QD 
 
 5 5 Tt ~ 
 
 2 * .ECTROCHEMICAL 
 
 EXPERIMENTS 
 ^1 OETTEL 
 
 UC-NI 
 
 277 7M7 
 
 GAR R SMITH 
 
LIBRARY 
 
 OF THE 
 
 UNIVERSITY OF CALIFORNIA. 
 
 Class 
 \ 
 
INTRODUCTION 
 
 TO 
 
 ELECTROCHEMICAL 
 EXPERIMENTS 
 
 BY 
 
 DR. FELIX OETTEL 
 
 TRANSLATED 
 (WITH THE AUTHOR'S SANCTION) 
 
 BY 
 
 EDGAR F. SMITH 
 
 )F CHEMISTRY IN THE UNIVERSITY OF PENNSYLVANIA 
 
 
 -DIVERSITY 
 
 OF 
 
 J^Smb {TwentE*sii Illustrations 
 
 PHILADELPHIA 
 P. BLAKISTON, SON & CO. 
 
 1012 WALNUT STREET 
 I8 9 7 
 
Q'D 
 
 COPYRIGHT, 1897, BY P. BLAKISTON, SON & Co. 
 
 PRESS OF WM. F. FELL &. CO., 
 
 I22O-24 SANSOM ST., 
 
 PHILADELPHIA. 
 
PREFACE. 
 
 THE purpose of this little volume is to furnish tech- 
 nical chemists and all persons interested in the ap- 
 plications of electricity to chemical manufacture with 
 a concise guide, containing in a compact form all 
 that is essential for the comprehension and solution 
 of problems arising in this comparatively new field of 
 chemical investigation. That it has in a measure 
 fulfilled its mission is evidenced by the hearty recep- 
 tion and favorable criticism accorded it in its German 
 home, and by its translation into Russian. That 
 it may prove equally helpful to all its English-speak- 
 ing readers is the hope of the translator, to whom it 
 is a pleasure to acknowledge indebtedness to Prof. 
 George F. Barker for advice and assistance in the 
 preparation of the text and correction of the proof 
 sheets. 
 
TABLE OF CONTENTS. 
 
 PAGE 
 
 A. SOURCE, MEASUREMENT, AND^REGULATION OF CURRENT, . 9 
 Sources of Current. 
 
 Galvanic Batteries Primary Batteries 10 
 
 Arrangement of Batteries, 14 
 
 Storage Cells Accumulators, 20 
 
 Thermopiles, 25 
 
 Dynamos, 27 
 
 Rules for Arrangement and Working of a Small Dynamo, 34 
 
 Current Measurement, 37 
 
 Silver Voltameter, 38 
 
 Copper Voltameter, . . 39 
 
 Oxyhydrogen Voltameter, 4 
 
 Tangent Galvanometer, 43 
 
 Torsion Galvanometer, 47 
 
 Technical Measuring Apparatus, 49 
 
 Ampere-hour'Meter, 5 1 
 
 Measurement of Pressure, . . 53 
 
 Regulation of Current, 57 
 
 B. ARRANGEMENT OF EXPERIMENTS. 
 
 Vessels, 63 
 
 Diaphragms, 65 
 
 Electrodes, 68 
 
 Conductors, 7 1 
 
 Electrolyte, 73 
 
 Sketch of Arrangement of Experiment, 75 
 
 vii 
 
Vlll TABLE OF CONTENTS. 
 
 C. PHENOMENA OBSERVED IN ELECTROLYSIS. PAGE 
 
 Decomposition Pressure. Polarization Current, 77 
 
 Law of Faraday. Current Efficiency, 88 
 
 Transference of Ions, . 92 
 
 Current Density, 94 
 
 Working Pressure, 96 
 
 D. PRELIMINARY EXPERIMENTS OF AN ELECTROLYTIC PRO- 
 
 CESS, 98 
 
 E. .CALCULATION OF NECESSARY POWER. CHOICE OF DY- 
 
 NAMO, 100 
 
 F. PRACTICAL PART. 
 
 1. Construction and Calibration of a Tangent Galvanometer, 104 
 
 2. Calibration of a Galvanometer by Means of a Shunt, . . 107 
 
 3. Construction of Simple Instrument for Measuring Pres- 
 
 sure, ... Ill 
 
 4. Calculation for and Construction of a Regulating Resist- 
 
 ance, 115 
 
 5. Working of an Arsenical Copper Liquor, 118 
 
 6. Arrangement for Electrochemical Analysis, 134 
 
 G. TABLES. 
 
 I. Electrochemical Equivalents of the More Important 
 
 Elements, 141 
 
 II. Thermochemical Data, 142 
 
 III. Wire Resistances, 144 
 
INTRODUCTION 
 
 TO 
 
 Electrochemical Experiments. 
 
 A. SOURCE, MEASUREMENT, AND REGU- 
 LATION OF THE CURRENT. 
 
 SOURCES OF THE CURRENT. 
 
 Four sources may be considered : ordinary galvanic 
 batteries (primary batteries), storage cells or accumu- 
 lators, thermopiles, and dynamo machines. The first 
 are most readily obtained, but constitute a rather in- 
 complete adjunct. Storage cells are decidedly more 
 advantageous, so far as regards constancy of current 
 and cleanliness in handling. Those who have em- 
 ployed this source of electric energy return to the 
 ordinary battery only in cases of absolute necessity. 
 Storage cells may be charged either by dynamos or 
 by thermopiles. The latter were, for a time, deemed 
 worthless, but since their improvement in construc- 
 tion have again come into favor. They are to be 
 recommended for experimental work on a small scale, 
 
 * 9 
 
IO ELECTROCHEMICAL EXPERIMENTS. 
 
 and for analytical purposes, but are only available 
 when illuminating gas can be had. A small dynamo 
 is the best source of current for various practical 
 experiments. 
 
 In undertaking an electrochemical investigation, the 
 most rational beginning is the selection of the proper 
 working conditions for the experiment, aside from 
 batteries or accumulators. With the data and experi- 
 ence thus gained, the next step is the arrangement of 
 a small plant provided with a dynamo. Difficulties, 
 as a rule, now appear; these are generally of a con- 
 structive nature. When they have been surmounted, 
 and the miniature experimental plant serves its 
 purpose properly, calculations for some definite pro- 
 cess may next engage the attention of the experi- 
 menter. 
 
 After this brief enumeration of the different sources 
 of electric energy and their general application, a 
 more detailed description of them may be given. 
 
 PRIMARY BATTERIES (GALVANIC CELLS). 
 
 Cells of this class, designed for the execution of 
 electrochemical experiments, should furnish a strong 
 and constant current. Many forms have been de- 
 vised, but there is not one which fully meets these 
 conditions, hence there can be no purpose in entering 
 into a detailed description of each variety. Two may 
 be mentioned the Bunsen cell and the Daniell cell. 
 
 The Bunsen cell is the zinc-carbon combination. 
 
SOURCES OF THE CURRENT. I I 
 
 It consists, usually, of a glass jar in which there 
 is a heavy, amalgamated zinc cylinder ( pole). 
 Within the latter stands a porous cup. This con- 
 tains concentrated nitric acid, in which is immersed 
 a bar of hard gas-carbon. The glass jar contains 
 dilute sulphuric acid (1:20). The action of the 
 current causes a reduction of the nitric acid, and 
 fumes of nitrogen dioxide arise, making it necessary 
 to keep the batteries in a good draught chamber. 
 This inconvenience may be partly obviated by drop- 
 ping into the cup, from time to time, chromic acid or 
 potassium bichromate. The electromotive force of 
 the cell is 1.8 V. At first the current from it is 
 very strong, but it grows considerably less in the 
 course of a few hours. This form of battery is very 
 well adapted for experiments requiring a strong cur- 
 rent for a relatively short time, and where high pres- 
 sures are necessary. It is not suited for experiments 
 extending through a period of days. 
 
 The following points should be observed when 
 using this battery. The zinc must be well amalga- 
 mated, otherwise a tumultuous evolution of hydro- 
 gen will occur, leading to a rapid consumption of the 
 zinc. In amalgamating, first dip the zinc cylinder 
 into very dilute sulphuric acid, then pour mercury 
 over it, and distribute the latter with a brush. The 
 clay cup should not be so porous that much nitric 
 acid can reach the zinc. In arranging the battery, 
 the sulphuric acid is first introduced, and when the 
 
12 ELECTROCHEMICAL EXPERIMENTS. 
 
 porous cup has become thoroughly permeated with 
 it, the nitric acid is introduced into the cup. As re- 
 gards the carbon bars, it may be said that those from 
 natural retort carbon are superior to those made 
 from pressed carbon. To prevent them from absorb- 
 ing nitric acid, which would eventually reach and 
 destroy the metallic binding-screw attachments at 
 their exposed ends, the bars should be heated or 
 dried thoroughly and the ends then immersed in 
 melted paraffin, the excess of the latter being re- 
 moved with a brush. The binding screws, in union 
 with the battery poles, should be clean and bright ; 
 the other parts may be covered with an asphalt paint, 
 and in this way be protected from acid vapors. The 
 current may be conducted from the battery according 
 to the experiment by a stout copper wire (not less than 
 one mm. in diameter). This is sometimes rolled into 
 spirals, although there is really no necessity for so 
 doing. 
 
 The Daniell cell possesses less electromotive force 
 but greater constancy than the Bunsen cell. It con- 
 sists of amalgamated zinc in dilute sulphuric acid, 
 and copper in a saturated copper sulphate solution, 
 the two liquids being separated by a porous cup. 
 The sheet of copper serving as the positive pole is 
 cylindrical in shape, and perforated. A copper wire 
 is welded to it. In order to maintain a concentrated 
 solution, a perforated bottom is sometimes placed in 
 the upper portion of the jar. Crystals of copper 
 
SOURCES OF THE CURRENT. 13 
 
 sulphate are, from time to time, placed upon it. If 
 zinc be on the exterior and copper be placed in the 
 porous cup, the combination will prove more energetic 
 than with the reverse condition. The pressure of 
 such a cell will be about 1.05 V. The nature of the 
 porous cup affects the efficiency of a battery very 
 much. If it be too dense, the internal resistance of 
 the cell will be too great. If it be too porous, then 
 the copper solution will penetrate to the zinc and 
 the latter will rapidly become covered with a film of 
 copper, in consequence of which the action of the 
 cell will be much diminished. 
 
 In testing a porous cup, fill it, when perfectly dry, 
 with water. It should be wet throughout within a 
 few minutes, but the water should pass through it very 
 slowly. It is a good rule to reject all porous cups 
 which, shortly after they have been moistened, show 
 drops of water on their external surface. 
 
 Another form of Daniell cell, arranged for con- 
 tinuous use, consists of a hollow zinc cylinder, not 
 amalgamated, standing in a concentrated zinc sul- 
 phate solution. In the porous cup there is a copper 
 cylinder immersed in a solution of copper sulphate. 
 By use, the latter loses color, indicating that it is 
 necessary to add crystals of blue vitriol. From time 
 to time the zinc solution is siphoned off, an equal 
 volume of water being added to prevent the separation 
 of crystals of zinc sulphate. Once or twice every 
 week the zinc cylinder should be taken out and 
 
14 ELECTROCHEMICAL EXPERIMENTS. 
 
 cleaned. An arrangement of this sort consumes 
 very little zinc, but has a greater internal resistance. 
 Its pressure does not exceed 0.9-0.95 V. 
 
 The disagreeable deposit of salts on the edges and 
 over the sides of the cells can be lessened by employ- 
 ing porous cups with glazed edges, or by paraffining 
 the portion of the cup extending beyond the liquid. 
 It can not, however, be wholly overcome. When the 
 battery is disconnected, the carbons and the porous 
 cups should be thoroughly washed, and put away 
 when dry. If the washing be omitted, the cups will 
 be cracked or at least be damaged to a marked 
 decree by the salts which crystallize out. 
 
 When a battery or cell is to be again put together, 
 the cup must first be thoroughly saturated with the 
 zinc sulphate solution, and the blue vitriol solution 
 then be introduced into it. If the cup be not dry, 
 but moist with water, it will, in consequence, yield but a 
 feeble current until its walls are filled by diffusion 
 with the better conducting vitriol solution. 
 
 ARRANGEMENT OF CELLS. 
 
 Having a number of cells, it is possible to arrange 
 them in three different ways : 
 
 (a) Parallel, when all the zinc poles are con- 
 nected with one another, and the copper poles in like 
 manner (Fig. i). 
 
 (b) In scries, when each zinc pole is connected with 
 the copper pole of the adjacent cell (Fig. 2). 
 
SOURCES OF THE CURRENT. 15 
 
 (c) In groups (inixed arrangement] ; an equal number 
 of cells are united into groups and the latter then 
 arranged as individual cells. The arrangement will 
 
 uu/ 
 
 FIG. 
 
 be different, depending upon whether that of (a) 
 Fig. 3 or (<) Fig. 4 is observed within the group. 
 The first is preferable, because slight defects in the 
 individual cells are less disturbing. 
 
 FIG. 2. 
 
 When should the one or the other combination be 
 chosen ? In answering this question, the following 
 points should be remembered : 
 
i6 
 
 ELECTROCHEMICAL EXPERIMENTS. 
 
 (1) The maximum work of a battery is obtained 
 when the resistance in the outer circuit is equal to 
 the total resistance of the battery. 
 
 (2) In the parallel arrangement of cells, the electro- 
 motive force of the battery is not altered. The inter- 
 
 FIG. 3. 
 
 FIG. 
 
 nal resistance is, however, diminished directly accord- 
 ing to the number of cells. 
 
 (3) When cells are arranged in series, both the 
 pressure and the resistance of the individual cells are 
 increased in the sum total. 
 
 If the electromotive force of a cell be represented 
 
SOURCES OF THE CURRENT. I/ 
 
 by e, and its internal resistance by w, then a battery 
 of n cells arranged parallel would have 
 
 the electromotive force e 
 
 and 
 
 the internal resistance . 
 
 n 
 
 A battery with cells in series would, on the con- 
 trary, have 
 
 the electromotive force ;/. e 
 
 and 
 the internal resistance n. w. 
 
 With a battery of n cells in series, composed of a 
 elements parallel, each group would have 
 
 the electromotive force e 
 
 and 
 
 the internal resistance , 
 
 so that in the entire battery of n such groups there 
 would be 
 
 the electromotive force n. e 
 
 and 
 
 the internal resistance n. 
 
 a 
 
 According to Ohm's law, the current strength in a 
 circuit is equal to the electromotive force divided by 
 the total resistance. The latter equals the sum of the 
 internal resistance Wj of the battery and the resistance 
 
I 8 ELECTROCHEMICAL EXPERIMENTS. 
 
 W a of the external circuit, so that the current strength 
 I may be expressed by the formula 
 
 E 
 
 = Wi + W a ' 
 
 This formula gives two possibilities for the increase 
 of current: either by increase of the numerator or the 
 diminution of the denominator. The first follows 
 from the arrangement of cells in series (increase of 
 electromotive force), the second from their parallel 
 arrangement (reduction of Wi). Which course should 
 be pursued depends upon whether W a is large or 
 small in proportion to W { . Several examples will 
 serve to demonstrate that in cases of great external 
 resistance a series arrangement of the cells is the 
 proper arrangement, while with low external resistance 
 the parallel arrangement should be chosen. 
 
 Examples : 
 
 The E. M. F. of a cell is 1.05 V. 
 
 Internal resistance of a cell is 0.5 $. 
 
 (a) External resistance is 10 Q. 
 
 Current strength with various combinations : 
 
 1.05 
 
 1 cell, I = - --=0.10 amp. 
 
 0.5 + 10 
 
 2. I.O5 
 
 2 cells in series, ....!= =0.19 amp. 
 
 2. 0.5 f 10 
 
 . 4. 1.05 
 
 4 cells in series, . . . . I = = o. 15 amp. 
 
 4. 0.5 -f 10 
 
SOURCES OF THE CURRENT. 
 
 Reverse : 
 
 1.05 
 
 2 cells in parallel, . . . I = - = o. IO2 amp. 
 
 1.05 
 4 cells in parallel, . . . I = = o. 104 amp. 
 
 *-+ 10 
 
 (b) External resistance .-= o.i Q. 
 Current strength with various combinations : 
 1.05 
 
 1 cell, ........ I = =1.75 amp. 
 
 0.5 + o.i 
 
 2. I.O5 
 
 2 cells in series, ....! = 1.91 amp. 
 
 2. 0.5 -j- o.i 
 
 4- I -5 
 
 4 cells in series, ....!=- - = 2.0 amp. 
 
 4- 0.5 + o.i 
 
 Reverse : 
 
 1.05 
 
 2 cells in parallel, . . , I = = 3.0 amp. 
 
 1.05 
 
 4 cells in series, ....! = = 4.67 amp. 
 
 0-5 
 
 1+0.1 
 
 4 
 
 After some experience in electrochemical work it 
 will be easy to determine whether one is confronted 
 in an experiment with a high or low resistance, and 
 the manner of cell arrangement will accordingly 
 follow. Until such experience has been acquired, the 
 safest, wisest course will be to experiment ! Intro- 
 
2O ELECTROCHEMICAL EXPERIMENTS. 
 
 duce a measuring instrument between the battery and 
 the experimental cell, and vary the arrangement of 
 the cells until the maximum current strength is ob- 
 tained. The mixed arrangement will be found prefer- 
 able if the cells employed, either in consequence of 
 their small size, or from any other causes, show a high 
 internal resistance. Several cells arranged in parallel 
 will give the same result as one large form of like type. 
 Meidinger cells are frequently met with in labora- 
 tories. They are only suitable for analytical opera- 
 tions in which great current strength is not required. 
 For electrochemical experiments of any other sort 
 they are worthless, because too many of them must 
 be arranged in groups to get currents of any great 
 degree of intensity. 
 
 STORAGE CELLS, ACCUMULATORS,* SECONDARY 
 BATTERIES. 
 
 While all primary batteries are more or less incon- 
 venient to handle, and, as a rule, furnish currents which 
 are not very strong, storage cells or secondary bat- 
 teries are excellent sources of electric energy, and 
 serve for the most varied electrochemical experiments. 
 
 Accumulators consist of a number of lead plates 
 which are covered with a so-called "active mass." 
 
 *A clear, concise description of the action and care of secondary 
 batteries will be found in Elbs' " Die Accumulatoren," Leipzig, 1893. 
 ffoppe, " Die Accumulatoren," 2te Aufl., Berlin, gives a more ex- 
 haustive and historical account of their development. 
 
SOURCES OF THE CURRENT. 21 
 
 On the anode (positive) plate this is lead superoxide, 
 while on the kathode (negative) plate it is spongy 
 lead. The liquid is pure, dilute sulphuric acid. 
 Secondary batteries are, therefore, galvanic batteries, 
 which, in consequence of lack of diaphragm, have 
 a vanishingly, low, internal resistance. When ex- 
 hausted, it is not necessary to take them apart ; for 
 by contact with a more powerful source of energy 
 they can be directly charged /. e., they can be again 
 restored to their normal, useful state. 
 
 The various modifications found in trade are prac- 
 tically the same. Nearly all are guaranteed for a 
 longer or shorter period. The Tudor and Correns * 
 types are among the best. There are both stationary 
 and transportable varieties ; the latter are closed, so 
 that no loss of acid occurs in transportation from one 
 place to another. 
 
 The capacity of a storage cell is given in ampere- 
 hours. The current strength varies with the individual 
 types, and so long as it is not exceeded, it is immaterial 
 how the electric energy is applied /. e., whether a 
 powerful current is used for a brief period or a feeble 
 current for a prolonged period. A storage cell with 
 a capacity of 100 ampere-hours with a maximum cur- 
 rent of 10 amp. may be discharged at the rate of 
 
 10 amperes for a period of 10 hours, or 
 5 " " " 20 " 
 i " " " " 100 " etc. 
 
 * The chloride accumulator is most widely known in this country. 
 TR. 
 
22 ELECTROCHEMICAL EXPERIMENTS. 
 
 Most factories give instructions as to the manner 
 in which their secondary batteries shall be handled. 
 General rules may, however, be given here. The 
 smaller types are usually mounted. The glass jars 
 should be thoroughly cleaned without removing the 
 lead plates. The latter are then completely immersed 
 in pure, dilute, cold sulphuric acid of 1.15 sp. gravity. 
 The purity of the acid is important. The presence of 
 arsenic or nitric acid in it is harmful. If the acid at 
 hand is not sufficiently pure, conduct hydrogen sul- 
 phide through it. Filter out the precipitated sul- 
 phides, and expel the hydrogen sulphide gas by means 
 of an air current. When the glass jars have been 
 filled, begin " charging." Connect the brown -f- plates 
 with the + conducting wire of a dynamo (or a ther- 
 mopile) and the gray plates with the pole. The 
 current recommended for the cell, by its manufacturer, 
 is next conducted into it. As a rule, this first charg- 
 ing will extend through a day or more. It is in this 
 way that the plates are first brought into normal con- 
 dition. The current will, apparently, be taken up 
 completely by the accumulator. The -f- plates grad- 
 ually change in color to a brownish black, while the 
 plates become a light gray. Eventually, a strong 
 evolution of gas will be observed, first on the positive 
 and later on the negative plates ("the acid boils"). 
 When the evolution of gas on all the plates remains 
 nearly the same for an hour, the first " charge " may 
 
SOURCES OF THE CURRENT. 23 
 
 be regarded as finished. The pressure for each cell 
 will have increased to 2.5 V. ; the specific gravity of 
 the acid will vary from 1:15 to 1.18-1.20. All of the 
 active mass upon the positive plates will now be com- 
 pletely oxidized, and that upon the negative plates 
 will be fully reduced. The first discharging can next 
 follow. It should be done with the current strength 
 previously mentioned. The pressure will fall rapidly 
 to 2.0 V. and then remain constant for a long period. 
 As soon as it decreases to 1.85 V., the discharging 
 should be interrupted. The cells should then be 
 charged a second time until the pressure reaches 2.5 
 V. and equal gas evolution is observable on all plates. 
 The cell is now ready for further use. 
 
