methods for the solution with butanolic potash at 110" C . ; in the case pound is heated under presa n a l y s i s of such mixtures. of volatile compounds the reacting mixture is covered Sprays frequently c o n t a i n sure to 100" C. with methwith a layer of solvent to prevent the escape of vapors anolic potash, using pressure carbon tetrachloride or p-dibefore decomposition. Total halogen is in this case bottles. Francois ( 1 2 ) has chlorobenzene or both, and determined by Method 2 followed by Method 1, using, thus the problem becomes recently reported good results however, a twenty to twenty-five fold excess of sodium. essentially the determination in the case of carbon tetraAnalytical data are presented to show that the method of alkyl chlorine in the preschloride by refluxing the cornis probably generally applicable. ence of aryl chlorine. Alpound with ethanolic potash though methods for the deunder a trap of glass beads. termynation of the two compounds separately h a r e been de- The writers, working independently, were ;nahG to obtain scribed, some of them require special apparatus or reagents, complete recovery by this means, even when a slow counterand do not appear to be suited t o the separation of the two current of solvent down the condenser was used. The "overlaying" procedure described below seems, however, to be effecconstituents. tive in preventing the escape of the volatile compound. Review of Methods ~PECIAL !lhrHoDs-Krishna and Swarup (16)have adapted GENERALRIETHoDs-The ultimate standard for halogen Kus's iodine method (1) to the determination of certain lidetermination is the Carius method, but many laboratories able halogens such as occur in acyl chlorides and chloramines, do not possess the equipment required for heating under pres- involving decomposition in alkaline solution. The separasure. The Parr bomb ignition Kith sodium peroxide and tion of p-dichlorobenzene from certain other halogenated comstarch has been adapted t o halogen determination ( I ? ) in- pounds by fractional distillation has been described by several cluding fluorine ( I S ) . Rlarcusson and Doscher (20) ignite investigators (6,12, 14). The authors were able to detect as the compound in a n atmosphere of oxygen and subsequently little as 2 per cent p-dichlorobenzene in a kerosene insecticide absorb the inorganic halide in alkali. R h e n liquid ammonia by suitable fractionation followed by the chilling of the 165is available the halogen compound may be determined ac- 175" C. fraction to - 10" C., but failed in the case of a solvent cording t o Clifford (4) or Dains and Brewster ( 7 ) by decom- naphtha preparation, the amount of petroleum hydrocarbon position with sodium. Several investigators have described distilling with the halogenated one being too great to permit methods involving catalytic reduction with various metals and separation of the latter. Cappenberg (3) has described a hydrazine (2, I S , 25'). Stepanom (26) has described a general method suitable in certain cases in which the compound is method based upon the action of a twenty-five fold excess of decomposed by potassium hydroxide in methanolic solution. sodium upon the compound in ethyl alcohol solution. Principles Involved VOLATILECovpoums-The combustion method of PlimpOf the above listed methods that of Stepanow, in conjuncton and Graves (24) appears to be well adapted to the treatment of volatile compounds, although a rather elaborate tion with the A. 0.A. C. alcoholic potash saponification, appeared to offer the most promise. A large number of trials 1 Received June 4, 1930. of various modifications finally led to the development of the Present address, The Fleischmann Laboratories, N e w York. N. Y.