 To insure long life to the cell, certain precautions 
 should be observed : 
 
 1. It must be preserved from " short-circuiting." 
 
 2. The maximum strength of the discharging cur- 
 rent must not exceed that given by the manufacturer 
 of the cell. 
 
 3. Do not discharge below 1.85 V. 
 
 4. It should not continue long in a discharged con- 
 dition. If not wanted for use, it should be held in a 
 charged state. 
 
 5. It is well occasionally to overcharge /. e., to con- 
 tinue charging for some hours with a feeble current 
 even after the " boiling" has begun. 
 
 6. Should it become necessary at any time to re- 
 move a plate from the acid, it must not be allowed to 
 
24 ELECTROCHEMICAL EXPERIMENTS. 
 
 become air-dried. It should be immediately im- 
 mersed in dilute acid. 
 
 7. Avoid heavy blows : they loosen and throw out 
 the " active masses." 
 
 By observing these rules, storage cells can be kept 
 in satisfactory condition for some time, especially if 
 the maximum effect be not constantly aimed at. It is 
 better to charge too frequently than not enough. In 
 time the surface of the liquid will sink below the 
 upper edge of the plates. Pure water should then be 
 added. The specific gravity of the acid should be 
 taken after charging and discharging. The use of the 
 hydrometer will furnish a means of ascertaining how 
 far the charge has been consumed. If, in time, it is 
 noticed that, after charging, the specific gravity of the 
 liquid is not the same as was observed at first, when 
 the cell was in this condition, it is evidence that acid 
 has been lost by spattering or in some other way. 
 Dilute acid should be added several times for the water 
 that evaporates until the normal condition is restored. 
 
 Storage cells answer well, both for analytical and 
 experimental purposes. The current and pressure of 
 any source of electricity can, by this means, be 
 altered in various ways. For example, a small 
 dynamo of 5 V. and 30 amperes, together with six 
 accumulators, each having a maximum discharge of 
 ten amperes, are at the disposal of the operator. As 
 
 the machine is only capable of charging ---- = 2 sec- 
 
SOURCES OF THE CURRENT. 2$ 
 
 ondary batteries in series, groups of three cells each 
 in parallel are formed, and these then connected in 
 series to the machine. When the charging is finished, 
 the following groupings may be made : 
 
 I to 6 in series will yield 2-12 V. and 10 amp. 
 I " 6 parallel " " 2V." 10-60 amp. 
 
 2X3 " " 4 V. " 30 amp. 
 
 3X2 6 v. " 20 amp. 
 
 Accumulators have found their way into labora- 
 tories slowly because dynamos with which to charge 
 them were rarely present. Recently, thermopiles * 
 have been used successfully for this work, hence 
 there now remains no good reason for their non- 
 adoption generally in electrolytic work. 
 
 THERMOPILES. 
 
 Thermopiles, which transform heat into electricity, 
 would be the most convenient sources of electric 
 energy for laboratory purposes if they could be built 
 in durable forms. In this respect the older modifica- 
 tions of Noe and Clamond were sadly lacking. They 
 required much attention and a constant gas pressure, 
 and, in spite of all care, rarely lasted for any great 
 length of time. The new modifications of Giilcher f 
 show decided improvement over the early types, and 
 the common verdict in regard to them is very favor- 
 able. Julius Pintsch, Berlin, O., Germany, makes 
 
 * Elbs, Chem. Ztg., 1893, 66. f D - R - p -> N <>- 44^6- 
 
 c 
 
26 
 
 ELECTROCHEMICAL EXPERIMENTS. 
 
 three varieties of this thermopile (Fig. 5). The 
 largest model, consuming 170 liters of gas per hour, 
 develops an electromotive force of 4 V. with an 
 internal resistance of 0.6 to 0.7 Q. The price of this 
 form is about $48. The smaller models consume 
 130-70 liters of gas, and have a pressure of from 3-1.5 
 
 FIG. 5. 
 
 V. If it be desired to use the larger form for charg- 
 ing storage cells, the latter should be arranged 
 parallel. This will result in the production of a cur- 
 rent of from 2-3 amperes. For ordinary purposes, 
 therefore, a thermopile of this description will be 
 ample. At present they are used in laboratories 
 
SOURCES OF THE CURRENT. 2? 
 
 chiefly for electrolytic analyses, but combined with 
 secondary batteries can be more widely applied. 
 
 DYNAMO-MACHINES. 
 
 The dynamo is the proper source of electric energy 
 in all experiments requiring a powerful current for an 
 extended period. 
 
 The action of such machines is dependent upon the 
 electric current induced in a coil of wire when brought 
 into a magnetic field i. e., when it is rotated between 
 the poles of a magnet. The portion that is rotated 
 is called the armature. Externally it has the form of 
 a flat ring or cylinder. The magnets about which 
 the armature rotates are not permanent, but are 
 electromagnets excited by a part of the current pro- 
 duced by the machine. The several current impulses 
 induced in the armature are collected in the commu- 
 tator, or collector, and are given up by the brushes 
 to the external conductors. 
 
 Two large classes of dynamos exist: direct current 
 machines, and those producing alternating currents. 
 In the first class, two successive current impulses have 
 the same direction, while in the second class they 
 proceed in opposite directions. For electrochemical 
 purposes, direct current machines are alone of conse- 
 quence. 
 
 The manner of winding also causes a division into 
 series machines, shunt machines, and compound 
 machines. In the first, the current produced in the 
 
28 ELECTROCHEMICAL EXPERIMENTS. 
 
 armature proceeds immediately about the electro- 
 magnets, then through the external circuit back to 
 the armature. In the second, the current divides 
 itself in the armature. The smaller portion of it 
 proceeds about the electromagnets, while the major 
 portion passes through the external circuit. In the 
 mixed or compound machines, a part of the winding lies 
 in the shunt circuit, the remainder in the main circuit. 
 
 Shunt machines alone interest us. They have this 
 important advantage, that in consequence of any 
 disturbances the poles can not reverse. They will, 
 therefore, be somewhat more closely considered. As 
 an example, or type, the Schuckert * flat-ring machine 
 (Fig. 6) may be mentioned. 
 
 The current circulating in the external circuit is the 
 main current, that about the magnets is the shunt cir- 
 cuit of the machine. The latter is interrupted at one 
 point for the introduction of the shunt-regulator, N. 
 This serves to change the pressure at the terminals 
 of the machine. If resistance be introduced by 
 means of this regulator into the magnet winding, the 
 current passing through it will be less than before ; 
 consequently the magnetism and the magnetic field 
 of the machine will be reduced and the pressure will 
 fall. By suitable winding of the shunt regulator the 
 pressure may be varied within wide limits. Another 
 means of altering the pressure consists in changing 
 the velocity. When the latter is increased, the pres- 
 
 * In this country the Edison machine is generally used. TR. 
 
SOURCES OF THE CURRENT. 
 
 2 9 
 
 FIG. 6. 
 
3<D ELECTROCHEMICAL EXPERIMENTS. 
 
 sure increases, and vice versa. This course is resorted 
 to only when extraordinary conditions prevail, 
 otherwise the velocity allowed by the builder of the 
 machine must be taken as normal. A dynamo with 
 an external resistance of 0.068 Q will yield 
 
 125 Amp. 8.5 V. with 500 revolutions. 
 215 " 14.7 V. " 800 " 
 
 280 " 19. o V. " 1000 " 
 
 Dynamos are constructed for the most varied cur- 
 rent strengths and pressures. In chemical operations, 
 strong currents are of prime importance the pres- 
 sures are relatively low. Chemical dynamos vary, as 
 a rule, from 4-20 volts and 100-500 amperes. For 
 electric lighting the customary pressures vary from 
 65-110 V., while the current strength falls below 100 
 amperes. 
 
 The product obtained by multiplying together the 
 voltage and the amperage represents the output of a 
 dynamo. The number of volt-amperes is, therefore, 
 a direct means of measuring the mechanical power of 
 a dynamo in each moment of its activity. 
 
 A chemical dynamo " running empty," i. e., when 
 it is not connected with a current circuit, rapidly at- 
 tains the pressure allowed by its prevailing velocity. 
 This is called the open-circuit pressure. It is always 
 greater than that observed when the machine is doing 
 work, or than the pressure on closed circuit. A wire 
 resistance connected with the two terminals at once 
 
SOURCES OF THE CURRENT. 3! 
 
 shows a current, which maybe calculated from Ohm's 
 law. According to the latter, the current strength 
 I is 
 
 E. M. F. 
 Sum of the resistances. 
 
 The electromotive force is the pressure of the ma- 
 chine. The resistance is on the outside the resist- 
 ance W, while within the machine there is the resist- 
 ance of the armature A and the resistance of the mag- 
 net coils M. These two latter resistances are arranged 
 parallel because the dynamo is a shunt machine. To- 
 
 A 1VI 
 
 gether they equal -^ ; hence the equation for the 
 current strength is 
 
 I = ___E_ 
 
 A + M 
 
 By lowering the external resistance, the current in- 
 creases and the pressure falls. The product of the 
 pressure and current strength the output of the 
 dynamo approaches the maximum. By further re- 
 duction of the external resistance the pressure will 
 fall still further ; the current will increase for a short 
 period, but will then decrease until, with no external 
 resistance (by short circuit of the dynamo), it becomes 
 zero. The machine then furnishes no current. The 
 voltage and amperage also rapidly drop from their 
 maximum to zero. 
 
32 ELECTROCHEMICAL EXPERIMENTS. 
 
 This action is related, too, with the flow of current 
 toward the limbs of the magnet. So long as the ex- 
 ternal resistance is great in comparison with the 
 resistance of the armature of the magnet, so long will 
 the greater portion of the current follow the more 
 convenient path about the electromagnets. It will 
 powerfully excite these, and acquire, in consequence, a 
 high pressure. As the external resistance gradually 
 grows less, the current which flows about the limbs 
 of the magnet will be diminished, the magnetic field 
 of the machine will become less powerful, also its 
 efficiency, so that, eventually, when the external 
 resistance has, by short circuit, fallen to zero, a cur- 
 rent will no longer flow about the magnets, and the 
 machine will cease to work. 
 
 The maximum work of a dynamo is attained when 
 the external resistance is slightly greater than its 
 internal resistance. This condition is the basis for 
 the normal work given by the manufacturer of the 
 machine. 
 
 Smaller machines, intended for experimental pur- 
 poses, models, are constructed with two separate 
 circuits. These carry a double coil upon the arma- 
 ture. The main coil, with low resistance, gives up its 
 current through the main brushes to the external cir- 
 cuit; the other coil has greater resistance, and the 
 current developed therein is delivered to the magnets 
 altogether in the side brushes. There are also 
 machines with separate magnet excitation. Should 
 
SOURCES OF THE CURRENT. 33 
 
 short circuiting be produced in such machines by 
 carelessness, the only resistance in the entire circuit 
 would be the main armature, and all the energy of the 
 machine would be consumed in bringing this resist- 
 ance to such a state that it would ignite. In other 
 words, the armature would be burnt out and the 
 machine be ruined. Direct current machines and 
 compound dynamos are damaged in the same manner. 
 It is only the true shunt machine that is non-sensitive. 
 
 The natural desire on the part of any one consider- 
 ing the introduction of a small dynamo, is that it shall 
 be suitable for every possible purpose. This wish 
 can not, of course, be wholly realized. It can not be 
 expected that one and the same machine will serve 
 both for electrolysis and fusion purposes, because, in 
 the first instance, low pressure and high current are 
 required, while in the second case the requirements 
 are directly the reverse high pressure and moderate 
 current. It is always well to have sufficient pressure, 
 as it can be easily reduced, but it is increased with 
 difficulty. But few electrolytic operations require 
 more than 5 V. pressure, so that this, in most cases, 
 would be amply sufficient ; yet, to render the machine 
 as widely useful as possible, it is well to double this 
 power for the following reason. 
 
 In all technical operations, this question must be 
 constantly kept in view : How can this work be done 
 on a technical scale ? The work of a dynamo can, 
 however, only be doubled by increasing the current 
 
34 ELECTROCHEMICAL EXPERIMENTS. 
 
 strength or by increasing its pressure. Reasons, to 
 be given later, will show that the latter course is pre- 
 ferable. However, to consume or exhaust the highest 
 pressure the electrolytic baths must be arranged 
 in series, as in the case of primary batteries. But 
 with this arrangement, phenomena frequently present 
 themselves which are not noticeable in a single bath. 
 If the machine, then, has a direct pressure of 10 V., it is 
 even possible, with a counter bath pressure of 5 V., to 
 try series arrangements. With baths of lower pressure, 
 it is, of course, understood that there can be more 
 baths arranged in series. 
 
 A machine having a pressure of 10 V. and a cur- 
 rent of 70-100 amperes will satisfy the greatest de- 
 mands which can be made upon a dynamo intended 
 for electrolytic experiments. The energy consump- 
 tion will, therefore, equal from iJ^-2 H. P. 
 
 RULES FOR THE ARRANGEMENT AND RUNNING OF 
 SMALL DYNAMOS. 
 
 The machine should stand upon a solid foundation, 
 which can not be easily shaken. When possible, this 
 should be a pier about J^ m. in height. It will render 
 the observation of and attention to the brushes much 
 easier. It would be very convenient and practicable 
 if the dynamo stood upon rails which would permit 
 of its movement in the direction of the driving belts. 
 Such a shifting is the simplest and most convenient 
 means of stretching a belt that has become loose and 
 
SOURCES OF THE CURRENT. 35 
 
 flabby ; cutting and sewing anew would otherwise 
 be necessary. An arrangement permitting this can 
 be furnished for every machine upon mere request to 
 the maker. 
 
 The transmission is best provided with a triple cone 
 pulley, so that the middle step affords the normal 
 number of revolutions, and the other two a somewhat 
 higher or a lower number. Avoid covered driving 
 belts. 
 
 The bed of the machine should be kept well lubri- 
 cated. The " self-oilers " should always be full. The 
 brushes should fit lightly but closely. When the 
 collector is pressed too closely it is unnecessarily 
 abraded and heated. To insure long life to the col- 
 lector, it should be constructed from hard bronze, 
 while the brushes should consist of soft copper. The 
 writer's experience warrants the statement that brushes 
 from sheet metal are superior to straight or twisted 
 wire. They consist of rolled sheets, thin as paper 
 and soft as feathers, which slide along the entire width 
 of the collector. In consequence of their great elas- 
 ticity and the absence of any points, such brushes do 
 not spark, and the collector continues smooth for 
 quite a long period. 
 
 New brushes are brought in contact with the col- 
 lector when the dynamo is running empty. Project- 
 ing points produced by wear should be cut off, because 
 they rapidly damage the machine. The brushes in 
 action require careful attention. When sparking sets 
 
36 ELECTROCHEMICAL EXPERIMENTS. 
 
 in, the binding screws should be eased and the brushes 
 withdrawn until the evil is corrected. In this adjust- 
 ment care must be taken not to raise the brushes from 
 the collector during the revolution of the latter. The 
 consequence of such an act would be not only the 
 production of a blinding spark, but, in all probability, a 
 very severe shock. In disconnecting the main current 
 avoid interrupting the conductors with both hands, 
 thus introducing the body into the circuit. This 
 always induces heavy shocks. In other respects the 
 various parts of a dynamo can be handled during its 
 action without experiencing any unpleasant sensation. 
 
 When the collector has become rough and worn, 
 the brushes are removed, and it is polished with 
 emery paper. When great inequalities exist, a smooth 
 file may be used, or the collector may be made to 
 revolve around a sharp file. 
 
 The abrasion of the brushes and of the collector 
 gives rise to a fine metallic dust which deposits on 
 all parts of the machine. It must be removed at 
 intervals. This is done, in the case of portions more 
 difficult of access, by use of a bellows. Gentle tap- 
 ping displaces it from the brushes. Copper dust 
 should not be allowed to accumulate on the collector 
 or its connections with the armature ; it gives rise to 
 short circuits in individual segments. 
 
 The current is best conducted to the baths by flat 
 metallic sheets of ample section. Branches are more 
 conveniently made from these than when rods of 
 
CURRENT MEASUREMENT. 37 
 
 metal are used. Their connection with the machine 
 is made with strips of copper cord, so arranged that 
 the machine is still movable on its foundation. 
 
 CURRENT MEASUREMENT. 
 
 Two courses maybe followed in measuring currents. 
 The chemical action of the electric current (volta- 
 meter) or its magnetic action upon a conductor 
 through which it passes (galvanometer) may be used 
 for this purpose. To determine the current strength 
 in a circuit, i. e., in an experiment, it is well to insert 
 one of the instruments just described into the circuit, 
 which consists of: battery experiment measuring 
 instrument battery. It would be wholly wrong to 
 have the following succession : battery instrument 
 battery, and then assume that the current strength 
 thus observed was identical with that from the arrange- 
 ment : battery experiment battery. 
 
 Electromagnetic measuring apparatus is calibrated 
 in an entirely empirical way by means of voltameters, 
 therefore the latter will be the first to receive a brief 
 description. Voltameters are based upon Faraday's 
 law, which reads : " The same current in equal units 
 of time decomposes equivalent quantities of chemical 
 compounds." As representatives of the latter, we 
 may choose silver nitrate, copper sulphate, or dilute 
 sulphuric acid, and then determine the quantities of 
 metallic copper, metallic silver, or electrolytic gas sep- 
 
38 ELECTROCHEMICAL EXPERIMENTS. 
 
 arated in a definite period of time. Consequently, we 
 distinguish silver voltameters, copper voltameters, and 
 the oxyhydrogen voltameters. Certain precautions 
 must be observed with each'to obtain accurate results. 
 
 SILVER VOLTAMETER. 
 
 This is regarded as the most accurate voltameter. 
 Secondary reactions do not occur in it. The high 
 equivalent of silver, and the fact that very considerable 
 quantities of metal are precipitated by comparatively 
 feeble currents, reduce the error in weighing to a min- 
 imum. A disadvantage which must be recognized is 
 that silver is greatly inclined to separate in crystals, 
 which are loosely attached to the kathode. Conse- 
 quently, stronger currents can not be sent through 
 the apparatus without the possibility of some of the 
 deposit becoming detached. In measuring strong 
 currents with this instrument, electrodes with a suffi- 
 ciently large surface must be provided, besides which 
 the current should not be allowed to act for more 
 than one to two minutes. 
 
 The voltameter usually consists of a silver or plati- 
 num dish serving as a kathode, in which there is a 
 moderately concentrated neutral silver nitrate solu- 
 tion. The anode, of pure silver, is a pencil dipping into 
 the liquid. This anode is surrounded by a muslin 
 bag serving to collect any particles of silver which may 
 separate, thus preventing them from falling upon the 
 dish and so falsely increasing the weight of the latter. 
 
CURRENT MEASUREMENT. 39 
 
 A more convenient form of this voltameter consists 
 of a small beaker glass containing the silver solution. 
 A sheet of pure silver serves as the anode, while the 
 kathode is a plate of platinum. The liquid should 
 be agitated with a glass rod during the action of the 
 current. Finally, the platinum plate is removed, 
 rinsed with water, dried, and weighed. If, after sev- 
 eral experiments, the crystals have become so large 
 that they threaten to fall off, clean the platinum 
 plate by immersion in nitric acid. 
 
 A current of one ampere precipitates 
 
 0.001118 gram Ag in one second, or 
 0.06708 " " " " minute. 
 
 COPPER VOLTAMETER. 
 
 In this instrument the electrodes are two copper 
 plates of equal surface. The thicker of the two 
 serves as the anode, the thinner plate as the kathode. 
 They dip into a solution of copper sulphate. In this 
 instance, also, the liquid should be thoroughly agi- 
 tated during the action of the current. The precipitated 
 copper should have a bright red color and be perfectly 
 adherent. It is first dipped into water, then into 
 alcohol, after which it is dried over a flame. When 
 strong currents are used anodes are placed on both 
 sides of the kathode. 
 
 Early data indicate that, as a filling liquid, an 
 almost saturated and perfectly neutral copper sulphate 
 solution answers best. It, however, increases the 
 
4O ELECTROCHEMICAL EXPERIMENTS. 
 
 resistance of the voltameter, in consequence of which 
 a relatively high pressure will be required. Further- 
 more, low results arise when feeble currents are em- 
 ployed, because then the copper which separates con- 
 tains cuprous oxide. It has been shown* that per- 
 fectly exact values, agreeing with those obtained 
 by the silver voltameter, have resulted by using cur- 
 rent densities varying from 0.06 to 1.5 ampere for a 
 sq. dm. of kathode surface. This was done by the 
 employment of the following solution : 
 
 15 grams of crystallized copper sulphate. 
 5 " " concentrated sulphuric acid. 
 5 c.c. " alcohol. 
 100 " " water. 
 
 The pressure (o. I o. 5V.) is only half as large as 
 that required for the neutral liquid. 
 
 Such an arrangement of the copper voltameter 
 is well adapted for most practical requirements. Its 
 results are correct. It is inexpensive and easily 
 made, and, in addition, can be thrown into a circuit 
 for almost any length of time. It can also be used 
 with advantage as an ampere-hour meter (see p. 5 i). 
 
 One ampere of current precipitates 0.0197 gram of 
 copper in a minute, or 1.181 gram of copper per hour. 
 
 THE OXYHYDROGEN VOLTAMETER. 
 
 In this voltameter dilute sulphuric acid (sp. gr. 
 1.1-51.2) is decomposed between two platinum 
 
 * Chemiker Zeitung, 1893, 543. 
 
CURRENT MEASUREMENT. 41 
 
 plates. The resulting electrolytic gas is measured. 
 An ampere liberates 10.44 c - c - f electrolytic gas (at 
 o and 760 mm. pressure) per minute. This kind of 
 voltameter is convenient, because it does away with 
 all weighings. The reduction of the volume of gas 
 
 FIG. 7. 
 
 can be taken from tables, and in a few minutes the 
 apparatus will again be ready for a new measurement. 
 This convenience, however, is offset by the disad- 
 vantage that the pressure required by the voltameter 
 is high. It varies from 1.7 to 2.5 V. according to the 
 
42 ELECTROCHEMICAL EXPERIMENTS. 
 
CURRENT MEASUREMENT. 43 
 
 size of the instrument. Therefore, to ascertain the 
 current strength in an experiment which demands a 
 pressure of 4 V., it would be necessary to employ 
 some source of electric energy with a pressure of 
 about 6 volts ! 
 
 If an oxyhydrogen voltameter is to be used, the 
 modification made by Kohlrausch (Fig. 7) is preferable. 
 The eudiometer vessel is filled by merely inverting 
 the apparatus. What is known as Schmitt's nitrogen 
 eudiometer (Fig. 8) can readily be converted into a 
 practical electrolytic voltameter. The electrodes are 
 introduced through rubber stoppers. The measuring 
 burette, during use, is pushed somewhat above the 
 rubber stopper. In reading, it is placed on the latter 
 and brought to equal pressure by the adjustable level- 
 ing tube. 
 
 Voltameters of this description are only in rare cases 
 adapted for current measurement by introduction into 
 the circuit. In order to read them the. experiment 
 must generally be interrupted, and they augment the 
 resistance of the circuit to a marked degree. These 
 inconveniences may be avoided by the use of measur- 
 ing apparatus based upon electromagnetic principles. 
 These will now be described. The 
 
 TANGENT GALVANOMETER 
 
 is the simplest example of its class. It consists of 
 a circular heavy copper wire. A magnetic needle, 
 attached to an unspun silk fiber, oscillates in this ring 
 
44 ELECTROCHEMICAL EXPERIMENTS. 
 
 over a graduated circle. The plane of the ring is 
 placed in the meridian in the direction of the needle. 
 The latter is short, compared with the diameter of the 
 ring (not more than one-sixth). The magnetic strength 
 of the needle does not affect the measurement. When 
 the needle is short, a more accurate reading can be ob- 
 tained, even when there is a large divided circle, by 
 attaching to the needle a lighter and longer indicator. 
 When the needle is deflected out of the meridian, 
 the current strength will be 
 
 1 = c, tang. o. 
 
 Here, c indicates a constant value for each instru- 
 ment. 
 
 To determine this constant c of the galvanometer, 
 introduce the latter, together with one of the volta- 
 meters, just described, into the circuit of some con- 
 stant source of electric energy (the order being : bat- 
 tery voltameter galvanometer battery). Close the 
 circuit for a few minutes, note the average deflection, 
 and from the reading of the voltameter calculate 
 the current in amperes. If, with a deflection of a, 
 the current strength was found to be I amperes, the 
 constant sought will be 
 
 I 
 
 c = ' 
 
 tang, a 
 
 Several such experiments are made with varying 
 currents, and from the mean thus found the value for 
 c is taken. A table is then calculated once for all to 
 
CURRENT MEASUREMENT. 45 
 
 serve for the different deflections. To alter the cur- 
 rent strength, an additional cell is added, or a German 
 silver wire is introduced into the circuit as resist- 
 ance. 
 
 Gaugain has made a modification of the tangent 
 galvanometer in which the magnetic needle does not 
 swing in the center of the ring, but is shifted from the 
 circular plane one-fourth of its diameter. The length 
 of the needle can now be increased without produc- 
 ing a variation from the law of tangents. 
 
 Having constructed a table for the galvanometer, it 
 will soon be observed that the measurements between 
 6o-9O are uncertain, because for each degree of 
 deflection the increase in current will become greater. 
 It is not well to exceed 65. In measuring larger 
 currents, two courses are open : either a second gal- 
 vanometer is used, which, with equal length of needle, 
 has a larger ring diameter, or, a so-called shunt is in- 
 troduced. 
 
 When two points in a circuit are connected, not by 
 a single wire, but by two wires of different resistances, 
 the current strength in the two wires will be inversely 
 as the resistances. For example, when the current I 
 between the two points A and B (Fig. 9) meets the 
 two wires I and II with the resistances W\ and W 2 
 respectively, it divides itself into the two currents 
 ii and i 2 , between which the following relations exist : 
 
 i. I* + 1 = 1. 
 
 2. ij : i 2 = W 2 : W r 
 
46 ELECTROCHEMICAL EXPERIMENTS. 
 
 If, for example, the resistance W T is one-third that of 
 W 2 , then three times as much current will flow 
 through the former as through the latter, or, when 
 referred to I, it would be: ij == ^ I and i 2 == J^ I. 
 On introducing, therefore, a galvanometer into a 
 circuit and connecting its terminals by means of a 
 wire, it will be possible, through the resistance of 
 the selected wire, to deflect the major portion of the 
 entire current through this shunt and to convey any 
 desired fraction of it through the galvanometer. In 
 
 FIG. 9. 
 
 this manner the capacity of the latter is correspond- 
 ingly increased. If the shunt has a resistance equal 
 to that of the galvanometer, this would receive but 
 one-half of the total current, hence its readings 
 should be doubled. If, in general, the resistance of 
 the shunt is -^ of that of the galvanometer, the read- 
 ings must be multiplied by n -\- i. When, on the 
 other hand, it is desired to conduct ~ of the total cur- 
 rent through the galvanometer, a shunt having ~- t of 
 
CURRENT MEASUREMENT. 47 
 
 its resistance must be introduced. If the resistance 
 of the shunt is not accurately known, the correct 
 readings of the galvanometer can be ascertained by 
 a re-calibration with a voltameter. The wire used as 
 shunt must not be stretched out straight, otherwise 
 the current passing through it will influence the 
 needle. It should be doubled and then coiled up. 
 In adjusting a galvanometer, care must be taken that 
 adjacent currents do not influence it. Consequently, 
 it is connected to the main circuit by two long wires, 
 lying closely together, so that their inductive action 
 can be disregarded. 
 
 By the aid of a shunt it is possible to employ very 
 sensitive galvanometers, having many turns of wire 
 (multipliers or galvanometers), for the measurement 
 of current strength. The readings of such an instru- 
 ment can not be deduced directly from a simple 
 formula, but the instrument itself must be calibrated 
 throughout its entire range with a voltameter (p. 106). 
 
 TORSION GALVANOMETER. 
 
 The torsion galvanometer of Siemens and Halske 
 (Fig. 10) is an exceedingly delicate instrument, and 
 for exact measurements is almost indispensable. A 
 bell magnet, provided with copper damping, is sur- 
 rounded by a coil of many turns, and attached to a 
 spring, by whose rotation the deflection of the magnet 
 is compensated and restored to its original position. 
 
 The magnitude of the angle of torsion, therefore, 
 
48 ELECTROCHEMICAL EXPERIMENTS. 
 
 serves as a means of measuring the current passing 
 through the instrument. The divisions are so selected 
 that the wire coils have a resistance exactly equal 
 to I 8, and one degree of deflection corresponds to 
 O.OOI amp. With a shunt of ^ Q the reading must be 
 
 FIG. 10. 
 
 multiplied by 10; i. e., i corresponds to o.oi amp.; 
 analogously, with a shunt of 
 
 fo 2 : i = o.i ampere, 
 
 lg 12 : 1 = I ampere. 
 
 As the greatest torsion angle equals 170, it is possi- 
 ble, with such an instrument and the three shunt 
 resistances, which have been mentioned, to measure 
 
CURRENT MEASUREMENT. 49 
 
 currents ranging from o.ooi-i/o.o amp., which is, 
 obviously, a very wide range. 
 
 MEASURING INSTRUMENTS FOR TECHNICAL PURPOSES. 
 
 These instruments carry an empirical scale, which 
 gives direct readings in amperes. The instruments 
 themselves can be constructed for all current strengths, 
 but their range can not be read with the same 
 degree of accuracy throughout. The first divisions 
 are usually contracted; hence their readings are less 
 reliable. Technical instruments of this class, called 
 amperemeters, are indispensable for the control of 
 work in a manufacturing establishment. They give a 
 reading of current strength in amperes at any moment. 
 They should always be introduced into the main cir- 
 cuit leading from the source of the current to the 
 electrolytes. As most amperemeters are entirely 
 independent of the direction of the current passing 
 through them, it is immaterial whether they are con- 
 nected with the negative or positive conducting wire. 
 
 Two phenomena, as a rule, are utilized in their con- 
 struction : (i) A solenoid, through which the current 
 passes, tends to draw an iron rod into its center ; or, 
 (2) a solenoid attracts an eccentrically arranged strip 
 of sheet-iron toward the periphery. The two attrac- 
 tive forces are opposed, either by gravity or by the 
 action of a spring. The modifications of this appa- 
 ratus are almost innumerable. Two typical forms 
 will be described : 
 
5O ELECTROCHEMICAL EXPERIMENTS. 
 
 The spring- galvanometer of Kohlrausch (Fig. 11) 
 consists of a vertical solenoid. In it there is sus- 
 pended a hollow sheet-iron cylinder, to which a spiral 
 spring is attached. When the current passes through 
 
 FIG. ii. 
 
 it, the iron cylinder is drawn into the solenoid until 
 equilibrium is established by the extension of the 
 spring. The latter then causes an indicator to move 
 
CURRENT MEASUREMENT. 51 
 
 over a vertical scale. The compact form of this 
 instrument recommends it. 
 
 The amperemeter of Hummel (Fig. 12), though one 
 of the earliest constructions, is yet one of the best. 
 The solenoid in it lies in a horizontal position. It 
 carries eccentrically a thin sheet of iron also in hori- 
 zontal position, bent and fixed at certain points. It 
 is provided with a long index, vibrating before a 
 scale. The index nearly balances the weight of the 
 iron strip, so that the sensibility of the instrument is 
 
 FIG. 12. 
 
 rather great. The absence of magnets and springs, 
 as well as the convenient and accurate arrangement 
 of the apparatus on the zero-point, when it is mounted, 
 contribute much to making this a trustworthy am- 
 peremeter. Its calibration is generally reliable. 
 
 AMPERE-HOUR METER. . 
 
 The electrolytic processes, proceeding quantita- 
 tively according to any single reaction, are few in 
 
52 ELECTROCHEMICAL EXPERIMENTS. 
 
 number. The majority are accompanied by secondary 
 changes which represent more or less loss in work. 
 Hence a prime factor in every electrolytic process is 
 the determination of the current efficiency ; i. e., the 
 ratio of the current active in the desired direction to 
 the total current passing through the experimental cell. 
 This demands an arrangement allowing the measure- 
 ment of the amount of current which passes through 
 the circuit in a definite period, and which is indepen- 
 dent of the fact, whether the current strength has 
 varied during the period of observation, or whether 
 periods of cessation have actually occurred. 
 
 This essential can be realized in two ways : By 
 use of the electromagnetic and the chemical action of 
 the current. Aron applies the first means in his reg- 
 istering amperemeter. Two synchronous clocks are 
 used : one has an ordinary pendulum, while that of 
 the second is controlled by an electromagnet. Con- 
 sequently, during the passage of the current, a time 
 difference will be noticed in the reading of the two 
 clocks, from which the amount of current, which has 
 passed through the instrument, can be calculated. 
 This apparatus is extensively employed in measuring 
 the current consumed in electric light plants. 
 
 The amperemeter of Kohlrausch can be re-arranged 
 so as to serve for the purpose of an ampere-hour 
 meter. This may be accomplished by attaching a 
 colored pencil to the indicator and allowing it to 
 write upon a strip of paper regulated in its move- 
 
CURRENT MEASUREMENT. 53 
 
 ments by clockwork. All the variations in the cur- 
 rent are recorded in this way. The surface traversed 
 by the pencil represents the ampere-hours. 
 
 The copper voltameter described on page 39 can, 
 without much expense, be changed into an am- 
 pere-hour meter. It should be retained continuously 
 in the circuit. A glass or earthenware vessel will 
 serve to contain the electrolyte. The anodes should 
 be copper plates varying from 3 to 10 mm. in thick- 
 ness. A thin copper plate will answer for the ka- 
 thode. It should be weighed before and after the 
 experiment upon an ordinary balance. One ampere- 
 hour corresponds to 1.18 grams of copper. If the 
 electrolyte is well agitated during the experiment, as 
 much as 2*/2 amperes can be allowed for 100 sq. cm. 
 of smooth kathode surface. The liquid may be agi- 
 tated either by a small stirrer or by conducting a 
 stream of hydrogen gas through it. It is not advis- 
 able to blow air through, because sulphuric acid con- 
 taining air oxidizes copper, and the results will, conse- 
 quently, be inaccurate. 
 
 MEASUREMENT OF PRESSURE. 
 
 Ohm's law is thus expressed : E = I . w. From 
 this it is evident that every pressure measurement can 
 be referred to a current measurement. To ascertain 
 the pressure E, prevailing at the points A and B of a 
 circuit, a shunt with a resistance w, should be at- 
 
54 ELECTROCHEMICAL EXPERIMENTS. 
 
 tached to them, and the current in this branch be 
 then measured. The chief condition is that the 
 quantity of current passing through the shunt should 
 be an exceedingly small fraction of the main current, 
 hence w must be very large in proportion to the 
 resistance of the main current between the points 
 A and B. The reason for this is very evident. By 
 the attachment of the shunt, the resistance of the 
 conductors between the points A and B is reduced, 
 because the current has now two outlets. The smaller 
 the resistance of the shunt, the more will the total re- 
 sistance A B be diminished. To produce the original 
 current strength in the now smaller resistance, a lower 
 pressure than at first will be required ; hence, by the 
 introduction of the shunt, the pressure is changed, and 
 is, indeed, lowered; or, in other words, low pressures 
 are obtained with the aid of a shunt of relatively less 
 resistance. The greater the resistance of the shunt, 
 the less will the total resistance between A and B be 
 altered, and, therefore, the more accurately will the 
 pressure be determined. 
 
 The instrument for the measurement of pressure 
 the voltmeter is, therefore, an amperemeter for 
 very low currents. It possess a high resistance in 
 itself, or a great resistance is inserted before it. Its 
 divisions, empirically deduced, do not correspond to 
 the current strengths which the individual deflections 
 produce, but they are the product of these current 
 strengths and the total resistance of the instrument; 
 
CURRENT MEASUREMENT. 
 
 /'. e. t I . w, which represents the pressure according 
 to Ohm's law, expressed in volts. 
 
 Every sensitive galvanometer may be calibrated to 
 do the work of a voltmeter by the insertion of a 
 sufficiently large resistance (p. 1 1 1). The capacity of 
 every voltmeter can be doubled by the insertion of a 
 resistance equal to that of the instrument itself; and 
 when the resistance is twice as great, the capacity is 
 trebled, etc., etc. 
 
 The methods of pressure measurement, which are 
 determined by the comparison of an unknown electro- 
 motive force with that of a normal element, can be 
 omitted here, as they are all rather inconvenient. 
 The chemist, engaged in electrochemical work, must 
 be able to know, in a very few minutes, both the 
 pressure and current strength, either by deflection of 
 a needle or by some other indicator. It is only by 
 this means that he can rapidly attain his purpose, and 
 have full oversight of his experiments at any moment. 
 
 The torsion galvanometer is not only an excellent 
 aid in determining current strength, but it can also 
 be applied in measuring pressure. This may be 
 accomplished by inserting resistances of varying 
 degree. This galvanometer has an internal resistance 
 of one ohm, and i of deflection represents o.ooi 
 ampere. When pressure is to be measured, i= 
 o.ooi X i o.ooi V. By the insertion of 9 Q t the 
 total resistance of the apparatus becomes 10 Q\ that 
 is, it is ten times greater than at first, and, in conse- 
 
56 ELECTROCHEMICAL EXPERIMENTS. 
 
 quence, 1 deflection corresponds to ten times its 
 original value, which also follows from the formula 
 of Ohm : E = o.ooi X 10 o.oi V. 
 
 The insertion of 99 Q raises the total resistance to 
 100 @, and 1 deflection corresponds to o.ooi X 100 
 = 0.1 V. 
 
 The insertion of 999 Q increases the entire resist- 
 ance of the apparatus to 1000 @, and 1 deflection 
 corresponds to o.ooi X 1000 == I V. 
 
 For convenience, the three resistances, 9, 99, and 
 999 $, are introduced into a box, and can be -inserted 
 in the circuit by means of a plug. To avoid injuring 
 the apparatus, when ascertaining unknown pressures, 
 it is best to throw in the greatest resistance at first, 
 then proceed downward to the lowest. The poles 
 should be so arranged that the magnetic needle is 
 deflected in a direction opposite to the graduation. 
 By turning the torsion screw in the line of graduation, 
 the indicator is again brought back to the zero mark, 
 and the angle of torsion may then be read. If this 
 be too small, the torsion spring is released by turn- 
 ing it back to the zero mark, and a plug is inserted at 
 the next highest point of resistance. This is repeated 
 until the angle of deflection is sufficiently large. 
 
 The principles of construction in all amperemeters 
 now in use permit of the application of the latter as 
 voltmeters, hence these occur in the greatest varieties 
 and with the greatest differences in capacity. Ex- 
 ternally, they resemble the amperemeters, but differ 
 
CURRENT MEASUREMENT. 57 
 
 from them in having many more coils of wire and 
 in their high resistance. 
 
 Amperemeters and voltmeters containing spirals or 
 springs should be tested, from time to time, as to their 
 accuracy, particularly if they are to be used con- 
 tinuously in a circuit. 
 
 REGULATION OF CURRENT. 
 
 The previous chapters have dealt with the produc- 
 tion of the current and its measurement, both as to 
 intensity and pressure. The methods by which it 
 can be altered within any desired limits will be next 
 
 considered. 
 
 p 
 
 It follows from Ohm's law: I = , that I can 
 
 lw 
 
 be altered in two ways : either by alteration of pres- 
 sure, the total resistance remaining the same, or by 
 changing the total resistance, the pressure remaining 
 the same. 
 
 A. ALTERATION OF CURRENT STRENGTH BY CHANGE 
 IN PRESSURE. 
 
 With primary or secondary batteries, the pressure 
 may be varied by arranging a smaller or larger num- 
 ber in series. It has already been stated that, in the 
 case of batteries with high resistance, the introduc- 
 tion of each cell into the circuit is accompanied by 
 an appreciable change in the expression Jw. 
 
 With dynamos it is possible to alter the pressure, 
 
58 ELECTROCHEMICAL EXPERIMENTS. 
 
 without disturbing the construction of the machine, 
 by changing the velocity or by altering the resistance 
 in the magnet coils. An example was given on page 
 30, showing how, by increasing the velocity, the pres- 
 sure on open circuit was increased, and mention was 
 also made that in setting up a dynamo for experi- 
 mental purposes it is well to provide it with a cone- 
 pulley, so that the middle pulley would cause the 
 normal velocity and the other two either a higher or 
 a lower speed. If the variations, above or below, 
 are maintained within 20 per cent, of the maximum 
 or minimum, there is no danger of doing harm to 
 the machine. This is furthermore true, because, in a 
 machine used in an experimental plant, the require- 
 ments are not continuous, but only temporary, and 
 for conditions that are extraordinary. 
 
 The second method of altering the pressure con- 
 sists in inserting a resistance, by means of the regu- 
 lating shunt, in the limb of the magnet (p. 28). This, 
 however, merely lowers the pressure. If the regulator 
 supplied by the manufacturer allows only a moderate 
 reduction, it can be connected with larger resistances. 
 
 B. ALTERATION OF CURRENT STRENGTH BY ALTERA- 
 TION IN TOTAL RESISTANCE. 
 
 Two distinct conditions may occur here. Increase 
 in current strength may be achieved 
 
 I. By reduction of the internal resistance of the 
 source of the current. This refers especially to bat- 
 
CURRENT MEASUREMENT. 59 
 
 teries. Their internal resistance may be diminished 
 by placing several of them parallel, or by the use of 
 a larger form of the same kind of battery. This is 
 also true of storage cells, but it must be observed 
 that such conditions rarely obtain, because this source 
 of current has but slight internal resistance. It can 
 only happen should the current required for an 
 experiment be greater than the heaviest discharge 
 allowed for this type of battery. 
 
 The internal resistance of dynamos can not be 
 diminished. 
 
 2. By reduction of the external resistance. Under 
 this head the conducting wires must be considered. 
 Those used in electrolytic experiments should be as 
 short as possible and not too light in weight. For 
 the minor experiments, good copper wires of 2 mm. 
 diameter will be sufficient. It is customary to allow 
 about from 2 to 3 amperes for a copper wire with a 
 cross-section of one sq. mm. 
 