HE g r o w i n g i m p o r -
T
October 15, 1930
I S D L ' S T R I A L A S D E S G I S E E R I S G CHEMISTRY
methods described below, in which the following principles appear t o be involved: R E L ~ T I V EDECOhlPOsITION RATEs-The reactivity or "liableness" of the halogen in the compounds acyl, alkyl, and aryl decrease in the order mentioned. The acyl chlorides, as well as certain alkyl alpha halogens, for example, are usually hydrolyzed by warm water (82). The alkyl halides upon treatment with alkali alcoholates are converted into ethers (26) with the elimination of the halogen as in the method of Cappenberg ( 3 ) . Aryl halides, however, require the uce of the alkali metal to bring about decomposition, as in the Wurtz ( 2 7 ) and Fittig ( I O ) reactions. Lowenhertz ( I S ) reports that 0.0048 per cent of monochlorobenzene was changed after 40 hours with sodium amylate at 25" C., corresponding to a reaction velocity constant of 2.3 X The data presented by Conant and Kirner ( 5 ) on the relative reactivities of PhC1, PhCH2CI, and PhCH2CH2C1as measured by their rates of decomposition by iodine in acetone show the relative reaction velocities at 50" C. to be 0 (no reaction), 200, and 1, respecti\-ely. The authors found butanolic potash to be without action on the three phenyl halides (Table I). ISHIBITIOS OF SIDE REacTIos-It has been repeatedly ohserved that aryl halogen would not succumb completely t o the twenty-five fold excess of sodium recommended by Stepanow, and other investigators advised a fifty-fold excess and a n increase in temperature (8). Favrel and Bucher ( 9 ) suggest the use of isoamyl alcohol, refluxing a t 140" C. Hovever, when any of the alcohols are used as the solvent, most of the sodium is consumed in its reaction with the alcohol; a layer of hydrogen constantly covers the surface of the metal and may prevent effective decomposition of the halogen compound. By progressively diluting the alcohol with a n inert solvent such as kerosene or xylene this side reaction may be largely suppressed. In fact, l l a r y o t t (21) describes a method in which a tenfold excess of potassium is used in 2:l benzeneethyl alcohol solution. The authors completely decomposed 0.2 gram of p-dichlorobenzene with 0.75 gram of sodium, a sixfold excess, b y refluxing in kerosene and adding through the condenser only sufficient butyl or amyl alcohol (5 cc ) to keep the surface of the metal bright.
395
traps to the prevention of the escape of vapors of volatile compounds from the refluxing halogen solution, the idea of overlaying the solution with solvent as described below was evolved. The success of the method depends upon these factors: (a) dilution of the volatile compound with inert solvent until its vapor pressure from solution a t room temperature is very small, and ( b ) the assumption that the rate of diffusion of the coinpound to the surface of the upper layer is less than the rate of decomposition. I t should be noted that while the vapor pressure may be small i t is not entirely negligible. I n a large number of test runs on carbon tetrachloride a constant loss of 3 to 4 per cent was finally traced to the use of a volumetric drainage pipet in aliquoting the diluted solution; substitution of a Llohr or measuring pipet gave theoretical results TT-ithinthe limits of error. Care should also he observed in weighing out an original sample which possesses a high content of volat~leconstituents; a weight pipet with a capillary tip or T-ictor lleyer glass-$toppered vial, s~ibsequentlyopened under the -olYent. is to be recommended. Details of Procedure