 The resistance of the experimental cell, or " bath," 
 as it is usually termed, may be reduced 
 
 (1) by increasing the electrode surfaces, 
 
 (2) by diminishing their distance from one another, 
 
 (3) by increasing the conductivity of the electrolyte, 
 
 (4) by using a thinner or more porous diaphragm, 
 if such is required by the bath. 
 
 It often happens, in using some source of electricity, 
 e.g, a dynamo or an aggregation of storage cells, 
 
60 ELECTROCHEMICAL EXPERIMENTS. 
 
 where there is abundant pressure and but slight inter- 
 nal resistance, that the current and pressure must be 
 reduced to some definite value. This can be accom- 
 plished by throwing resistances into the circuit. 
 German silver wires (or nickelin or rheotan), of vary- 
 ing diameters, arranged in spiral or zigzag form upon 
 a frame of wood, answer well for this purpose. At 
 suitable distances solder on short, thick copper wires, 
 which dip into mercury cups. When such a resist- 
 ance is introduced into the circuit, the current is com- 
 pelled to pass through all the wire divisions ; and in 
 order that the resistance may be lowered, it is usual to 
 throw out one or several of the divisions. This is 
 generally done by connecting the corresponding mer- 
 cury cups with a copper wire having this shape | 1. 
 To screw the individual wires to metal blocks, and 
 throw out the several divisions by introducing plugs, 
 is less worthy of recommendation, because usually 
 the metallic blocks soon become worn and the plugs 
 do not fit well. Furthermore, such plug contacts do 
 not long remain bright and clean in a laboratory at- 
 mosphere. The adjustable rheostats (Fig. 13) are 
 better. In them the individual resistance divisions 
 are directed to metal buttons ; over these passes the 
 movable arm. 
 
 The resistances of the several divisions of wire are 
 numbered as is done in the case of a box of weights : 
 I, 2, 2, 5, IO, 20, 20, 50, etc., U ; or, by doubling each 
 
CURRENT MEASUREMENT. 
 
 6l 
 
 value, i, 2, 4, 8, 16, Q. By the latter plan more is 
 accomplished with the same number of resistances, 
 but the arrangement is less convenient. 
 
 The resistance of a wire is greater, the thinner it is ; 
 hence, it might be thought that it would be more 
 advantageous to use a very light wire, for then the 
 
 FIG. 13. 
 
 minimum quantity of wire would be consumed. This 
 conclusion is, however, false. The electric energy 
 annihilated by a wire is transformed into heat. On 
 sending a current of moderate strength through a 
 very thin wire, the increase in temperature could rise 
 to the ignition point, or even to the point effusion of 
 
62 ELECTROCHEMICAL EXPERIMENTS. 
 
 the wire. In making calculation for a current regu- 
 lator, the intensity of the current passing through 
 each division or section must be considered. The 
 following table gives results obtained in experiment- 
 ing with nickelin wires; it may prove of assistance in 
 making the calculation for current regulators : 
 
 DIAMETER IN 
 
 RESISTANCE FOR 
 
 IGNITION BEGINS 
 
 MM. 
 
 i M. LENGTH IN Q. 
 
 WITH AMPERES. 
 
 O.2 
 
 13 
 
 i-7 
 
 0.4 
 
 3-2 
 
 4 
 
 0.6 
 
 1.41 
 
 7 
 
 0.8 
 
 0.79 
 
 9 l /2 
 
 I.O 
 
 0.51 
 
 14/2 
 
 1-25 
 
 0.33 
 
 20 
 
 1.50 
 
 0.23 
 
 32 
 
 i-75 
 
 0.16 
 
 40 
 
 2.O 
 
 0.13 
 
 45 
 
 The current strength given here should not be 
 actually reached, because ignition always proves 
 destructive to resistance wires (p. 1 16). With power- 
 ful currents, such as dynamos give, wires will no 
 longer answer, because the cooling of a wire by in- 
 creasing its thickness is always unfavorable. Hence, 
 it is preferable and better to take short strips of thin 
 sheet nickelin. These, with a section equal to that 
 of a wire, can carry much heavier currents and yet 
 
 not ignite. 
 
THE VESSELS. 63 
 
 STRIPS OF NlCKELIN, 0.3 MM. IN THICKNESS. 
 
 WIDTH IN MM. RESISTANCE FOR MAXIMUM CHARGE 
 
 i M. LENGTH IN U. IN AMPERES. 
 
 IO 
 
 0.133 
 
 40 
 
 15 
 
 0.0889 
 
 60 
 
 20 
 
 0.0667 
 
 80 
 
 25 
 
 0-0533 
 
 95 
 
 30 
 
 0.0444 
 
 no 
 
 35 
 
 0.0381 
 
 130 
 
 40 
 
 0.0333 
 
 145 
 
 45 
 
 0.0296 
 
 160 
 
 5o 
 
 0.0267 
 
 175 
 
 The maximum charges given above are so measured 
 that the sheets of metal, under the ordinary methods 
 of cooling, never reach the ignition point. They will 
 not fuse through until the current is doubled or 
 trebled. Several metal sheets can be screwed or sol- 
 dered together. 
 
 B. ARRANGEMENT OF EXPERIMENTS. 
 
 THE VESSELS. 
 
 In the initial experiments, when electrolysis is to 
 be employed, it is best, where possible, to use glass 
 vessels, simply because the latter permit the eye to 
 observe many of the changes occurring. For exam- 
 ple, by transmitted light, alterations in concentration 
 at the electrodes may be detected by the formation 
 
04 ELECTROCHEMICAL EXPERIMENTS. 
 
 of currents in the liquid; alterations in color, the in- 
 tensity of the gas evolution at different points on the 
 electrodes, the separate stages in the production of 
 precipitates at the kathode, and the solution of the 
 anode, as well as other points of interest, can be noted. 
 The general introduction of storage cells of many 
 sizes has made it possible to obtain glass jars of the 
 
 FIG. 14. 
 
 most varying dimensions. Smaller vessels may be 
 prepared by painting cigar-boxes, etc., on the interior 
 and exterior with paraffin. Such baths answer for 
 both acid and alkaline liquids. They will last for 
 several weeks at least. 
 
 With experiments conducted on a larger scale, 
 earthenware troughs or boxes, such as are now 
 used in galvanizing establishments, are much em- 
 ployed. They are, however, being gradually sup- 
 
DIAPHRAGMS. 65 
 
 planted by wooden vessels lined with lead (Fig. 14). 
 The latter are made of stout planks, provided with 
 tongue and groove, with addition of a tarred string, 
 and are tightly bound together and screwed up with 
 bolts. The inner surface of these receptacles is cov- 
 ered with tar or pitch, or, what is better, with sheet- 
 lead. The separate lead sheets are then welded 
 together at their edges with an oxyhydrogen flame, 
 so that the receptacle is really a water-tight lead box, 
 resting in a frame of wood. 
 
 DIAPHRAGMS. 
 
 When, in an electrolytic process, substances are 
 produced at one or at both electrodes, which prove 
 to be soluble in water, it is impossible, unless suitable 
 means are adopted, to prevent the separated material 
 from mixing. Substances thus separating and again 
 mixing destroy one another, or react upon one another 
 in a very undesirable way. To obviate this, a porous 
 diaphragm is introduced between the electrodes. It 
 should, first of all, prevent the products, appearing at 
 the electrodes, from mixing. Next, it should offer 
 little resistance to the passage of the current, and it 
 should, from a mechanical standpoint, be firm and 
 durable, and not affected either by the chemicals 
 present in the bath, or by any which may be pro- 
 duced in it. The material composing it should be 
 easily obtained and inexpensive. These requirements 
 
66 ELECTROCHEMICAL EXPERIMENTS. 
 
 are fulfilled with difficulty, hence anything approxi- 
 mating the chief requirements must suffice. The 
 preparation of a diaphragm, satisfactory in every par- 
 ticular, is still a problem of the future. Wide fields 
 will be opened up to electrolysis when once this de- 
 sideratum is found. The discovery of a diaphragm 
 suitable for acid and also for alkaline liquids, would 
 certainly bring a high pecuniary reward. Recently, 
 efforts have been made to conduct, on a technical 
 scale, operations requiring diaphragms, but without 
 success, and the consequence has been that manufact- 
 urers have turned again to those processes and ma- 
 terials in which these disturbing factors (diaphragms) 
 are not present. 
 
 Bunsen, in preparing metallic magnesium from the 
 fused chloride, brought into notice the simplest form 
 of diaphragm. He introduced a partition from above 
 into the fused mass ; two communicating chambers 
 were thus formed, and into them he introduced the 
 electrodes. Chlorine can not remain dissolved in the 
 fused mass; it rises immediately to the surface, where 
 it distributes itself and escapes with much foaming. 
 The diaphragm prevents it from passing into the 
 second chamber, where the magnesium collects in 
 little globules. 
 
 In galvanic batteries we noticed that the porous cup 
 acted really as a diaphragm. These cups answer well 
 or poorly for the work for which they are constructed, 
 depending upon the material and the temperature at 
 
DIAPHRAGMS. 67 
 
 which they have been baked. A method for testing 
 them has already been given on p. 13, and it is not 
 necessary to repeat the same here. For acid liquids 
 porous cups answer well enough, but they are useless 
 with alkaline liquids, because the latter decompose 
 and soon destroy them. When experimenting on a 
 small scale, the porous cups are satisfactory enough as 
 diaphragms, but in technical operations their compara- 
 tively high price, and their short life, due ofttimes to 
 their fragility, prevent their general adoption. When 
 the products sought are costly, the use of diaphragms 
 is to be recommended. The cylindrical form of por- 
 ous cup is usually quite inconvenient in the construc- 
 tion of apparatus, hence it will be found to be more 
 practical to use plates of porous clay ; these should 
 be placed in the baths as diaphragms. The formation 
 of such chambers as would result in this way must 
 produce no insurmountable difficulties. 
 
 Quite frequently the sole purpose of the diaphragm 
 is to prevent pulverulent substances separated at one 
 electrode from passing to the other electrode. In such 
 instances the diaphragm is really nothing more than 
 a filter, so that a bag of silk or muslin can be attached 
 to the respective electrode (see old form of silver vol- 
 tameter, p. 38). 
 
 The material used in technical operations, at least 
 in an experimental way, for diaphragm purposes, has 
 been parchment paper, felt, or asbestos. 
 
68 ELECTROCHEMICAL EXPERIMENTS. 
 
 THE ELECTRODES. 
 
 To save space, electrodes are generally in plate or 
 sheet form. These are hung parallel in the bath. 
 This arrangement allows the use of both sides of the 
 electrodes. When the purpose is to throw a metal 
 out of solution, a sheet kathode of the same metal 
 should be used to get rid of impurities. Otherwise 
 the choice of kathode material is merely dependent 
 upon the nature of the liquid which it is proposed to 
 decompose. A favorable circumstance which maybe 
 mentioned in this connection is, that the kathode is 
 protected from corrosion under the liquid, by the re- 
 ducing action of the current. ' In acid liquids lead or 
 copper is employed, while iron is used in alkaline 
 liquids. The electrodes should dip far into the liquid, 
 first, in order that the material may be used, second, 
 because the salts of the solution creep up and strongly 
 corrode that portion of the electrode which is not im- 
 mersed. Particular attention should be directed to 
 the points of contact, otherwise great resistance may 
 be introduced, and, indeed, the current can eventually 
 be entirely interrupted. Exposed points should be 
 protected by a layer of varnish. 
 
 Anodes, according to their deportment toward a 
 current, are divided into two groups. The first group 
 comprises those anodes which decompose and are dis- 
 solved the soluble anodes. In the second group are 
 the insoluble anodes. Soluble anodes should be used 
 
THE ELECTRODES. 69 
 
 if it can possibly be done in the process under study. 
 The insoluble anodes are platinum, lead, and carbon 
 for acid and neutral solutions, while iron is used in al- 
 kaline liquids. 
 
 At the ordinary temperatures platinum anodes are 
 not attacked in solutions of any kind, not even by the 
 halogens ; hence they are the most satisfactory ma- 
 terial for experimental purposes. Their high price 
 prevents their general adoption in technical work. 
 The attempt has been made to substitute platinum 
 coated sheets for plates of the pure metal, without, 
 however, any degree of success. The platinum coated 
 material is not sufficiently dense and adherent to pre- 
 vent the solution of the metal beneath it. With 
 platinum anodes, the union between the plate and the 
 wire is always effected by rivets or by welding, but not 
 by soldering, as the gold used in soldering is not only 
 dissolved by the halogens, but also by the oxygen 
 acids during the electrolytic process. 
 
 Sheets of lead form very satisfactory anodes in 
 solutions of pure sulphuric acid. They soon become 
 coated with a brownish-black, protecting layer of 
 lead dioxide (PbO 2 ), otherwise oxygen appears upon 
 them just as it does with platinum. (The author has 
 observed that the oxygen liberated upon a lead anode 
 is free from ozone.) The lead dioxide slowly sepa- 
 rates in leafy scales, which are increased by interrup- 
 tion of the current. The consumption of lead, how- 
 ever, is in no sense more appreciable. In the presence 
 
70 ELECTROCHEMICAL EXPERIMENTS. 
 
 of halogens and nitric acid, the lead is attacked more 
 rapidly, but irregularly, with the production of white 
 layers of chloride or sulphate (p. 87). 
 
 When there is an evolution of chlorine, the third 
 substance, carbon, is stable, but it is strongly attacked 
 in all cases where oxygen is evolved. Carbon anodes 
 serve admirably in hydrochloric acid solutions, but in 
 sulphuric acid they are visibly affected, large black 
 scales falling off, while a brown dye-stuff dissolves. 
 The resistance of carbon being so much greater than 
 that of the metals, the carbon anodes must, conse- 
 quently, be of much larger cross-section. The con- 
 ductivity and the power of resisting the action of 
 chemicals are greater, the harder and denser the car- 
 bon is; therefore, cut retort-carbon is better than the 
 artificial and compressed carbon. The creeping of 
 solutions on the porous carbon plates is disagreeable 
 and troublesome, causing destruction of the contact 
 points. It may be overcome by paraffining the carbons 
 
 (p. 12). 
 
 Thus far no material has been discovered which 
 will answer as a durable anode in electrolytic pro- 
 cesses where mixtures of chlorine and oxygen are 
 produced. Platinum is too expensive and carbon is 
 consumed too rapidly. A process of this class a 
 process of eminent importance in which this diffi- 
 culty is keenly felt, is the electrolysis of sodium 
 chloride for the manufacture of chlorine and caustic 
 soda. 
 
THE CONDUCTORS. /I 
 
 THE CONDUCTORS. 
 
 Soft copper is invariably used to conduct the cur- 
 rent. In minor experiments, the ordinary, refined cop- 
 per with a conductivity of from 52 to 56 will suffice. 
 In technical operations, electrolytic copper is used. 
 Its conductivity varies from 58.5 to 60. If, at any time, 
 it is necessary to throw in a flexible piece, experi- 
 ments on a small scale will require only a thin spiral, 
 but in the larger operations a piece of copper cable will 
 be the most suitable. The contacts must all be kept 
 bright. When they become dull, as happens at the 
 electrodes, it should be the care of the operator to 
 inspect them frequently and brighten them up. The 
 cross-section is so chosen that in conductors of mod- 
 erate length, say up to 20 meters, not more than 3 
 amperes will fall to I sq. mm. It follows from this 
 that in using powerful currents large cross-sections 
 are requisite, and consequently that conducting rods 
 of very considerable cost are necessary. It is best, 
 in general, not to use currents exceeding 500 amperes. 
 To increase the production, and yet use a single 
 machine, it is better to increase the pressure than to 
 increase the current strength. To conduct or dis- 
 tribute the current from the main line, screw clamps 
 of the most varied construction may be used (Fig. 
 
 is). 
 
 The screw a is used to connect two wires ; it is 
 pushed over the one wire, b and c are very conveni- 
 
72 ELECTROCHEMICAL EXPERIMENTS. 
 
 ent modifications for the construction of apparatus. 
 They are either screwed directly into wood (b), or, 
 like the table connector c, are fastened to a table by 
 means of two screws, d serves to connect a sheet 
 and wire conductor, e is used to connect larger con- 
 ducting rods. The hinge arrangement permits of its 
 attachment from without. 
 
 Cylindrical rods are not well suited for conducting 
 purposes, because the borings in the binding screws 
 
 FIG. 15. 
 
 are only intended for a single wire. A much more 
 convenient arrangement is the flat conducting wire. 
 This can then be attached to the machine, and there 
 being a larger contact surface, one can be assured of 
 a more complete distribution of the current. The 
 " rider " binding screw / can be clamped to this. 
 The binding screw g facilitates conduction by means 
 of the round wire. 
 
THE ELECTROLYTE. 73 
 
 An essential for every electric conductor is the 
 " key " or " switch." At the completion of every ex- 
 periment, the main current must be interrupted. This 
 is necessary in order to prevent a reverse discharge 
 of the experimental cell. In minor experiments the 
 wires are merely withdrawn from the binding posts or 
 screws. In larger or technical operations an adjust- 
 able rheostat is used. The contact of the latter 
 breaks the entire current. To renew the electrolysis, 
 the dynamo is permitted to run until it has gained 
 its maximum speed and pressure ; then the crank or 
 arm of the switch is slowly moved over the individual 
 resistances until the point is reached at which full 
 current again sets in. The current and energy con- 
 sumption are gradually raised to the maximum; a 
 damaging loading of the machine, by jerks, is thus 
 avoided ; for this would happen were the full current 
 turned on at once. 
 
 THE ELECTROLYTE. 
 
 Not much in general is to be said concerning the 
 electrolyte. It may be either crude material to be 
 changed by the electrolysis, and then its properties 
 are regarded as assumed in the purchase ; or, it is a 
 means to an end e.g., the purification or refinement of 
 metals. Then the effort is made to have its chemical 
 properties such that they will answer for the require- 
 ments of the designed process. In the refinement of 
 
74 ELECTROCHEMICAL EXPERIMENTS. 
 
 copper, the electrolyte is the sulphate, because lead, 
 antimony, tin, and gold dissolve in it with difficulty. If 
 a chloride solution should be employed, the metals 
 mentioned would first pass into solution, and later they 
 would reappear in the precipitate. 
 
 The electrolyte should be as good a conductor as 
 possible, so that little energy will be required to 
 overcome its resistance. Conductivity increases 
 with increasing concentration, by acidulation, and 
 by the application of heat. A maximum conduc- 
 tivity is imparted to many substances by increas- 
 ing their concentration. The application of heat to 
 the electrolyte is permissible when no undesirable 
 secondary reactions and no appreciable expense are 
 incurred. As a rule, the liquors are sensibly heated 
 by the current itself. 
 
 In experiments with organic substances, it may be 
 added that they must, in all cases, be dissolved before 
 they are subjected to electrolysis. When they are 
 insoluble in water, they should be brought into a 
 suitable condition for the action of the current by 
 means of acids, alkalies, etc. 
 
 An advantage in many electrolytic processes is 
 found in maintaining a thorough agitation of the elec- 
 trolyte; this can be effected by circulation or by 
 mechanical means. It is in electrometallurgy par- 
 ticularly that a more or less complete circulation of 
 the liquor plays an important role. 
 
THE ARRANGEMENT OF EXPERIMENTS. 75 
 
 THE ARRANGEMENT OF EXPERIMENTS. 
 
 FIG. 16. 
 
 The regulating resistance R is first thrown into the 
 circuit. It is followed by A an apparatus intended 
 to measure the current strength. This is either a 
 technical amperemeter, a galvanometer, or the shunt 
 of a galvanometer. If the experiment is to be car- 
 ried out quantitatively, the ampere-hour meter is next 
 inserted; it is the copper voltameter K. The current 
 is then introduced and passes through the experi- 
 mental cell V, the screw-clamp U, used to interrupt 
 the current, and then back to the battery. To ascer- 
 tain the pressure in the experimental cell the instru- 
 ment S is attached to the two electrodes by means of 
 a thin copper wire. This instrument is either a volt- 
 
76 ELECTROCHEMICAL EXPERIMENTS. 
 
 N 
 
 FIG. 17. 
 
 FIG. 18. 
 
PRESSURE OF DECOMPOSITION. 77 
 
 meter or a torsion galvanometer, provided with a series 
 resistance. The latter instrument can be used both 
 for the measurement of current and of pressure, by at- 
 taching its conducting wires to the shunt-resistance 
 N or to the series resistance W, in connection with 
 the experimental cell (Figs. 17 and 18). 
 
 All such aids as are used in a chemical examina- 
 tion of the precipitates, solutions, and gases arising 
 in an electrolytic experiment, are to be considered as 
 a part of the necessary outfit. 
 
 C. PHENOMENA ARISING IN ELECTRO- 
 LYSIS. 
 
 PRESSURE OF DECOMPOSITION. POLARIZING 
 CURRENTS. 
 
 When water acidified with sulphuric acid is elec- 
 trolysed with insoluble anodes and a varying electro- 
 motive force, it will be observed that its decomposi- 
 tion begins when the pressure =1.5 V. Water 
 can not be resolved into its components hydrogen 
 and oxygen with a lower pressure. The higher 
 this factor rises, the more energetic will the current 
 be. As in the case of water, so, too, in every other 
 chemical compound, there is a certain minimum pres- 
 sure, below which decomposition does not occur. 
 When hydrogen and oxygen combine to yield water, 
 
78 ELECTROCHEMICAL EXPERIMENTS. 
 
 heat is evolved, energy is dissipated. The water 
 molecule has less energy than its components pos- 
 sessed when they were free and before they united. 
 If, then, the water molecule is to be decomposed, this 
 will only be possible by applying an amount of en- 
 ergy equal to that which is lost in its formation. 
 This energy is supplied by the electromotive force of 
 the current. The law of the conservation of energy 
 would lead to the hope that the decomposition- 
 pressure might be calculated. This is, indeed, 
 possible. 
 