METHOD1. TOTAL HALOGES IS ABSSSCE OF 1-OLATILE CoIisTITcEsTs-Place a n aliquot of the halogen solution in kerosene or xylene containing 0.002 to 0.003 equivalent of halogen in a 100-125-cc. Erlenmeyer flask, dilute to 50 cc. with kerosene, and add 1 to 1.5 grams of metallic sodium in very small pieces. Connect to a reflux condenser and apply heat. -4s the temperature begins to rise above 100' C. and the sodium loses its luster, add 3 to 5 cc. of amyl alcohol through the condenser. Reflux a t a moderate rate for 2 or 3 hours. Cool, disconnect from the condenser, add 20 cc. of water, and shake. If a n y globules of sodium remain. allow the flask to stand until they are completely decomposed. Alcohol is ineffective in the decomposition of the sodium under these conditions. The decomposition may be hastened by adding more water and shaking vigorously, unstoppered. Then transfer quantitatively to a separatory funnel, wash the flask thoroughly with more than sufficient dilute nitric acid to neutralize all the alkali, and add the washings to the
Table I-Analysis
of V a r i o u s Compounds" AMT. AMI. CALCC). FROM . 4 s i ~ u s r s Method 1 Method 2 Method 3 Gram Gram Grain Grain Gram CaHsCl 0,01125 0.2182 0,2190 Nil 0.2167 Chlorobenzene CsHsBr 0.0157 0.3708 0.3690 Nil 0.3700 Bromobenzene CaHsI 0,0204 0.4248 0.4197 Traceb Iodobenzene CeHaCI: 0.00735 0,1400 0,1403 Xi1 0.1392 0-Dichlorobenzene CoHIBr: 0.0118 0.2200 0.2152 Si1 0.2170 0-Dibromobenzene CnHsBr 0 0109 0.2512 .... 0.2483 Ethyl bromidee CaHrBr 0.0123 0.1780 .... 0.1793 .... n-Propyl bromide CHICI. CH2CI 0,00495 0.08908 .... 0.05865 .... Ethvlene dichloride Carbon tetrachloride cci, 0.00385 0,1015 .... 0 1005 .... 1odoformd~CHI3 0.0131 0.2600 .... 0.2564 .... p-Ntrobenzyl bromide PiOzCbHaCHzBr 0.0216 0.4020 0 3996 0.4060e .... p-Bromophenacyl bromide BrCsHaCO. CHzBr 0.0139 0.2800 0.2780 .... 0.2815 P-Bromophenacyl bromide BrCsHaCO, CHzBr 0.0278 0,5600 .... 0.5640 .... a These analyses were made through t h e courtesy of t h e Fleischmann Laboratories. b T h e compound after purification by distillation was still slightly colored with free iodine c Room temperature during this determination was 32' C. T h e boiling point of ethyl bromide is 38 J 0 C d Solid iodoform weighed o u t for this determination. c l'olhard method: colored decomposition products obscured t h e chromate end point.
FoRM~-L.\
coMPouso
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1 cc. 0 1 S .4gN03
T.AKES
....
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Loss OF VOLATILECOIisTITUESTs-The most thoroughly satisfactory treatment in the case of volatile compounds is, of course, the sealed tube, in spite of certain difficulties of manipulation. The only objection t o the use of pressure Iiottles is their inconvenience and notorious unreliability. Indeed, the authors obtained good results in the case of carbon tetrachloride by heating to 100" C. for 2 hours with butanolic potash in a tightly stoppered flask, the advantage of this method over that described by the A. 0.A. C. being the attainment of higher temperature without the development of escessive pressure. After failing in the attempt to adapt various forms of
PI:R CEST CALCD. FROM ANALYSIS 3Iethod 1 X e t h o d 2 Method 3 % c7, 7 ." ,~ . ", 100.4 0 99 3 99.5 0 99.8 98.8 0 ... 100.2 0 99 4 99.2 0 9s ;I ... 98.9 ... ... 100.9 ... ... 99.5 ... ... 99.0 ... 98.6 . . 99.4 101.0 99 3 100: : ... 166: 7 ...
main solution. Separate the acid aqueous layer and wish the contents of the funnel twice with water. Determine inorganic halides in the combined extracts b y any standard method. METHOD2 . ALKYLHALOGES IS PRESESCE OR ABSESCE O F hRYL--l-'repare a saturated solution of potassium hydroxide in n-butyl or amyl alcohol (commercial butanol is the most economical) by refluxing for 30 minutes a quantity of the alcohol with a considerable excess of the solid caustic. The final solution should have a gravity of approximately 0.93 and contain 14 to 15 per cent KOH b y weight. .4dd a suitable aliquot of the halogen solution (in kerosene or xy-
396
ANALYTICAL EDITIOX
To]. 2, KO.4