 The heat modulus (thermal value) is the number of 
 calories, in grams, set free in the formation of a mole- 
 cule of a chemical compound referred to one gram of 
 hydrogen as a unit. The thermal value of water is, 
 therefore, -f- 68, 360 c. : /. e., in the union of 2 grams of 
 hydrogen with 16 grams of oxygen, forming 18 grams 
 of water, heat is evolved sufficient to raise 68,360 
 grams of water i C. This is expressed symbolically 
 by placing a comma between the symbols of the 
 several elements and enclosing the whole in brackets : 
 
 (H 2 ,0) = + 68,360. 
 
 Water of solution is represented by " aq." This is 
 always very large in proportion to the dissolved sub- 
 stance. (KCl aq) represents the aqueous solution 
 of a molecule of potassium chloride. 
 
 Knowing the thermal value (heat modulus) attached 
 to an electrolytic process, the decomposition-pressure 
 
POLARIZING CURRENTS. 79 
 
 may be calculated in the following manner : * Divide 
 the thermal value (heat modulus) W by the product 
 of 23,067 multiplied by the number of valences dis- 
 solved by the current: 
 
 Examples : 
 
 1. In the case of (H 2 ,O): 
 
 W = 68,360. 
 n = 2, 
 
 68,360 
 
 therefore Z = - r = 1.48 V. 
 2 . 23,067 
 
 2. In the decomposition of copper sulphate (Cu 
 SO 4 , aq), with insoluble anodes, into copper, oxygen, 
 and sulphuric acid (Cu,O,SO 3 aq) we find 
 
 W = 55,960 
 n = 2, 
 
 55,960 
 consequently Z = - r = 1.21 V. 
 
 * 2 . 23,067 
 
 3. In the decomposition of (AgCl) into (Ag,Cl): 
 
 W =-. 29,380 
 n = I, 
 
 hence Z = 1.27 V. 
 
 If the current supply in an electrolysis be inter- 
 rupted and a galvanometer be connected with the 
 electrodes, it will be observed, from the deflection, 
 
 * The deduction of this formula is given in any complete volume on 
 Physics. 
 
8O ELECTROCHEMICAL EXPERIMENTS. 
 
 that a current proceeds from the electrodes, which, 
 as regards direction, is the opposite of that which 
 was originally sent through the electrodes. At first 
 it will be quite strong, but it will gradually grow 
 less. This phenomenon has long been known under 
 the name of "current polarization." It is attribu- 
 table or due to the ions, separated at the electrodes, 
 seeking to re-unite to the original compound. 
 
 The current produced in this way must manifestly 
 be opposed to the original current. If the products 
 of electrolysis are not removed, should they remain 
 in the solid or liquid condition in the vicinity of the 
 two electrodes, the polarization current can, depend- 
 ing upon its quantity, enrich the original primary 
 current very materially (storage cells). If gas evo- 
 lution has occurred at one or both of the electrodes, 
 the polarization current can carry only a fraction of 
 the primary current. At first, so long as the elec- 
 trodes are covered by a gaseous layer, this current 
 is quite powerful, but it diminishes rapidly. The 
 polarization current is also responsible for another 
 phenomenon. If primary batteries are used in carry- 
 ing on an electrolytic process, a process well adapted 
 from the preceding statements for the production of a 
 continuous polarization current, it may occur in time 
 that the electromotive force of the elements will be 
 exhausted and become equal to that of the polariza- 
 tion current Then, equilibrium would be established 
 between the battery and the experimental cell, with 
 
POLARIZING CURRENTS. 8 1 
 
 the consequent cessation of current. If, for any 
 reasons, the pressure of the battery continues to fall 
 even lower, the direction of current will be reversed ; 
 the experimental cell will discharge itself, like a stor- 
 age cell, into the battery. A like occurrence, dis- 
 charge of the experimental cell, with annihilation of 
 all that has already been accomplished, will be ob- 
 served, if, when a dynamojs employed for the source 
 of current, the circuit is not interrupted at the close 
 of the work. The armature then acts like a short 
 circuit. The polarization current can, under certain 
 conditions, become so powerful that it will be strong 
 enough to again set the inactive machine into motion. 
 
 The decomposition-pressure previously mentioned, 
 is also the maximum electromotive force of the polari- 
 zation current. It can usually be determined by permit- 
 ting the electrolysis to proceed for some time, then in- 
 terrupting the source of current and as quickly as 
 possible attaching the torsion galvanometer. In this 
 manner it may also be demonstrated that the electro- 
 motive force of the polarization current gradually 
 grows less. 
 
 When soluble metallic anodes are employed in 
 an electrolysis, the conditions become complicated. 
 The anion the constituent deposited at the anode 
 is not liberated as such, but combines in statu nas- 
 cendi with the substance of the anode. This union 
 develops heat which is favorable to the process. 
 Consequently, the current is not called on to furnish 
 
82 ELECTROCHEMICAL EXPERIMENTS. 
 
 all the energy required : it is only necessary that this 
 should equal the difference between the two thermal 
 values (heat moduli), i. e., the heat of decompo- 
 sition of the electrolyte minus the heat of formation 
 of the new compound produced at the anode. If 
 the anode consists of the same metal as that which 
 is to be deposited from a solution, e. g., as in a 
 copper or silver voltameter, then the pressure of 
 decomposition becomes zero, for the energy con- 
 sumed at the kathode in the decomposition of copper 
 sulphate into Cu -f- SO 4 is just as great as that pro- 
 duced at the anode by the union of SO 4 with Cu. 
 When the pressure of decomposition equals zero, 
 naturally there can be no polarization current; hence 
 it is said of such combinations that " the polarization 
 is removed by the choice of soluble anodes." In prac- 
 tice, the decomposition-pressure never entirely disap- 
 pears, but is only more or less reduced, hence the 
 polarization current, arising from the reaction, may 
 be constantly detected. 
 
 Attention may here be called to a misunderstand- 
 ing which the author has frequently encountered. 
 We read, and sometimes hear, that the polarization 
 may be removed by blowing a current of air into the 
 experimental cell. If a gas is developed in a pro- 
 cess the scheme proposed can accomplish merely this, 
 viz. that after the short circuit of the cell only a very 
 small fraction of the current first introduced reap- 
 pears as polarizing current. This is because the one 
 
POLARIZING CURRENTS. 83 
 
 product of the electrolysis the gas is removed, 
 therefore the material for the prolongation of the po- 
 larizing current is absent. It is a mistake, however, 
 to suppose that by the mere blowing-in of air it is 
 possible to obtain a lower decomposition-pressure. 
 The chemical reaction which the current completes, 
 is, as has been observed, ample for this purpose, and 
 this is in no sense influenced by blowing in air. 
 
 A second special case arises in the use of soluble 
 anodes, consisting, not of metal, but of some chem- 
 ical compound. As a typical case, the process of 
 Marchese* may be here introduced. In it plates 
 of copper, serving as kathodes, are hung in a 
 solution of blue vitriol, while the anodes consist of 
 plates made from copper-matte. The latter is mainly 
 cuprous sulphide (Cu 2 S), with a small amount of 
 iron sulphide (FeS). The following reactions occur : 
 Copper, from the blue vitriol solution, separates at 
 the kathode. It has the thermal value (Cu,SO 4 aq). 
 Sulphur appears at the anode, while copper dis- 
 solves as sulphate CuSO 4 . The course of the 
 reaction may be imagined to progress in this way : 
 At first the copper sulphide breaks down into 
 sulphur and copper; and for this change a quantity 
 of energy corresponding to the thermal value (Cu 2 ,S) 
 will be requisite. After this the liberated copper will 
 be converted into copper sulphate, and in so doing 
 
 *D. R-P., Nr. 22429. 
 
84 ELECTROCHEMICAL EXPERIMENTS. 
 
 produces energy corresponding to the thermal value 
 (Cu,SO 4 aq). Since the thermal value of the electro- 
 lysis is equal to the algebraic sum of the thermal 
 values developed at the two electrodes, we then have, 
 if the heat consumption be represented by , and 
 the heat production by +, 
 
 KATHODE ANODE 
 
 (Cu,S0 4 aq) (Cu,S) + (Cu,SO 4 aq). 
 
 The first and third members fall away, so that in 
 the process the actual energy-quantity is that corre- 
 sponding to the decomposition of the copper sul- 
 phide. As (Cu 2 ,S) equals 20,270 c, the decomposition- 
 pressure will be 
 
 .. 
 
 2 . 23067 
 
 This diminution in the pressure of decomposition 
 only occurs when the material to be dissolved is a 
 conductor of electricity and serves, at the same time, 
 as anode, so that the solution occurs in consequence 
 of the direct action of the current. For example, 
 this end would not be attained if insoluble anodes 
 were used, and the ore were to be suspended in the 
 bath by means of baskets. A solution of the ore 
 occurring in this way, if indeed it did happen, would 
 be a secondary reaction, which would have absolutely 
 nothing to do with the electrolysis, hence would not 
 affect the required pressure. 
 
 Recently several suggestions have been made for 
 obtaining metals electrolytically which seek to reduce 
 
POLARIZING CURRENTS. 85 
 
 the pressure of decomposition by a very peculiar 
 method. Thus, insoluble anodes have been used and 
 these are hung in the solution of some readily oxi- 
 dizable substance. The electrodes are, therefore, 
 separated from one another by a diaphragm. Indeed, 
 Siemens (D. R.-P., 42243 and 48959) electrolyzed a 
 mixed solution of ferrous sulphate and copper sul- 
 phate by allowing the solution first to pass through 
 the kathode chamber, and then through the anode 
 compartment. In this way copper was deposited at 
 the kathode, after which the solution of ferrous sul- 
 phate passed to the anode where it took up the 
 atomic group SO 4 the anion separated from the 
 copper sulphate and became ferric sulphate. The 
 latter was next digested with finely divided chalco- 
 pyrite or copper-matte. Sulphur separated, the ferric 
 salt was reduced, and copper sulphide dissolved, thus 
 regenerating the original liquid, which in turn was 
 again subjected to electrolysis. 
 
 The thermochemical changes were : 
 
 ANODE KATHODE 
 
 + (2FeS0 4 aq,S0 4 ) (Cu,SO 4 aq). 
 
 The thermal values needed here are not exactly 
 known. The difference in question can, however, be 
 calculated by writing the reactions differently : 
 
 + (2FeS0 4 aq,0,S0 3 aq) - (Cu,O,SO 3 aq). 
 
 The heat of oxidation resulting from the conver- 
 sion of ferrous into ferric sulphate may be ascertained 
 
86 ELECTROCHEMICAL EXPERIMENTS. 
 
 by subtracting the heat of formation of two molecules 
 of ferrous sulphate from the thermal value (heat 
 modulus) of ferric sulphate : 
 
 (Fe 2 ,O 3 ,3SO 3 aq) = 224,880 c. 
 2(Fe,O,SO 3 aq) = 186,460 c. 
 
 (2FeSO 4 aq,O,SO 3 aq) = 38,4800. 
 
 Hence the heat consumption at the kathode (Cu,O,- 
 SO 3 aq) equals 55,960, the heat produced at the anode 
 38480, and the difference would be 17,480 c, which 
 would correspond to a pressure of 0.38 V. 
 
 In fact, the author, experimenting on a small scale 
 and observing the precautions laid down on p. 80, 
 was able to confirm a pressure of decomposition 
 equal to 0.380.41 V. by using platinum anodes, and 
 on substituting anodes of carbon found 0.37-0.40 V. 
 A sheet of lead, used as anode, gave an evolution of 
 oxygen and a decomposition pressure of 1.26 V. 
 This also occurred when the experiments were varied 
 in every possible way. Therefore Siemens' process 
 is only practicable with platinum or carbon anodes, 
 but not with lead anodes ! There is no explanation 
 of this rather singular deportment. 
 
 Hopfner (D. R-P., 53782), using cuprous chloride, 
 and Seegall (D. R-P., 53196) employing ferrous chlo- 
 ride, pursued an idea similar to that of Siemens. The 
 suggestion that oxidizable salts should be used in 
 electrolysis is deserving of great consideration. 
 
 As observed in the Siemens process, the nature of 
 
POLARIZING CURRENTS. 8/ 
 
 the electrodes also exercises an influence upon the 
 pressure. An example will show this. When a 
 copper sulphate solution is electrolyzed with plati- 
 num anodes the pressure of decomposition will be 
 observed to be 1.26 V. On substituting lead anodes 
 these will gradually become coated with dioxide 
 (PbO 2 ) and on making a test, the pressure of decom- 
 position will be found to be 1.55-1.60 V. This is 
 due to the reactions which give rise to the polariza- 
 tion current. The reverse electrolysis takes place 
 with platinum anodes, while with lead anodes there 
 comes into play the . formation of lead sulphate 
 (PbSO 4 ) from the oxide (PbO) and the sulphuric acid 
 (H 2 SO 4 ). The dioxide at the anode first loses an 
 atom of oxygen and passes into oxide (PbO), which 
 unites with the free sulphuric acid to form lead sul- 
 phate and water. In fact, the heat, resulting from the 
 formation of the lead dioxide (PbO 2 ) from lead (Pb) 
 and oxygen (O 2 ) should tend to diminish the pressure 
 of the primary electrolysis; yet nothing of this kind 
 is observed, because the main reaction evolves oxy- 
 gen, and but little of it is consumed in the formation 
 of PbO 2 . With the polarizing current, however, the 
 gradually accumulating dioxide immediately comes 
 into evidence with its entire mass. 
 
 The magnitude of the pressure of decomposition 
 and its eventual diminution by means of aid-reactions, 
 are of the greatest importance for the commercial 
 success of a process or method. In the case of a 
 
88 ELECTROCHEMICAL EXPERIMENTS. 
 
 metallic conductor the pressure E, which is necessary 
 to preserve a current of I ampere in a resistance w, 
 
 equals : 
 
 E = I. w. 
 
 In the electrolysis there is added to this the pres- 
 sure of decomposition Z, so that for every electro- 
 lysis the necessary pressure is 
 
 E=: I. W + Z. 
 
 In this expression Z is a constant, determined only 
 by the nature of the electrolytic process and wholly 
 independent of the dimensions of the bath. The 
 value I. w., however, is most intimately connected 
 with the dimensions of the bath and is smaller, the 
 lower the resistance w of the bath. Consequently Z 
 is usually greater than I. w. and will, in large meas- 
 ure, determine the pressure necessary for carrying on 
 the work of the bath. The higher the pressure, how- 
 ever, the greater will the energy consumption for a 
 definite production be. Hence, in every electrolytic 
 process, care must be exercised to have the pressure of 
 decomposition as low as possible, i. e., use soluble 
 anodes whenever it is possible, or if this be not prac- 
 ticable, endeavor to facilitate the electrolytic process 
 by other aid-reactions. 
 
 FARADAY'S LAW. CURRENT EFFICIENCY. 
 
 When a current is passed through different solu- 
 lutions in series, the products separated in equal peri- 
 
FARADAY'S LAW. CURRENT EFFICIENCY. 89 
 
 ods of time are in proportion to their equivalent 
 weights (law of Faraday). 
 
 For example, the same current in an equal period 
 of time, decomposes 
 
 I molecule of HC1, AgNO 3 , KOH ; 
 y, " ZnCl 2 , CuSO 4 , H 2 O; 
 y z " AuQ 3 , NH 3 , etc. 
 
 As a current of one ampere precipitates 4.02 grams 
 of silver per hour, it is easy to calculate the number 
 of grams of any substance which will be decomposed 
 or deposited in a definite period of time. 
 
 Example. How much blue vitriol can be decom- 
 posed daily, in a bath, using a current of 50 amperes? 
 How much copper will be deposited ? 
 
 In accordance with the law just stated above, for 
 every atom, or 107.6 parts by weight of silver, one- 
 half molecule = 124.6 parts of CuSO 4 ,5H 2 O will 
 be decomposed, and consequently j Cu = 31.6 
 parts by weight of copper will be deposited. One 
 ampere-hour corresponds to 4.02 grams of silver, 
 
 hence 4.02 . ' = 4.65 grams of blue vitriol and 
 
 4.02 ' = 1. 1 8 grams of copper; so that in 
 
 24 hours, with a current of 50 amperes : 24 . 50 . 4.65 
 55^ grams of CuSO 4 ,5 aq will be decomposed 
 and there will separate 24 . 50 . 1.18 = 1416 grams of 
 copper. 
 
 When a metal enters its compounds with varying 
 
9O ELECTROCHEMICAL EXPERIMENTS. 
 
 valence, the quantities of that metal deposited by 
 the current will naturally vary according to the va- 
 lence. With the same current, Cu 2 Cl 2 will yield twice 
 as much copper as CuSO 4 . In most instances the 
 reductions first proceed to the lowest state of oxida- 
 tion before the metal begins to separate. 
 
 Very few electrolytic processes proceed in a simple 
 way. In most cases secondary reactions play an 
 important role. There is a double possibility for this : 
 unintentional oxidations can occur at the anode or 
 reductions at the kathode. In such instances the 
 law of Faraday serves for the sum of the reactions 
 occasioned by the current. In the decomposition of 
 copper nitrate, metallic copper appears at the kathode, 
 while a portion of the nitric acid is reduced to lower 
 oxides of nitrogen, or even to ammonia. Oxygen is 
 liberated at the anode, and is partially consumed in 
 the reoxidation to nitric acid of the nitrogen oxides 
 formed at the kathode. When ferric chloride is 
 electrolyzed, ferrous chloride and metallic iron occur 
 at the kathode, which is, in turn, converted at the 
 anode into ferric chloride. A similar phenomenon 
 explains why manganese can not be quantitatively 
 precipitated at the anode from a liquid containing 
 iron, because the salts, in a lower state of oxidation, 
 which have been simultaneously formed, favor the 
 re-solution of portions of the manganese. 
 
 It is of the utmost importance to know the quan- 
 titative course of an electrolytic process. That por- 
 
FARADAY'S LAW. CURRENT EFFICIENCY. 91 
 
 tion of the total current sent into a bath, which is 
 active in the direction desired, represents the current 
 efficiency, while the remainder, consumed in undesired 
 secondary reactions, is the current loss. In every 
 electrolytic process the secondary reactions should 
 be determined both qualitatively and quantitatively, 
 and by alteration of conditions the effort should be 
 made to eliminate these disturbing factors. The 
 character of the side reactions may be discovered by 
 a qualitative examination of the electrolyte after the 
 completion of the experiment, and the amount of 
 loss by a direct determination of the current yield 
 (efficiency). 
 
 To this end insert an ampere-hour meter (a copper 
 voltameter) in the circuit in series with the experi- 
 mental cell (see p. 52). Fill a large beaker or glass 
 jar with copper sulphate (p. 40), observing- the ar- 
 rangement of apparatus and the size of the kathode, 
 as indicated on p. 53. When the experiment is 
 finished, determine the increase in weight of the 
 kathode, and by Faraday's law calculate how much 
 of the desired product should be present in the ex- 
 perimental cell. The actual amount is learned by 
 weighing, or by titration, and its percentage may 
 then be calculated. 
 
 Example. In the electrolysis of a solution of salt, 
 after a certain time, 650 c.c. of liquor, containing 4 
 per cent. NaOH, were obtained. During this same 
 period 30.4 grams of copper had been deposited in 
 
92 ELECTROCHEMICAL EXPERIMENTS. 
 
 the copper voltameter. Calculate the current yield 
 (efficiency). 
 
 According to Faraday's law, 2 molecules, or So 
 parts by weight of caustic soda, result for each atom, 
 or 63.2 parts by weight of copper; hence, for the 
 
 30.4 grams of copper obtained in the experiment, 
 
 38.5 grams of NaOH would be produced (63.2 : 80 = 
 30.4 : x = 38.48). The actual yield was 
 
 650 . = 26.0 grams. 
 
 100 
 
 Hence the current efficiency would be 
 26 
 
 == 67.5 per cent. 
 3-5 
 
 The rest of the current, equaling 32.5 per cent, was 
 therefore expended upon other reactions. 
 
 ION TRANSFERENCE. 
 
 In electrolysis the components of a compound are 
 not only liberated from their molecular union, but 
 they'undergo a rearrangement. If the movement of 
 the liquid particles is not interfered with, this phe- 
 nomenon will not be noticed, but if it is restricted 
 by a diaphragm, alterations in concentration will be 
 observed at the electrodes. 
 
 This interesting phenomenon, which can not be 
 considered theoretically at this time, may be best 
 understood by an example. Decompose dilute sul- 
 
ION TRANSFERENCE. 93 
 
 phuric acid (56.7 grams acid to the liter) between two 
 semicylindrical sheets of lead, by a current of one 
 ampere. Electrolytic gas will be set free, and at the 
 expiration of the experiment the acid will show the 
 same concentration which it first had. A porous cup 
 is next introduced between the electrodes. In three 
 hours the acid in the cup will contain 63.25 grams 
 of sulphuric acid per liter, while the external liquid 
 will contain 55.47 grams per liter of the same acid. 
 After two additional hours the contents will be found 
 to be 68.77 grams and 52.86 grams respectively. 
 Hence a transference has occurred on the part of the 
 sulphuric acid, toward the anode ; and it has been 
 withdrawn from the vicinity of the kathode. By re- 
 versing the experiment, it will be observed that this 
 phenomenon is independent of the varying sizes of 
 the electrode surfaces. On mixing the two portions 
 of acid there will result an acid containing 57.1 grams 
 per liter. Its slight difference from the original content 
 is due to the acid of unknown concentration absorbed 
 by the porous cup. Next, take the smaller sheet of 
 lead, which has been hung in the porous cup, and 
 let it serve as kathode while the larger is made the 
 anode. In three hours the concentration will be as 
 follows: 
 
 in the porous cup ( ) 50.65 grams H 2 SO 4 per liter 
 outside the porous cup (-(-) 60.04 " " " " 
 
 Again, the acid in the anode chamber has been en- 
 riched. Similar phenomena invariably show them- 
 
94 ELECTROCHEMICAL EXPERIMENTS. 
 
 selves if diaphragms are used in the work. They 
 must be considered as a part of the investigation 
 because they influence the concentration of the 
 liquors either favorably or unfavorably. 
 
 CURRENT DENSITY. 
 
 The quantity of the products formed in any elec- 
 trolytic process accords with the law of Faraday and 
 the current strength; the quality, however, as well 
 as the nature of the reactions, depends in many in- 
 stances chiefly upon the size of the electrode surfaces. 
 Current density is the current strength for a unit of 
 surface of an electrode. In technical operations the 
 unit of surface is the sq. m y but in scientific researches 
 it is ordinarily the sq. dm. Whether a metal is de- 
 posited electrolytically in a spongy, crystalline, or 
 amorphous form depends very much upon the cur- 
 rent density prevailing at the kathode. The smaller 
 the electrode surface the more energetically must the 
 action of the current show itself. Thus, Bunsen, 
 by using exceedingly high current densities, actually 
 succeeded in precipitating metallic chromium from an 
 aqueous solution. 
 
 In working with soluble anodes, the influence of 
 current density will also find expression. When the 
 current density is low, the individual constituents of 
 the anode will dissolve one after the other. Metals 
 in solution will be precipitated one after the other, in 
 
CURRENT DENSITY. 95 
 
 accordance with the law that the metal which devel- 
 ops the greatest energy in its solution will first dis- 
 solve, and that one which for its deposition requires 
 the consumption of the least quantity of energy will 
 be first precipitated. As the density increases, these 
 reactions take place almost simultaneously. From a 
 neutral solution containjjig copper and zinc, a current 
 of low density will first throw out the copper, while 
 with higher density an alloy of copper and zinc, i. e., 
 brass, will be deposited. 
 
 In an electrolytic process in which metals are not 
 precipitated, but where gases or substances soluble 
 in water are separated, it might be assumed that the 
 current density was of no moment. This, however, 
 is not the case. If there is a possibility of producing 
 different stages in the oxidations or reductions occur- 
 ring at the electrodes, as, for example, is often the 
 case with organic bodies, the higher density will 
 always produce a more far-reaching oxidation or re- 
 duction than the current with low density. 
 
 In addition to the physical character and the chem- 
 ical nature of the depositions which are formed, the 
 current density further exercises an influence on the 
 pressure of the bath. If, in arranging for an experi- 
 ment, the size and distance of the electrodes from 
 one another, as well as the nature of the electrolyte, 
 remain unchanged and the current density alone be 
 varied, it will be observed that the bath pressure will 
 increase simultaneously with the density of the cur- 
 
96 ELECTROCHEMICAL EXPERIMENTS. 
 
 rent. This phenomenon is not singular or strange 
 because, inasmuch as the resistance of the bath is 
 not altered, a higher current-strength can, according 
 to Ohm's law, only be attained by a higher pressure. 
 
 THE WORKING PRESSURE. 
 
 Although the dependence of the pressure upon 
 various circumstances has been touched upon at 
 many points in the preceding chapter, these relations 
 must again be considered from a common point of 
 view. 
 
 The working pressure depends : 
 
 (1) Upon the decomposition-pressure developed by 
 the thermal value of the process under consideration. 
 
 (2) Upon the sum of all the resistances of the 
 bath. 
 
 (3) Upon the current density which must be main- 
 tained. 
 
 Economy demands that the working pressure shall 
 be low. Let us see how this can be accomplished. 
 
 (a) The decomposition-pressure of a process is 
 dependent upon the chemical changes producing it: 
 hence it can not be changed at will. However, as 
 indicated upon p. 82, it is possible in many instances 
 to bring in a reaction which will aid the electrolysis 
 one which will produce heat and thereby dimin- 
 ish the consumption of external energy in the form 
 of electricity. The use of soluble anodes and the 
 
THE WORKING PRESSURE. 97 
 
 continuous addition of oxidizable substances to in- 
 soluble anodes may also be mentioned in this connec- 
 tion. 
 
 (b) The resistance of the bath may be reduced 
 by greater concentration of solution, the addition of 
 a free acid, if this is advisable or allowable, by heat- 
 ing the electrolyte, and by bringing the electrodes 
 nearer together. In arranging the electrodes care 
 should be exercised that short-circuiting between ad- 
 jacent electrodes is not only avoided but is impossi- 
 ble. The resistance in a liquid can be diminished 
 not only by bringing the electrodes nearer together, 
 but also by increasing their surface, because thereby 
 the liquid layer to be traversed by the current will 
 acqVire a larger cross-section. This is, however, 
 scarcely allowable because the size of the electrode 
 surfaces has already been fixed by the current density 
 most favorable for the operation. A slight advantage 
 can be obtained by having the electrodes of unequal 
 size. The occasion will be rare when the current 
 density at both electrodes must have a definite value. 
 This is necessary usually for only one electrode, 
 either the negative or positive, depending upon the 
 nature of the process. Therefore, the other electrode 
 may be made larger, and in this way the resistance in 
 the liquid will be diminished. It is only possible to 
 make an essential gain in this manner when the two 
 electrodes show decided differences in size. 
 
 (c) When the density and composition of the so- 
 
98 ELECTROCHEMICAL EXPERIMENTS. 
 
 lution in an electrolytic process are given, the pres- 
 sure value is at once fixed and only the minor details 
 can now be considered, such as may be produced by a 
 greater or less distance between the electrodes and a 
 higher or lower bath temperature. With such provi- 
 sions the pressure can no longer be varied at will with- 
 out changing both the current density and the separa- 
 tion of the electrodes. With unchanged solution and 
 unchanged distances between the electrodes, there is of 
 necessity a lower density for a lower pressure, and 
 vice versa. 
 
 D. THE PRELIMINARY EXPERIMENTS 
 NECESSARY FOR AN ELECTRO- 
 CHEMICAL PROCESS. 
 
 If electricity is to be used in carrying out a process, 
 answers should be given to the following questions : 
 
 1. What are the most favorable conditions for the 
 experiment? 
 
 2. What expense will attend the process ? 
 
 The experiment should not be conducted on too 
 contracted a scale. The selected electrodes should 
 have I oo sq. cm. surface per side. This is particularly 
 desirable where metal precipitations are concerned. 
 The metallic deposits on the edges of the electrodes 
 are generally different from those appearing on the 
 middle of its surface, so that a false judgment might 
 
PRELIMINARY EXPERIMENTS. 99 
 
 easily follow if the electrode had, so to speak, only 
 an edge surface. 
 
 As electrolyte choose a solution apparently suit- 
 able for the purpose, and at first vary the current 
 density. Note the pressures which are produced, 
 and observe the physical and chemical nature of the 
 deposits ; examine the secondary reactions both 
 qualitatively and quantitatively. Next alter the com- 
 position of the solution let it be more dilute, more 
 concentrated, neutral, acid, alkaline. Change the salts 
 until a suitable solution and proper current density 
 have been found. When the most favorable condi- 
 tions have been discovered, make a few preliminary 
 experiments upon the current efficiency, and when it 
 is possible let this be done with extremes in tempera- 
 ture (high and low) which are likely to figure in the 
 practical application. During the course of the ex- 
 periment it will no doubt become patent that the 
 construction of the apparatus must be altered. In 
 all such changes consider the question : Will this 
 arrangement prove satisfactory if the dimensions 
 are those required when working on a large scale? 
 Technical experiments should not be performed with 
 apparatus whose principles can not be applied practi- 
 cally. Results obtained with such apparatus have 
 little, if any, technical value, simply because they can 
 not be directly introduced into practice. 
 
 In the whole course of the experiments such fre- 
 quent opportunity is offered to exercise the ingenuity 
 
IOO ELECTROCHEMICAL EXPERIMENTS. 
 
 as chemist, physicist, and engineer, that it is unneces- 
 sary to speak of all the possible conditions. 
 
 When the most favorable experimental conditions 
 have been found and suitable apparatus has been 
 constructed, advance is made to the next stage of the 
 experiment by having a small dynamo act for several 
 weeks or months on a number of baths arranged in 
 series. A more extended view of the process is thus 
 gained, and the deficiencies in construction appear 
 which in the case of smaller apparatus would be over- 
 looked. When the faults have been largely removed 
 and the plant works well, a calculation for larger 
 conditions can be undertaken without fear of stumb- 
 ling upon great deceptions or mistakes. 
 
 E. CALCULATION OF THE NECESSARY 
 POWER. CHOICE OF DYNAMOS. 
 
 To calculate the power required for an electro- 
 chemical process it is necessary to know the pressure 
 and current efficiency for each bath. 
 
 Suppose that the bath pressure equals s volts, and 
 the current yield <r per cent., then the work of a horse- 
 power would be 
 
 V V amperes r -' 
 
 I HP = ; with a pressure, therefore, of s 
 
 volts there would consequently result amperes 
 
CALCULATION OF THE NECESSARY POWER. IOI 
 
 to the horse-power. If a grams of some definite sub- 
 stance is produced per ampere-hour (see Table i), then 
 
 for each horse-power hour there would result - - . 
 
 a grams. Since the current efficiency is never 
 quantitative, but only equal to <r per cent., the value 
 
 above must be further multiplied by and then it 
 becomes Finally, a correction must be 
 
 S . 100 
 
 added for the efficiency v of the dynamo. In the 
 case of small dynamos v = 75 to 80 per cent., whereas 
 with larger machines and normal loads this value 
 rises to 93 per cent. 
 
 Hence the actual yield for each horse-power hour is 
 
 736 a . ff . v 
 
 grams; or introducing the limiting values 
 
 of v 75 per cent, to 93 per cent. given above, the 
 production p for a horse-power hour becomes, depend- 
 ing upon the size of the machine used 
 
 552 a 684 a 
 
 p = . a . - -to . a . - . 
 
 S 100 S 100 
 
 Having thus deduced the production for a horse- 
 power hour, the daily production, having a definite 
 power at command, can easily be ascertained, or the 
 reverse, viz., the power requisite to manufacture daily 
 ICO kg. of any product. 
 
 In projecting a plant, the result above indicated 
 will not be sufficient. Regard must also be had 
 
IO2 ELECTROCHEMICAL EXPERIMENTS. 
 
 to the mixing and transportation of the liquor, 
 to the working of the adjunct machinery, etc. 
 If a steam boiler is necessary, attention must be paid 
 to the consumption of steam in heating the liquor, 
 and at least to the heating of the working-rooms by 
 steam during the winter. 
 
 As regards the choice of dynamos, it may be said 
 that, so far as its efficiency goes, it is immaterial 
 whether medium pressure and powerful current or 
 high pressure and low current be employed. How- 
 ever, the following must be observed : If the nature 
 of the process in any manner permits, i. e., if the baths 
 are alike and regularly served, they should be ar- 
 ranged in series. The number thus introduced into 
 
 o 
 
 the circuit, and the pressure that the machine will 
 consequently have, will depend upon whether fre- 
 quent disturbances occur, due to cleansing of the 
 baths or electrodes, fresh charges, etc. In electro- 
 lytic copper refining, 40 to 50 baths are arranged in 
 series, but with a less active plant 15 to 20 constitute a 
 very respectable number. If each bath requires a 
 pressure of s volts and if ;/ baths of this kind are to 
 be used in series, a pressure of V = n . s volts will be 
 required of the dynamo. As a rule, the pressure of 
 the machine should be higher than that calculated, 
 because by constant work it falls in consequence 
 of the heating of the armature, and also because of 
 imperfect contacts here and there. The latter can 
 scarcely be avoided, so that an increased pressure 
 
CHOICE OF DYNAMOS. 1 03 
 
 is necessary to maintain an undiminished current 
 strength. When the entire electromotive force of 
 the dynamo is not needed, the excess is cut off by 
 being sent through a shunt regulator. 
 
 A reserve dynamo should always be on hand, so 
 that in case of repairs to the running machine the 
 action of the current need not be interrupted for days 
 or weeks. It is recommended to use the one ma- 
 chine for day work, and the other for work at night. 
 In this way both machines are better protected than 
 if they be run until some part or parts are completely 
 used up. By an arrangement such as the preceding, 
 the periods required for cleaning can also be minim- 
 ized, and the person in charge, finally, can have a 
 better oversight of the dynamos. Each workman 
 can then supervise his own machine, and can not con- 
 cgal any mistakes which may have occurred in its 
 operation. Again, it will be easy in this way to 
 arouse the sense of honor of each attendant so that 
 he will strive to keep his machine in the best con- 
 dition. 
 
 In addition to the expense incurred in obtaining 
 the power it is necessary, when calculating the cost of 
 the manufacture, to include* interest on the purchase 
 sums for the entire plant (buildings, machinery, instru- 
 ments, baths, conductors), the cost of chemicals con- 
 
 * See also Berg- u. Hiittenmann. Zeitung, 1893, 5 2 - 
 
IO4 ELECTROCHEMICAL EXPERIMENTS. 
 
 sumed in the process, and wages, as well as loss in 
 interest due to the storage of the necessary supplies 
 of crude material. 
 
 F. PRACTICAL PART. 
 
 i. CONSTRUCTION AND CALIBRATION OF A TAN- 
 GENT GALVANOMETER. 
 
 A very simple contrivance shown in the accom- 
 panying Fig. 19, and answering every purpose, may 
 
 FIG. 19. 
 
 be made by any experimenter. It requires no special 
 description. Notches are made in the little table 
 at the points where the wire passes through it. It 
 must, however, have margin enough to support a 
 glass cover, intended to protect .the needle from air- 
 currents. The needle is a short magnetic steel rod 
 (a knitting needle). That the angle of deflection on 
 
CALIBRATION OF GALVANOMETER. IO$ 
 
 the largely divided circle may be conveniently read, 
 attach to the needle by means of shellac or wax a thin 
 hollow glass thread filled with ink. A fiber of the 
 finest sewing silk will answer for the suspension fila- 
 ment. This also is placed upon the sealing wax, and by 
 momentary contact with a hot wire is attached to the 
 needle. A drop of wax or paraffin brought upon 
 the glass tube will serve to balance the needle. This 
 device, it is true, is very primitive, but it will suffice 
 for many purposes, especially when it is only in- 
 tended to obtain an approximate idea of the current 
 strength, or whether it changes during the experiment. 
 Tangent galvanometers are calibrated by bringing 
 them into circuit with a voltameter (arrangement of 
 experiment : battery galvanometer voltameter 
 battery) and observing the deflections a and a. 2 , with 
 varying current strengths, I and I 2 . According to 
 
 I = c . tang, a, 
 
 consequently 
 
 tang, a 
 
 The values for c, deduced from the two experi- 
 ments, must agree fairly well. Determine their mean 
 and calculate a table for the different deflections. 
 
 EXAMPLE. The length of the needle is 28 mm., and 
 the diameter of the circle is 167 mm. 
 
 Experiment i. In six minutes 0.0508 gram of me- 
 tallic copper was obtained in the copper voltameter; 
 
 H 
 
IO6 ELECTROCHEMICAL EXPERIMENTS. 
 
 the deflection in the galvanometer averaged 12. 
 The precipitation of copper per hour would, there- 
 fore, be 0.508 gram, and as 1.181 grams copper, per 
 hour, correspond to I ampere, the calculated cur- 
 
 0.508 
 rent strength would be = 0.430 ampere. The 
 
 constants of the galvanometer may, therefore, be cal- 
 culated from the equation 
 
 0.430 
 
 c - - = 2.023. 
 tang. 12 
 
 Experiment 2. In six minutes 0.0963 gram of 
 copper was precipitated. The average deflection was 
 22. Therefore the current strength was 
 
 0.963 
 
 I = = 0.815 ampere, 
 1.181 
 
 hence 
 
 0.815 
 
 c = - - 2.017. 
 tang. 22 
 
 hence 
 
 The mean of the two experiments is c = 2.02. 
 This is, at the same time, the current strength for the 
 deflection-angle 45 (tang. 45 = i). A table for the 
 galvanometer can now be constructed : 
 
 1 deflection : I = 2.02 tang. 1 = 0.035 Amp., 
 
 2 " I = 2.02 tang. 2 = 0.070 " 
 
 3 " I = 2.02 tang. 3 = 0.106 " 
 
 etc. 
 
CALIBRATION OF GALVANOMETER. IO/ 
 
 2. CALIBRATION OF A GALVANOMETER BY 
 MEANS OF A SHUNT. 
 
 If it is desired to enlarge the capacity of this very 
 sensitive instrument, recourse may be had to a shunt. 
 This must be considered and arranged as a part of 
 the main circuit. As all sensitive galvanometers have 
 a great number of turns, and as these are usually 
 wound flat, the constants can not be established by 
 one or two experiments, but must be determined em- 
 pirically. This is done by making a number of read- 
 ings with different current strengths, the results being 
 then transferred to millimeter paper on which the 
 current strengths appear as the abscissas and the cor- 
 responding deflections as the ordinates. The points 
 fixed in this manner are then united into a continuous 
 curve, from which, by inverting the order, current 
 strengths corresponding to all deflections can be ob- 
 tained. 
 
 The lower the resistance of the shunt, the stronger 
 may the currents be for which the instrument is avail- 
 able. It should be remembered that in consequence 
 of the sensitiveness of the apparatus the needle may 
 be appreciably influenced by the main current if the 
 galvanometer be set up in too close proximity to the 
 latter. Whenever possible, place the galvanometer 
 from ^ to I m. from the main current. Should the 
 galvanometer always occupy the same position, the 
 disturbance may be disregarded, because it has 
 
io8 
 
 ELECTROCHEMICAL EXPERIMENTS. 
 
 been fully allowed for in the curve previously con- 
 structed. If, however, the galvanometer and its shunt 
 are to be used in different localities, it is quite neces- 
 sary to remove them from the main current to the 
 distance mentioned. 
 
 When a delicate galvanometer can not be had, a very 
 simple form can be constructed after the style of the 
 tangent instrument described in the preceding chap- 
 
 FlG. 20. 
 
 ter. Instead of the single copper ring, use a coil 
 of covered copper wire having numerous turns. 
 The needle may be large in proportion to the diam- 
 eter of the ring, as no use will be made of the tangent 
 law. 
 
 Example. A delicate instrument provided with a 
 bell magnet and copper damping was used as the gal- 
 vanometer (Hartmann and Braun, Frankfort-on-the- 
 
CALIBRATION OF GALVANOMETER. 
 
 ICQ 
 
 Main) ; its resistance was 3.35 Q. The shunt con- 
 sisted of I m. of copper wire 2 mm. in diameter, at 
 each end of which a plate of metal with two clamps 
 was attached by solder. This shunt N, a copper 
 voltameter K, and a resistance W, serving to vary the 
 current strength, were arranged in series in the same 
 circuit. The galvanometer G, was connected with the 
 shunt N by means of two copper wires, I m. in length 
 (Fig. 20). (These two connecting wires form a 
 part of the branch resistance, hence, in subsequent 
 measurements, can not be exchanged for any other 
 two wires.) 
 
 Produce with W currents of different strengths ; 
 note the deflection, and weigh the copper deposited in 
 a unit of time (6 minutes, preferably, as they equal 
 ^ of an hour). 
 
 DURATION OF 
 EXPERIMENT. 
 
 DEFLECTION. 
 
 PRECIPITATED 
 COPPER 
 
 CURRENT 
 STRENGTH. 
 
 MINUTES. 
 
 DEC. 
 
 IN GRAMS. 
 
 AMPERES. 
 
 6 . . 
 
 . 5-6. .. 
 
 . . 0.0114 . . 
 
 . 0.096 
 
 6 ... 
 
 . . 7-5 .-. 
 
 . . 0.0154 . . 
 
 . 0.130 
 
 6 ... 
 
 . . 10.8 . 
 
 . . 0.0222 . . 
 
 . 0.187 
 
 6. . . 
 
 . . 14.3 . . . 
 
 . . 0.0307 . . 
 
 - 0.259 
 
 8 ... 
 
 . . 20.0 . . . 
 
 . . 0.0599 
 
 -379 
 
 6 ... 
 
 . . 22.1 . . . 
 
 . . 0.0502 . . 
 
 . 0.423 
 
 7 . 
 
 . . 24.5 . . . 
 
 . . 0.0661 . . 
 
 . 0.478 
 
 6 
 
 28 o 
 
 . o 0668 
 
 o. ^6^ 
 
 6 . . . 
 
 . . 32.0 . . . 
 
 . . 0.0802 . . 
 
 . 0.676 
 
 6 ... 
 
 37.O . 
 
 . . 0.0961 . . 
 
 . 0.810 
 
 6 . . . 
 
 . . 43.0 . . . 
 
 . . O.I2OO . . 
 
 . 1. 012 
 
 6 . . . 
 
 . . 49-0 . . 
 
 . . o. 1488 . . 
 