lene) to an equal volume (at least 15 to 20 cc.) of the butanolic kerosene, and dlluted to 100 cc. h vie,ght p:pet with a capilpotash reagent in a 100- to 125-cc. flask, lary tip was used for all volatile liquids. Unless otheruise If the compound is a t all volatile, as in the case of carbon stated, a 20-cc. aliquot was taken. After decomposition the tetrachloride, the following precautions must be observed: halides were determined by the hlohr method of direct titraUse a Mohr pipet or a buret to measure the aliquot, discard- tion in neutral solution with silver nitrate using chromate ing the last drainings. Avoid all unnecessary exposure to the indicator. air, swirling the solution with the delivery tip of the pipet Table I1 gives the results of some variations in the procejust under or close to the surface. Immediately overlay the dure described illustrating the necessity of observing certain solution with 50 CC. of a 20:l solution of kerosene and butan- precautions in following the method. The analyses were olic potash as follows: Hold the flask upon the top of the made in the manner described for those in Table I. bench with one hand and with the other slowly pour the soluT a h l e 11-Effect of V a r i a t i o n s i n P r o c e d u r e tion on to the rim of the flask from a 250-cc. beaker by means COMAMOCST of a stirring rod across the top of the beaker, following around POUND METHODTAKEN NATUREO F VARIATION RECOVERY the rim of the flask in such a manner that the thread of liquid Gram Per cenl 1 0 2200 0 5 gram Na, equivalent t o fivedescends the side of the flask in a spiral. There should be p-CsHaBr2 fold excess 80 5 1 0 2000 0 7 5 gram Na. eauivalent t o SIXvery little mixing of the layers and the boundary should be 9-CsHClz fold excess 99.2 quite distinct. An efficient pressure bottle or stoppered flask P-C~HICIZ 3 0,1400 2.0 gram S a , heated over gauze, much undecomposed S a 83 0 at 100' C. for 2 hours has been found to be effective in the CsHsBr 3 0.3710 1.5grams Na, heated over asbestos mat, N a not all decomposed 86.8 case of carbon tetrachloride in lieu of the overlaying procedure, ~H&I 3 0 4248 Same 93.2 If the vapor pressure of the compound is negligible at llO°C., CCCll 2 0 1015 10 cc. reagent overlaid with 20 cc. 20:l s o h . 92.2 the overlaying procedure may be omitted. Using care not CzHaClz 2 0 1034 Heated rapidly t o mixing of layers 88.0 2 0.1034 Isobutylic K O H ; held a t 95to disturb the layers, stopper lightly and heat gradually to CzHiClz 100' C. for 15 minutes 73.0 110" C. in a glycerol bath. The surface of the bath should be above that of the liquid in the flask in order to prevent The values obtained in the case of carbon tetrachloride and convection currents. Maintain a temperature of 10% p-dichlorobenzene are presented in Table 111. The Volhard 115' C. for a t least 15 minutes. If a t any time the layers be- method of analysis was used. come mixed, the final determination will be low. Then gradually increase the temperature just to the beginning of T a h l e 111-Carbon T e t r a c h l o r i d e a n d p - D i c h l o r o b e n z e n e , A l o n e a n d in M i x t u r e s . E f f e c t of V a r i a t i o n s in P r o c e d u r e the dissolution of the boundary, cool, tronsfer to a separatory NATUREOF VARIATION RECOVERY funnel, and determine the halides as described under Method 1. Per cenl CARBOK TETRACHLORIDE, 0.10 T O 0.20 G R A M hlETHOD 3. TOTAL HALOGEN I N PRESENCE OB 170LSTILE with propyl alcoholic KOH, countercurrent flow of CoxsTITuExTs-Proceed exactly as in Method 2 to the point Refluxed solvent through trap of glass beads 93.0 K O H in stoppered flask a t 99.5' C. for 2 hours, voluat which the solution, after heating to incipient mixing of the Butanolic metric pipet used to measure aliquot 97.4 layers, is cooled. If not already present, add sufficient kero- Same, using hlohr pipet 99.3 sample added in gelatin capsule 100.1 sene to make the volume of the solution 7 5 to 100 cc., then Same, hlethod 2, volumetric pipet used t o measure sample 97.2 99.2 add 2 to 2.5 grams of metallic sodium in very small pieces, Same, using hlohr pipet 9-DICHLOROBEKZENE. 