 1-255 
 
110 
 
 ELECTROCHEMICAL EXPERIMENTS. 
 
 In Fig. 21 the preceding current strengths appear 
 as abscissas, the corresponding deflections as ordi- 
 nates, and the resulting points of intersection indicated 
 by crosses form the connecting curve : 
 
 50r 
 
 O s 
 
 FIG. 21. 
 
 The following table can be read from this curve as 
 a result of the calibration : 
 
 O.I 
 
 o. 2 
 
 0-3 
 0.4 
 
 0.5 
 0.6 
 0.7 
 0.8 
 0.9 
 
 I.O 
 
 I.I 
 
 1.2 
 1-25 
 
 DEFLECTIONS. 
 
 16.3 
 21. 
 
 25- 
 2 9 . 
 
 33- 
 
 36.5 
 
 39-5 
 
 43-o 
 
 46. 
 
 48. 
 
 49- 
 
INSTRUMENT TO MEASURE PRESSURE. I I I 
 
 3. CONSTRUCTION OF AN INSTRUMENT TO MEAS- 
 URE PRESSURE. 
 
 If several storage cells are at the disposal of the 
 operator, the galvanometer used in the preceding 
 experiment may be calibrated so as to measure pres- 
 sure. When a single accumulator has lost a small 
 part of its charge, its pressure may be accepted as 
 fairly accurate if taken as 2.0 V. If the cell be 
 discharged by a German silver wire, the resistance of 
 which has been so chosen that the wire never becomes 
 too hot and the current allowed for the cell is never 
 exceeded, there will exist between the terminals 
 of the German silver wire a pressure of two volts ; at 
 the middle it will have one volt, and between the one 
 terminal and the first quarter 0.5 volt, etc. If a wire 
 I m. in length has been selected, each cm. of it will 
 correspond to T i 7 of the pressure prevailing at the 
 terminals. Hence it will only be necessary to suc- 
 cessively connect the galvanometer by its wires to the 
 points of the German silver wire representing o.i, 0.2, 
 0.3, etc.,V. pressure, and note the deflections in order to 
 use the instrument as a voltmeter graduated in tenths. 
 In order to obtain suitable deflections of the galvan- 
 ometer it must be provided with a series resistance 
 the size of which will depend upon the pressure and 
 the delicacy of the instrument. As mentioned on p. 
 54, the use of a high series resistance increases or en- 
 
112 ELECTROCHEMICAL EXPERIMENTS. 
 
 larges the applicability of the instrument. Hence, 
 having the choice between a less delicate galvanom- 
 eter with lower series resistance, and one that is very 
 delicate, and for the introduction of which a high 
 resistance will be necessary, it will be wise to choose 
 the latter. 
 
 Example. A piece of fine German silver wire, 2 m. 
 in length, is stretched over a narrow board, which at its 
 terminals is soldered to binding screws, connected by 
 stout copper wires to two accumulators, in series, hav- 
 ing a maximum discharge capacity of ten amperes. 
 As the resistance of the wire equals 6.67 Q, there 
 exists in it, during the'experiment, a current strength of 
 
 - = 0.6 ampere. To obtain a proper angle of de- 
 6.67 
 
 flection for the pressure of 4.0 V. prevailing at the ter- 
 minals use a series resistance of about 550 Q. Under 
 the stretched wire is placed a scale 2 meters in length 
 and divided into 40 equal parts. The division lines are 
 five cm. apart and the pressures are written at these 
 points. The calibration from T x to -fa V. is made in 
 such a manner that the one binding screw of the gal- 
 vanometer is connected by a fine wire to the first 
 binding screw of the German silver wire, while the 
 other binding screw with the inserted resistance W is 
 connected with a long, flexible wire. This is carried 
 along the German silver wire and at each division the 
 corresponding deflection of the galvanometer noted. 
 Fig. 22 represents the arrangement as projected. 
 
INSTRUMENT TO MEASURE PRESSURE. 
 
 + 
 
 -\ 
 
 , 1 . , , 1 , , , , 1 1 , 
 
 
 < 1 1 1 1 | 1 1 1 1 1 4-4- 
 1.0 
 
 < * 1 ' 1 | 1 1 1 | 1 1 1 1 1 1 1 1 1 1 i 1 1 1 * 
 
 2.0 V-, 3' 4o 
 
 FIG. 22. 
 
 The values obtained when W = about 550 U 
 
 are: 
 
 PRESSURES. 
 o.i V. 
 
 0.2 
 
 o-3 
 0.4 
 
 0-5 
 0.6 
 0.7 
 
 DEFLECTIONS. 
 2.1 
 
 4-5 
 6.6 
 
 8-7 
 10.8 
 12.8 
 14.6 
 
 3-6 
 3-7 
 3-8 
 3-9 
 4.0 
 
 51.6 
 52.3 
 53-o 
 53-7 
 54-4 
 
114 ELECTROCHEMICAL EXPERIMENTS. 
 
 When a second series resistance of only 100 Q was 
 used to obtain a greater accuracy for a lower ca- 
 pacity, the following deflections were observed : 
 
 PRESSURES. DEFLECTIONS. 
 
 o.io V. 12 
 
 0.15 17.2 
 
 O.2O 22.5 
 
 0.25 27.4 
 
 0.30 31.7 
 
 0.80 56.5 
 
 0.*5 58.1 
 
 O.QO 59-4 
 
 0.95 60.7 
 
 1. 00 61.8 
 
 The pressure corresponding to each degree of the 
 galvanometer may be ascertained by carefully con- 
 structing a curve, as shown on p. no, from the 
 observations made and then taking from it the num- 
 bers not directly observed. 
 
 Series resistances may be constructed by winding 
 covered nickelin or rheotan wires,* of 0.2 mm. 
 diameter, upon wooden spools. 
 
 Each wire has an approximate resistance of 15 Q 
 per meter. Stout copper wires should be soldered to 
 the terminals and the whole apparatus be then placed 
 
 * May be obtained from F. A. Lange, Berlin C., Seydelstrasse. 
 
, 
 
 OF THE 
 
 UNIVERSITY 
 
 REGULATING RESISTANCE. 
 
 in a box or under a glass cover, the terminals of the 
 copper wire alone projecting beyond the latter. 
 
 4. THE CALCULATION FOR AND CONSTRUCTION 
 OF A REGULATING RESISTANCE. 
 
 A battery of storage cells is at the disposal of the 
 chemist. Each cell is allowed a maximum discharge 
 rate of ten amperes. The resistance in circuit may be 
 so regulated that only a portion of the four cells need 
 be connected for pressure. This, however, would be 
 a crude arrangement, and, furthermore, the cells would 
 be discharged unequally, so that after the lapse of a 
 certain period some of the cells would be exhausted, 
 while others would still be in a workable condition. 
 It would be best, if possible, to utilize all the cells in 
 every experiment. They would then sustain equal 
 discharge, and could subsequently be equally charged. 
 A wire is therefore run from the battery to the experi- 
 ment table. Somewhere in the line introduce a reeu- 
 
 o 
 
 lator, which will enable the operator to reduce the 
 current in every possible way and without making any 
 great jumps. When several experiments are to be 
 conducted simultaneously by means of the same 
 battery, throw in such a resistance between each 
 individual experiment and the main circuit. The 
 resistance wires are of such a size that the passing 
 currents are not sufficient to ignite them. The wires 
 should not be too long and not too different in kind. 
 
Il6 ELECTROCHEMICAL EXPERIMENTS. 
 
 The ordinary arrangement of the battery is such 
 that the four cells are placed in series. Their yield 
 would then be 8 V. and 10 amperes. The group 
 arrangement of 2 X 2 cells with a yield of 20 
 amperes and 4 V. rarely occurs. The maximum 
 current strength, due to the resistance in question 
 is, therefore, 20 amperes. Disregarding altogether 
 the resistance in the experiment under consideration, 
 
 the 20 amperes appear with a resistance of - - = 0.2 
 
 Q and 10 amperes flow in a circuit where the re- 
 
 g 
 
 sistance is = 0.8 &. The wire for the resistance 
 
 10 
 
 subdivisions under I Q should be so selected that 
 it may safely carry 20 amperes. From the table 
 on p. 63 nickelin wire 1.5 mm. in diameter would 
 answer. If it is desired to regulate down to 0.2 
 
 o 
 
 ampere, a total resistance of =40 Q would be 
 
 requisite. Indeed, with 40 @ it is possible to 
 get even below this point, because the resistance of 
 the conducting wires in the particular experiment 
 must always be added, and its decomposition-pres- 
 sure must be deducted from the 8 or 4 V. of the 
 battery. 
 
 Selecting the subdivisions 0.25, 0.25,0.5, i, 2, 2, 5, 
 10, 20 Q, the corresponding current strengths in the 
 separate subdivisions, and the diameter of the nickel- 
 in wire, suitable for the same, may be found as 
 follows : 
 
REGULATING RESISTANCE. I \J 
 
 SUBDIVISION. CURRENT STRENGTH. DIAMETER OF WIRE. 
 
 Up to I G 20 10 amperes 1.5 mm . 
 
 2 5 " 41.6 " 0.7 " 
 
 10 40 " 0.8 0.2 " 0.3 " 
 
 The entire regulating resistance then arranges it- 
 self as follows : 
 
 SUBDIVISION. LENGTH AND SIZE OF NICKELIN WIRE. 
 
 0.25 Q 1.09 m. 
 
 0.25 " 1.09 " V 1.5 mm. 
 
 0.5 " 
 
 1. " 
 
 2. " 1.92 " }- 0.7 
 2. 
 
 5- " 
 
 10. " 1.79 " f 0.3 
 
 20. " 
 
 The first of these wires is arranged in a long loop, 
 provided with a sliding contact, so that its resis- 
 tance can be changed from o. to 0.25 Q by mere altera- 
 tion in position. Two thin copper plates, bound 
 together, answer for the sliding contact. They are 
 pushed over the wire loop as indicated in Fig. 23, a. 
 The remaining wires are wound in spirals and at- 
 tached to a frame, each of their ends being soldered * 
 
 * Every person engaging in electrolytic experiments should possess 
 some skill in soldering. Contacts which are to act well for long 
 periods are best prepared by soldering. The bright, clean terminals of 
 the two wires are wrapped about each other, moistened with a solution 
 of zinc chloride, heated in the flame of a Bunsen or alcohol lamp, and the 
 heated point touched with a thin hammered piece of tin, until the 
 latter melts and flows smoothly over the soldered spot. If the solder 
 does not spread, the wires are again moistened with " solder-water," 
 
115 ELECTROCHEMICAL EXPERIMENTS. 
 
 to a piece of stout copper wire which dips into a cup 
 of mercury. 
 
 To throw out a resistance subdivision, unite the 
 corresponding mercury cups with a copper wire hav- 
 ing this form 
 
 FIG. 23. 
 
 5. WORKING A COPPER LIQUOR CONTAINING 
 ARSENIC. 
 
 Problem. A factory produces daily 5 cb. m. of 
 liquor, which contains 40 grams of copper as sulphate 
 
 and the manipulation repeated. With practice the end may soon be 
 attained with ease. Each solder-point must be washed to remove any 
 adherent acid, which would gradually destroy the metal. 
 
COPPER LIQUOR. I IQ 
 
 and 10 grams of arsenic as arsenic acid, per liter. 
 Other heavy metals are present only in small quan- 
 tity, while chlorine and nitric acid are absent. The 
 attempt is to be made to purify the copper as far as 
 possible in the electrolytic way, and to separate it in a 
 marketable form. Make an estimate of the probable 
 cost of the plant with the necessary power, keeping in 
 view its future expansion. 
 
 It is a fact well known to every chemist that copper 
 can not be separated quantitatively, as metal, in the 
 electrolytic way from solutions containing arsenic 
 without some of the latter being carried down with it. 
 At the beginning of the electrolysis pure copper, 
 bright red in color, separates ; but it gradually 
 grows paler, and in a comparatively short time it is 
 steel gray, or even black, from the co-precipitated 
 arsenic. The bright blue color of the solution, at 
 this point, is evidence that the liquid is not yet free 
 from copper. 
 
 In the electrolytic separation of two bodies, such as 
 are presented in this case, two considerations must be 
 observed : the proper current density nx\A the agitation 
 of the liquor. When the current density is high, the 
 velocity with which chemical reactions must occur at 
 every point of the kathode will leave very little time 
 for the current to select from among the separate 
 bodies in the solution, so that two or even more of 
 them will be simultaneously precipitated. With low 
 current density, however, the possibility of such a 
 
I2O ELECTROCHEMICAL EXPERIMENTS. 
 
 selection will occur. Hence a very definite relation 
 between the copper and arsenic content of a solution 
 exists for every current density beyond which point 
 the copper precipitated will contain arsenic. These 
 limits should be determined. 
 
 The liquid at the kathode, from which copper has 
 been deposited, becomes specifically lighter and rises 
 to the surface. This upward movement of liquid at the 
 kathode can be readily shown by carrying out the ex- 
 periment in a glass vessel and examining the solution 
 by transmitted light. The liquid rising in the vicinity 
 of the kathode becomes, in consequence of the dimin- 
 ished copper content, relatively richer in arsenic, as 
 compared with the major portion of the solution. 
 Unfavorable mixture conditions, therefore, exist about 
 the kathode unless the difference is destroyed by 
 constant stirring. Thorough agitation of the liquor 
 becomes necessary in proportion as the kathode 
 plates are longer. 
 
 With the preceding observations before us, the 
 course of the investigation is clearly indicated. To 
 obtain as much pure copper as possible, the liquor must 
 be constantly agitated. It must also be determined, 
 with varying current densities, at what copper-content 
 of the liquor the experiment should be interrupted to 
 prevent the deposition of arsenic. The current effi- 
 ciency and the bath pressure should be ascertained by 
 an inserted voltameter. The last two factors form a 
 basis for the calculation of the requisite power, while 
 
COPPER LIQUOR. 121 
 
 the most favorable current density determines the 
 necessary size of plant. 
 
 Execution of Experiments. Two paraffined cigar 
 boxes will answer for containing vessels. They should 
 be 1 1 cm. in depth and in length and be 6 cm. wide. 
 The one contains the solution to experiment upon, 
 while the other does service as a copper voltameter 
 (see p. 40). The electrodes of the experimental cell 
 consist of two plates of lead as anodes, between which 
 is suspended a thin plate of copper, serving as kathode. 
 Each metal plate is 10 X 10 cm. The electrodes of 
 the voltameter should be a thick and a thin plate of 
 copper of equal size. Agitate the solution by a stirrer 
 which constantly moves two glass rods back and forth 
 between the electrodes (30 revolutions per minute). 
 A horizontal, not vertical, movement is preferable, 
 because in actual practice the former is cheaper. With 
 an up-and-down movement the weight of the agitator 
 must be raised with each lifting ; this means a con- 
 siderable consumption of energy. With a to-and-fro 
 movement of the agitator, however, a rake can be 
 attached to a bar passing over firmly attached carry- 
 ing-rolls. This does away with the weight of the 
 agitator, and the work required for stirring, occa- 
 sioned by the resistance of the liquid and the friction 
 of the carrying rolls, is reduced to a minimum. 
 
 The current from two storage cells is made to flow 
 through a regulating resistance, and afterward passes, 
 in regular succession, the shunt of a galvanometer, 
 i 
 
122 ELECTROCHEMICAL EXPERIMENTS. 
 
 the experimental cell, the copper voltameter, and then 
 back to the battery. The liquor contained 39.29 
 grams of copper and 9.9849 grams of arsenic per liter. 
 The experimental cell contained 530 c.cm., corre- 
 sponding to 20.822 grams of copper and 5.291 grams 
 of arsenic. 
 
 The first experiment was conducted with a current 
 I = 1.257 amperes (calculated from the voltameter), 
 the kathode surface immersed in the liquor was 10 X 
 8.4 cm., hence the surface used was 168 sq. cm., and 
 
 the current density was D = ~ = o .748 ampere 
 
 for each sq. dm. In trade, whole numbers are pre- 
 ferred ; hence, in calculating the current density, the 
 square meter, one hundred times as great, is taken 
 as the unit. The current density in the preceding case 
 would then be " 75 amperes per sq. m." The pres- 
 sure remained nearly constant at 1.92 V. 
 
 The experiment was interrupted at the close of 
 eight and one-half hours. The precipitated copper 
 had a perfectly bright color, and weighed 12.109 
 grams. In the voltameter 12.613 grams of copper 
 were deposited, consequently the current efficiency 
 
 was 
 
 12.109 
 
 12.613 
 
 = 96 per cent. 
 
 After two additional hours the plate of metal in 
 the voltameter had increased 2.8925 grams in weight, 
 while the sheet in the experimental bath had in- 
 
COPPER LIQUOR. 123 
 
 creased 2.820 grams. Hence the carrent efficiency 
 was 97.49 per cent., the mean current strength 1.224 
 amperes, and the current density 73 amperes per sq. 
 m. The precipitate was perfectly satisfactory in 
 every respect. 
 
 The following morning the experiment was con- 
 tinued for two hours more. The pressure equaled 
 1.95 V. In the voltameter 2.8195 grams of copper 
 had been deposited, and in the experimental bath the 
 quantity of copper equaled 2.7828 grams. The cur- 
 rent strength, therefore, was 1.194 amperes, the cur- 
 rent density 71 amperes, and the current efficiency 
 98.7 per cent. 
 
 As the copper now began to take on an earthy 
 color, it was assumed that the limit for the current 
 density, so frequently mentioned, had been attained. 
 The copper precipitated in all was 
 
 12.109 grams. 
 2.820 " 
 2.783 
 
 Total, 17.712 " 
 
 hence, there still remained in solution 20.822 
 17.712 = 3.11 grams of copper and 5.29 grams of 
 arsenic, or, as these quantities were dissolved in 530 
 c.c. of liquor, the latter contained, per liter, 5.87 
 grams of copper plus 9.98 grams of arsenic i. e., 
 in round numbers, 6 grams of copper for 10 grams of 
 arsenic. 
 
124 ELECTROCHEMICAL EXPERIMENTS. 
 
 Again, a new plate of copper was suspended in the 
 solution. The experiment was then continued with 
 a more feeble current, consequently with a lower cur- 
 rent density. At the expiration of two hours there 
 was an evident gray-colored deposit of arsenic. The 
 observations were as follows : 
 
 The copper deposited in the voltameter was 1.4485 
 grams. 
 
 The copper deposited in the experimental cell was 
 I -3974 grams. 
 
 Pressure equaled 1.85 V. 
 
 The current strength, deduced from these data, 
 equaled 0.613 ampere; the current density equaled 
 36.5 amperes per sq. m., and the current efficiency was 
 96.47 per cent. The liquor still contained 3.11 I 40 
 1.71 grams of copper, together with 5.29 grams of 
 arsenic, or in every liter there remained 3.23 grams 
 of copper plus 10 grams of arsenic. 
 
 The gray-coated metallic plate was now replaced by 
 another, and the current was still further diminished 
 to 0.25 ampere for a kathode surface of 144 sq. cm. 
 The current density, calculated from this, equaled 18 
 amperes per sq. m. The pressure was 1.69 V. After 
 three hours the copper precipitate was still a beau- 
 tiful red in color. The deposited quantities were 
 0.9089 gram and 0.8354 gram, from which the cur- 
 rent efficiency of 92 per cent, was calculated. 
 
 At the end of another hour and seven minutes the 
 copper became coated with a gray deposit of arsenic, 
 
COPPER LIQUOR. 
 
 125 
 
 so that the experiment was now definitely interrupted. 
 The solution contained 2 grams of copper and 10 
 grams of arsenic per liter. 
 
 These results can be best understood from the 
 following tabular statement : 
 
 CURRENT 
 DENSITY PER 
 SQ. METER. 
 
 71 Amp. 
 ?6 " 
 18 " 
 
 PRESSURE. 
 
 '95 
 
 1.85 
 1.69 
 
 V. 
 
 CURRENT 
 EFFICIENCY. 
 
 97 % 
 
 THE FINAL LIQUOR. 
 
 6 g. Cu -)- 10 g. As per L. 
 
 3-2 g. " " " " " " 
 
 2 rr " * 
 
 It may be again mentioned that these numbers 
 relate to liquors which were well agitated during the 
 electrolysis. 
 
 Assuming that the preceding results have been 
 confirmed by several controls, steps can then be 
 taken for the calculation of the plant. The lower 
 the current density selected, the more completely can 
 the liquor be freed of copper. The current density 
 of 1 8 amperes, tested last, is certainly not to be recom- 
 mended. It gave only 4 grams more of copper than 
 the current density 71, but to do this it required an 
 electrode surface four times as large, hence a larger 
 plant, and the efficiency also was about 5 per cent, 
 less. The middle current density, that of 36 am- 
 peres, is more favorable. This, or in round numbers 
 40 amperes, may be made the basis of the following 
 calculation, taking the pressure as 1.9 V. and the final 
 liquor as containing 4 grams of copper per liter. 
 According to these conditions, the liquor at first con- 
 
126 ELECTROCHEMICAL EXPERIMENTS. 
 
 tains 40 grams of copper per liter, and is to be 
 worked down to 4 grams of copper per liter, hence 
 for each liter or each cubic meter there should be 
 obtained 36 grams or 36 kg. of copper respectively. 
 The working of the daily production of 5 cb. m. 
 should then yield 180 kg. of copper per day. Since 
 the current efficiency is 96 per cent., there would be 
 deposited for each ampere hour, not the theoretical 
 
 i.i 8 1 grams, but only I.i8l X ~ ~ = I-I33 grams of 
 
 copper. Consequently, the precipitation of 180 kg. 
 of copper would require a quantity of electricity 
 
 180,000 
 equal to = 158870 ampere hours. As the 
 
 electrolysis demands a pressure of 1.9 V., therefore 
 the work would be 158,870 X 1.9 = = 301,853 volt- 
 ampere hours. This calculated into horse-power 
 
 hours is L = 410.1 H. P. hours. If twenty-four 
 hours are to be worked per day, the machine must 
 have - = 17.1 H. P. The efficiency of a 17 
 
 horse-power dynamo can be considered equal to 
 about 90 per cent., so that eventually the power re- 
 quired for the work will be found equal to 
 
 0.90 
 
 19.0 H. P. 
 
 In calculating the size of dynamo necessary for the 
 above purpose, it may be assumed that there is 
 a daily consumption of 301,853 volt-ampere hours. 
 