0.20 GRAM connect to a reflux condenser, and heat rapidly in a glycerol Stepanow method 86.2 or oil bath to the temperature of reflux. Do not lieat over a Same, using butanol 93.3 1 xylene as solvent 100.5 wire gauze or an asbestos mat, as under these conditions a Method hlethod 1: 20 cc. butyl alcohol instead of 5 cc. of amyl 91.2 crust of solid alcoholate, enmeshing the sodium globules, is C A R B O N TETRACHLORIOE, 0.10 GRAM; f i - D I C H L O R O B E N Z E X E , 0.12 G R A M 3, stoppered flask in lieu of overlaying, 1.5 gram 100.1= formed over the bottom of the flask. Reflux a t a moderate hlethod Method 3, 10 cc. butanolic K O H , 0.75 gram Pia 99 5 rate for 2 hours or until the sodium is consumed. Then cool, Method 3, 20 cc. butanolic K O H , 1.0 gram S a 95.5 a Total chlorine. add water to destroy any excess of sodium, transfer to a separatory funnel, and determine halides as described under Conclusions Method 1. The experience of the writers as presented indicat'es that I n the absence of inorganic halogen, aryl halogen may be taken as the difference between total and alkyl halogen. the modifications of the Stepanow and Cappenberg methods described herein may prove to be of general application to Results of Analyses organic halides with the possible exception of certain minor Some of the data obtained by the authors in this investiga- classes of compounds not yet investigated as mentioned above. tion which is pertinent to the description of the method are However, close attention t o detail and careful manipulation a t certain points in the procedure are necessary to obtain satisgiven in the accompanying tables. Table I gives the results obtained in the analysis of a lim- factory results with some compounds. They have been shown ited number of several classes of compounds. Members of to be applicable to the separation of carbon tetrachloride and neither the acyl halides nor the halo-amines, in view of their p-dichlorobenzene in fly sprays. known "liable" character, were subjected to analysis. The Literature Cited halides of saturated cyclic and unsaturated aliphatic com(1) Bauman and Kux, Z anal. Chem., 32, 129 (1893). pounds were also omitted, but since groups attached to a carbon (2) Busch, Z . ange:cl. Chem., 31, 232 (1918). atom alpha to a polar group are unusually reactive it is be(3) Cappenberg, Pharm. Zlg., 56, 667 (1911). lieved that the method will be applicable a t least to this second (4) Clifford, J . A m . Chem. Soc., 41, 105 (1919). ( 5 ) Conant and Kirner, Ibid., 46, 232 (1924). class. Conant and Kirner (5) have shown that the reactivity ( 6 ) Coppin and Holt, Analyst, 44, 226 (1919). of halogen atoms in straight-chain compounds does not change (7) Dains and Brewster, J . A m . Chem. Soc., 42, 1573 (1920). with increase in the length of the chain beyond the beta car(8) Duin, van, Rec. trao. chim., 45, 363 (1926). bon atom in the case of several polar groupings, and hence, (9) Favrel and Bucher, Ann. chim. a ~ p i i c a t a 9, , 321 (1926). since n-propyl bromide is decomposed by the treatment de- (IO) Fittig, A n n . , 121, 363 (1862). (11) Francois, A n n . faEs., 22, 226 (1929). scribed in Method 2, the higher homologs should also be (12) Frankland, Carter, and Webster, J. Soc. Chcm. Ind., 38, 153T (1919). amenable to the method. (13) Hahn and Reid, J. A m . Chcm. Soc., 46, 1645 (1924). The compounds mentioned were weighed out, dissolved in (14) Jones, J. SOC.Dyers Colourisls, 35, 4 5 (1919).
IL\TD USTRIAL AiYD EAVGINEERISG CHEJfISTRY
October 15, 1930
397
(22) Meyer and Jacobson, “Lehrbuch der organischen Chemie,” 2nd ed., Vol. I, Pt. I , pp. 363, 563. (23) blonthule, A n n . Chim. Anal., 17,133 (1912); Chem.-Zlg., 36, 339 (1912). (24) Plimpton and Graves, J . Chem. SOL.,42T, 119 (1883). ( 2 5 ) Stepanow, Bcr., 39, 4056 (1906). (26) Williamson, Ann., 77, 37 (1851). (27) Wurtz, Ibid., 96, 364 (1855).
(15) Krishna and Swarup, J . A m . Chem. SOC.,60, 790 (1928). (16) Kunke, J . Assocn. Oplciol Agr. Chem., 11, 355 (1928). (17) Lemp and Broderson, J . A m . Chem. Soc., 39, 2069 (1917). (18) Lowenhertz, Z . p h r s i k . Chem., 29, 413 (1899). (19) Macbeth, Chem. News, 126, 305 (1922). (20) Marcusson and Doscher, Chem.-Ztg., 34, 417 (1910). (21) Maryott, Chem. News, 103, 1 (1911).
Titration of Lead by Means of a Thermionic Titr ometer’ R. W. Gelbach and K. G. Compton STATECOLLEGEOF WASRIWGION~ PULLMAN, WASH.
If the apparatus is set up according to the diagram and the for determining lead are not of practical value. Sev- value of Ez is adjusted until about 2.5 milliamperes flow in eral adaptations have been made of the thermionic the plate circuit of the second tube, then a change of 120 electron tube to this and other titrations ( 1 , 3, 4). The de- millivolts in the potential of the titration cell will cause 1 vices discussed in this article were designed to increase the .milliampere deflection of the milliammeter. This is about speed and accuracy with which potentiometric titrations the order of potential change expected from the oxidation of might be made. An ideal potentiometric titration apparatus ferrous ammonium sulfate when titrated with 0.05 N poshould indicate the equivalence point-i. e., where A E / A V be- tassium permanganate. Such sensitivity will be satisfaccomes a maximum-without the necessity of constructing a tory for most titrations. If greater sensitivity is desired, i t may be obtained by recurve. A device sufficiently sensitive to changes in potential to show a large change in the deflection of the indicating in- ducing the resistance of R3 to zero. If this is still insufficient, an increase in the values of Rz and Ed will increase the voltage amplification of the ux L U I A first tube. By means of this adjustment a voltage amplification of 80 to 1 may be obtained. Using a value of 0.5 megohm for Rz and 190 volts a t E4 a change of 1 millivolt on the grid of the first tube caused a deflection of 60 microamperes on the milliameter. This amplification is applicable to null-point determinations in conjunction with potentiometric work.
HE electrometric methods that have been developed
T
Materials
F i g u r e 1-Three-Tube
The electrode system consisted of a bright platinum wire in conjunction with a calomel electrode. The calomel was prepared according to the method of Ewing ( 2 ) . Solutions were prepared from e. P. chemicals produced by leading chemical companies.
Titrometer
E1 and E z 4 5 - v o l t B battery Ra--2000-ohm potentiometer Ea-90-volt B battery Ra-50,000-ohm resistor R6-3-ohm rheostat or 0.75 ampere amperite E4-1.5-volt battery Ej-6-volt storage battery S-filament switch RI-30,000-ohm electrad resistance V-voltmeter (in case amperites are not used) .M-milliammeter with 1.5 milliampere range Rz-10,OOO-ohm electrad reqistance Ra-500-ohm potentiometer In case i t is not desirable t o use a bimetallic electrode system, parts Rd, Rw8and S may be omitted.
strument, thus showing the approach and location of this point, has been constructed. Apparatus The three-tube titrometer (Figure 1) will operate over a 2-volt range and is applicable to any system of electrodes other than glass. This particular circuit was employed by the authors for titrations in which the potential change did not exceed 80 millivolts. Closing the switch, S , adapts the circuit to polarized bimetallic systems. The advent of the screen-grid tube has made possible the circuit shown in Figure 2, in which only two tubes are employed. This device has been used successfully with electrodes having a resistance greater than 130 megohms. 1
Received July 3, 1930.
M
’
x
-9
R3
+
-
€4
x
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r
F i g u r e 2-Two-Tube
fl4
RI--l5-ohm potentiometer Kz-0 2.5 megohm resistor R~-2000-ohm resistor Rd -1000-ohm rheostat R$--G-ohm rheostat El-1 5.boIt C battery Ez--38 5-volt C battery Ea and Er-22.5-volt B batteries Es--SO-\olt B battery Es-6-volt storage battery E?--l.5-bolt battery dl--milliammeter with 1.5 milliampere range 1’---voltmeter Circuit