COPPER LIQUOR. I2/ 
 
 This would require in 24 hours a steady output of 
 - = 12,577 V. amperes. As I bath requires 1.9 
 
 V., the work projected could be done with a machine 
 of 1.9 V. and 6620 amperes (as 1.9 X 6620 = 12578). 
 Two baths, in series, would require 2 X 1.9 = 3.8 V., 
 
 and - = 3310 amperes. Four baths, with similar 
 
 3- 8 
 arrangement, would demand: pressure = 4.19 = 
 
 7.6 V., and current strength: = 1655 amperes, 
 
 etc. 
 
 The greater the number of baths arranged in series, 
 the greater must the pressure of the machine be, but 
 the current strength can be correspondingly low. 
 The current strengths mentioned above are too high 
 for a machine. It is better, as already mentioned on 
 p. 72, not to exceed 500 amperes. In using a current 
 of 300 amperes the corresponding pressure would be 
 
 = 41 .9 V., and with this pressure it would be pos- 
 
 4! .Q 
 
 sible to arrange - = 22 baths in series. With this 
 
 relatively simple working of baths twenty-two will not 
 be too many, and the current strength will not be too 
 high, therefore a machine of the model 42 V. AND 300 
 AMP. can be selected for the work. It is always well 
 to allow a certain latitude in such matters ; therefore 
 it is best to consider propositions for a machine 
 which, with the lowest velocity possible, will yield a 
 
128 ELECTROCHEMICAL EXPERIMENTS. 
 
 current of 300 amperes with a pressure of 42-45 V., 
 but it must still be possible to increase it, without 
 danger, to 350 amperes. 
 
 After the energy consumption for the process has 
 been calculated as 19 H. P. and the dimensions of the 
 machine are of such proportions as to give 42 V. and 
 300 amperes, the size of plant can be next considered 
 i. e., the number and size of baths and the number 
 and size of electrodes. The number of baths to be 
 
 42 
 arranged in series is = 22. As the current density 
 
 must equal 40 amperes per sq. m. there should be 
 
 = 7.C sq. m. of electrode surface in each bath. A 
 40 
 
 suitable and convenient size for the electrodes is 50 
 X 50 cm. As both sides of the electrodes are used, 
 the surface becomes 0.5 sq. m. Therefore, to get 7.5 
 sq. m. surface for each bath it will be necessary to use 
 
 - = 15 such plates. The lead plates, serving as 
 
 anodes, are of equal size; in number they are 16, or I 
 more than the kathode strips, this is because an anode 
 should be suspended opposite the reverse side of the 
 last kathode plate. The lead plates should be about 2 
 mm. thick and the copper plates 0.3 mm. in thickness. 
 It will be necessary to purchase the copper plates 
 only in the initial experiment, because as the work 
 progresses these will be produced in the process. 
 If the kathode surfaces be covered with an exceed- 
 
COPPER LIQUOR. 
 
 I2 9 
 
 ingly thin layer of fat, a little thicker on the edges, 
 they soon become coated with layers of copper, 
 which can readily be removed in sheet form. 
 
 FIG. 24. 
 
 The electrodes are hung parallel in the bath, with 
 alternating anodes and kathodes, the copper plates 
 being connected with one another, and the plates of 
 
I3O ELECTROCHEMICAL EXPERIMENTS. 
 
 lead in like manner. The current is carried to the 
 adjacent bath by the device a or b, Fig. 24. 
 
 Directions as to the manner of fastening the elec- 
 trodes, etc., will not be here given. They can be 
 found in the various volumes of Berg- u. Huttenman- 
 nischen Zeitung, as well as in Balling's Grundriss der 
 Electrometalhirgie^ Stuttgart, 1888. 
 
 Dimensions of the Bath. In the preliminary experi- 
 ments the electrodes were removed 3 cm. from each 
 other. This separation is somewhat too small. It 
 would be better to make the distance between them 
 from 4 to 5 cm., so that by the buckling of the elec- 
 trodes beneath the liquid surface short circuits may 
 not arise. It is especially at the beginning of the ex- 
 periment, when the copper sheets are still thin, that 
 this danger exists. Later, when they have become 
 thicker and firmer, it is less to be feared. When an 
 agitator is used, it may diminish short circuiting, but 
 in spite of this the separation of the electrodes 3 cm. 
 from each other can still be made the basis of cal- 
 culation. Thirty-one electrodes give 30 intermediate 
 places 90 cm., and the distance of each terminal 
 electrode from the side of the trough being 4 cm., 
 there would in all be a full length of 98 cm. The full 
 width of the trough, granting equal separation of 
 electrodes, would be 4 -f- 50 -f- 4= 58 cm. To pre- 
 vent the lead peroxide, which slowly disintegrates, 
 from becoming incorporated with the copper plate, the 
 latter should not be allowed to reach entirely to the 
 
COPPER LIQUOR. 13! 
 
 bottom of the vessel. Its distance from the bottom 
 should be 4 cm. Further, the trough must never be 
 filled to overflowing. A spare of 7 cm. should exist 
 between the edge and the liquid surface. The depth, 
 consequently, of the trough would then be 4 -f 50 
 -f- 7 = 61 cm., and the depth of liquor in it would be 
 54 cm. The full size of trough should, therefore, be 
 in length, breadth, and depth 98 X 56 X 61 cm. The 
 volume of liquor would then be 9.8 X 5.8 X 5.4 = 
 307 liters. The volume of the electrodes would 
 cause a reduction of 10 L., leaving in all 297 liters. 
 
 Should the dimensions of the bath, using the pre- 
 ceding calculation as basis, prove too large, it can 
 be divided into two, or several, smaller baths. 
 These should be arranged parallel, and the group 
 that results can then be introduced, in series, as one 
 single, large bath. 
 
 An interesting question is, In what time will the con- 
 tents of the bath be exhausted ? As 300 amperes are 
 active in the bath and 1.133 grams of copper are pre- 
 cipitated with the.observed current efficiency of 96 per 
 cent, per ampere hour, there would result an hourly 
 yield, from the bath, of 300 X 1.133 34 grams of 
 copper. A liter of liquor would yield 40 4 = 36 
 grams of copper, while the contents of the trough, con- 
 taining 297 L., would give, consequently, 36 X 297 = 
 10,692 grams of copper. The time requisite for this 
 
 would be - - = 31^ hours. At the end of this 
 340 
 
132 ELECTROCHEMICAL EXPERIMENTS. 
 
 period the entire 22 troughs would have to be re- 
 plenished. This would require 22 X 297 = 6534 liters 
 of liquor. During this period there have also been pro- 
 duced 5 X 6.563 cb.m. of liquor. The calcu- 
 
 24 
 
 lation, therefore, agrees. The working of the liquor 
 keeps pace with its production during the operation, 
 but care should be had for a receptacle, in which can 
 be collected the liquors obtained from two days' 
 operation. 
 
 The example may be amplified by calculating the 
 daily production of the plant on the basis that copper 
 is precipitated in 22 baths arranged in series, and 
 that the dynamo yields 300 amperes with 42 V. 
 Such are the statements which generally reach the 
 public, while the actual process is held as secret. 
 Let us see how, from such statements, the daily 
 production, at least, can be calculated. The pressure 
 need not be taken into account in this calculation ; it 
 is sufficient for the energy consumption. The current 
 strength and the number of baths arranged in series, 
 are, however, important. With 22 baths thus ar- 
 ranged and a current strength of 300 amperes, 300 
 amperes would be active in each of these baths, or a 
 total of 22 X 300 = 6600. It is quite immaterial 
 whether the 22 baths are simple, or whether 22 
 groups of 2, 3, or 4 baths each, in series, are worked. 
 The only difference being that with 22 simple baths 
 300 amperes fall upon a single bath, whereas with 22 
 
COPPER LIQUOR. 133 
 
 quadruple baths 300 amperes will fall to one quadruple 
 bath, a group, so that a single bath will take 
 
 = 75 amperes. But in the entire circuit there will be 
 a total of 22 X 300 = 6600 amperes doing actual work. 
 For each ampere hour there is a theoretical produc- 
 tion of 1.181 grams of copper, while the entire plant 
 would yield, consequently, 22 X 300 X 1.181 per 
 hour, and 22 X 300 X 1.181 X 24 daily, which 
 equals 187,070 grams of copper. This number must 
 yet be multiplied by the current efficiency. In the 
 preceding example it was known to be 96 per cent., 
 so that the daily production would be 179,587 = 
 
 Instead of attempting to free the liquor from cop- 
 per in a single operation, it may be worked up in 
 two stages, at first with greater current density, 
 reducing this toward the end, or the attempt may be 
 made to arrange the process for continuous work. 
 This can be effected by letting the liquor slowly 
 enter the first bath, then conducting it into the next, 
 and so on until it flows from the last bath fully 
 deprived of its copper. The first baths, containing 
 rich liquors, may have, under certain conditions, 
 greater current densities, consequently less electrode 
 surface than the end baths. These statements merely 
 aim to show the possibility of reaching the desired 
 goal by various methods. 
 
 The few grams of copper per liter, remaining in 
 
134 ELECTROCHEMICAL EXPERIMENTS. 
 
 the final liquors, are removed by hydrogen sulphide, 
 by cementation, or by any other good method. 
 
 The evolution of oxygen during the electrolysis 
 causes a disagreeable spattering of the liquor. This 
 may be much diminished by swimming a sheet of 
 oiled paper on the bath. Agitation of the liquor is 
 then out of the question, but it can be mixed by cir- 
 culation or by similar means. 
 
 6. ARRANGEMENT FOR ELECTROCHEMICAL 
 ANALYSIS. 
 
 As a rule, the pressure needed in analyses by elec- 
 trolysis does not exceed 4 V., therefore, we may use, 
 as sources of electric energy, either 4 large Daniell 
 cells, arranged in series, or a thermopile (largest 
 model with 4 V.), or two secondary batteries arranged 
 in series. The decided advantage of electrochemical 
 analysis, its extreme accuracy, as well as the cir- 
 cumstance that the work in the main is self-acting, 
 leaving the chemist free to perform other tasks, are 
 recognized on all sides. As a consequence, electroly- 
 sis is pushing its way more and more into technical 
 establishments. In these places many analyses are 
 conducted simultaneously, hence the source of the 
 energy must yield a rather powerful current and still 
 have low internal resistance. The three arrange- 
 ments previously described render this possible. 
 Meidinger cells are not well adapted for the work, as 
 
ELECTROCHEMICAL ANALYSIS. . 135 
 
 they have great internal resistance. The writer, 
 using a battery consisting of 4 large Daniell cells (22 
 cm. high), conducted simultaneously 5 copper deter- 
 minations, or two nickel estimations, and with two 
 secondary batteries made simultaneously 12 elec- 
 trolytic determinations of the most varied character. 
 To render each electrolysis independent of the rest, as 
 far as regards current strength, it is only necessary to 
 insert between each experiment and the main current 
 a suitable regulating resistance. This is most simply 
 made as follows (Fig. 25): Two conducting strips, 
 in the form of flat wire, are run from the battery, on 
 the wall, along the experiment table. One of the two 
 electrode stands is connected directly with one of the 
 metallic conducting strips.while between the other con- 
 ducting strip and the second stand-support a resistance 
 wire is inserted (see p. 117 and Fig. 23). The brass 
 clip, serving as a movable contact, is held firmly by 
 pushing a short piece of rubber over it. The author 
 uses for this purpose 1.5 m. of rheotan wire, 0.4 mm. 
 in thickness, which represents 5.5 U. If, occasion- 
 ally, greater resistance is necessary, connect the 
 stands by the conducting metal strips, not with cop- 
 per wire, but by means of a spiral of German silver. 
 By pushing the movable contact back and forth the 
 current strength can be sufficiently regulated. It 
 may be determined by inserting, during the experi- 
 ment, a measuring instrument between one of the 
 stands and the main current. The amperemeter of 
 
136 
 
 ELECTROCHEMICAL EXPERIMENTS. 
 
 Kohlrausch * (Fig. n) answers admirably for this 
 purpose. 
 
 The customary electrodes are of platinum. A few 
 
 FIG. 25. 
 
 words may be mentioned in regard to their form. 
 The electrodes, originally proposed by Luckow and 
 
 * Compare Classen, Quantitative Analyse durch Electrolyse. 3te 
 Auflage, S. 52 ; also Smith, Electrochemical Analysis, second edition. 
 
ELECTROCHEMICAL ANALYSIS. 137 
 
 later introduced into the Mansfield works, consisted 
 of a closed cylinder or cone. This was used as 
 kathode, while the anode was a platinum wire. Alex. 
 Classen, who has rendered marked service in the 
 extension of electrolysis, particularly recommends 
 (loc. cit.) platinum dishes. 
 
 The writer does not favor this idea, because the 
 dish allows of no other interruption of the analysis 
 than to siphon out the liquid contained in it, water 
 being poured in at the same time to replace that 
 which was removed. If the liquid remaining from 
 the electrolysis is to be still used in the determination 
 of other metals, nothing further remains than to 
 again concentrate the diluted liquid to a suitable 
 volume. In scientific work this is not a very disturb- 
 ing factor, but in practical operations, where the 
 question of " time " represents an expensive factor, 
 the plan can not be adopted. By using cylindrical or 
 cone-shaped electrodes the siphoning can easily be 
 avoided. The electrodes should have side slits and 
 should not be suspended from below in the stands, 
 but from above (Fig. 26). 
 
 When the analysis is finished, loosen the screw 
 with the left hand, pressing the electrode with the 
 right to the stand. Raise it vertically, taking care 
 not to touch the anode spiral. The moment the 
 edge of the beaker glass is passed, the little liquid 
 adhering to the edge is removed and the electrode 
 quickly immersed in a glass containing water. The 
 J 
 
133 
 
 ELECTROCHEMICAL EXPERIMENTS. 
 
 anode is then immediately removed. It is washed with 
 water from a wash bottle. The electrodes after washing 
 with water are dipped into alcohol and dried directly 
 over a flame. This procedure will perhaps cause a 
 slight loss in liquid, but it is so slight that in techni- 
 cal operations it need not be at all considered. The 
 loss equals about 0.5-0.8 c.cm. ; consequently, with a 
 residual liquid of 150 c.cm. it would be 0.3-0.5 per 
 
 FIG. 26. 
 
 cent, of the remaining metals. For example, in the 
 case of a sample of German silver containing 20 per 
 
 20 0.3 
 
 cent. Ni, the error would be 
 
 0.06 per 
 
 100 100 
 
 cent. Ni. In contrast with this slight inaccuracy, 
 falling within the limit of error of other methods, we 
 have the saving of time, which is very appreciable 
 and of importance. 
 
 The platinum cylinders and cones should have a 
 number of perforations, so that the lines of force 
 
ELECTROCHEMICAL ANALYSIS. 139 
 
 arising at the central anode can exert themselves on 
 the outer surface of the kathode. If this is neglected, 
 the major portion of the precipitate will be found on 
 the inner kathode surface, especially when there is 
 not much liquid between the electrodes and the glass 
 vessel. Since much depends, in the separation of 
 some metals, on the current density, perforation of the 
 kathode surface is to be recommended, so that little 
 difference will exist between the current density upon 
 the inner and outer surface. A wire is usually made 
 to serve as the anode. It is bent in the most various 
 forms. Any one in practice who would bend the 
 anode into fantastic shapes would justly arouse 
 astonishment. In scientific laboratories this is ap- 
 parently a matter of no moment, because we find it to 
 be almost universally the case that the bending of the 
 anode rarely accords with the kathode. The two 
 electrodes should be equidistant at all points, there- 
 fore a cylindrical anode is by far the most satisfactory 
 when using a cylinder. When a cone is used, the 
 bent wire should have the form of a cone-shaped 
 screw (Fig. 26). The anode wires should always 
 stand or rest on the bottom of the beaker glass, be- 
 cause the gas evolved about them will then effect the 
 mixing of the solution; consequently, the lower liquid 
 layer would stagnate did not the anode extend to the 
 bottom of the beaker. The forms of electrodes just 
 described and the manner of work just mentioned, 
 have been used and followed by the writer for years, 
 
I4O ELECTROCHEMICAL EXPERIMENTS. 
 
 and in several thousand electrolytic determinations 
 have answered excellently. They fulfill all practical 
 demands. 
 
 In communicating an electrolytic method for the 
 determination or separation of metals, the nature of 
 the solution, the approximate concentration, but espe- 
 cially the current density should be mentioned. Cylin- 
 drical or conical electrodes must be considered with 
 both surfaces. If there be references with the reverse 
 statement in literature, then introduce an ampere- 
 meter between the electrolytic stand and the main 
 current, and by means of the resistance-strip, or with 
 a rheostat, produce the current strength which has 
 been calculated from the given current density and 
 the size of the electrodes in use. In this way alone 
 is it possible to use the observations of others, with- 
 out necessitating a minute copying of the details of 
 the first author. Having once ascertained for a new 
 process how much resistance must be thrown in with 
 the arrangements at hand, in order to arrive at the 
 prescribed current density, it will not be absolutely 
 necessary, in later trials, to insert the amperemeter in 
 the circuit. It will suffice to use the resistance of 
 the amperemeter as found. 
 
TABLES. 
 
 141 
 
 G. TABLES. 
 I. 
 
 ELEMENT. 
 
 SYMBOL 
 
 AND 
 
 VALENCE. 
 
 ATOMIC 
 MASS. 
 
 QUANTITY DE- 
 POSITED PER 
 AMPERE HOUR. 
 
 Aluminium, 
 
 Al'" 
 Sb"' 
 
 As"' 
 Ba" 
 Bi'" 
 Br' 
 <M" 
 Ca" 
 Cl' 
 Cr'" 
 Co" 
 Cu" 
 Cu' 
 FT 
 Au'" 
 H' 
 If 
 Fe" 
 Pb" 
 Li' 
 Mg" 
 Mn" 
 Hg" 
 Hg> 
 Ni" 
 N'" 
 O" 
 
 p t iv 
 
 K' 
 
 Ag' 
 Na' 
 Sr" 
 S" 
 Sn" 
 Zn" 
 
 27.04 
 119.6 
 
 74-9 
 136.86 
 207.5 
 79.76 
 111.70 
 39-91 
 35-37 
 52-45 
 58.6 
 63.18 
 
 19.06 
 196.2 
 i 
 126.54 
 55-8 
 206.39 
 7.01 
 23-94 
 54-8 
 199.8 
 
 58.6 
 14.01 
 I5-9 6 
 194-34 
 3903 
 107.66 
 23.00 
 87.30 
 31-98 
 JI7-35 
 64.88 
 
 0-337 gr- 
 1.491 
 
 0-934 
 
 2-559 
 2-587 
 2-983 
 2.088 
 0.746 
 
 1-323 
 0.654 
 1.096 
 1.181 
 
 2.363 
 0.713 
 2.446 
 0.0374 
 4-732 
 1.045 
 3-859 
 0.262 
 0.448 
 1.025 
 3.736 
 7.472 
 1.096 
 
 o.i75 
 0.298 
 1.817 
 
 1-459 
 4.026 
 0.860 
 1.632 
 0.598 
 2.194 
 1.213 
 
 im 
 
 Antimony, 
 
 Arsenic, 
 Barium, 
 Bismuth, . 
 
 Bromine, . 
 
 Cadmium, 
 Calcium, 
 Chlorine, .... 
 
 Chromium, . . 
 
 Cobalt, 
 
 Copper, 
 Fluorine, . 
 
 Gold, . 
 
 Hydrogen, . 
 Iodine, 
 
 Iron, ... . .... 
 
 Lead, 
 
 Lithium, 
 
 Magnesium, 
 
 Manganese, 
 
 Mercury, . 
 
 Nickel, 
 
 Nitrogen, 
 
 Oxygen, 
 
 Platinum, 
 
 Potassium, 
 Silver, ... 
 
 Sodium, 
 Strontium, 
 
 Sulphur, 
 
 Tin, 
 
 Zinc. 
 
142 
 
 TABLES. 
 
 II. THERMOCHEMICAL, DATA. 
 
 (from JVaumann's Thermochemie.} 
 
 HYDROGEN. 
 
 
 ARSENIC. 
 
 
 (H 2 ,0) 
 
 68360 
 
 (As 2 , 0.) 
 
 154590 
 
 CHLORINE. 
 
 
 (As 2 ,O 3 , aq) 
 (As 2 , 6 ) 
 
 147040 
 219400 
 
 (Cl, H) 
 
 22OOO 
 
 (As 2 , 5 , aq) 
 
 225400 
 
 (Cl, H, aq) 
 
 39320 
 
 
 
 (C1 2 , 5 ,aq) 
 
 - 20480 
 
 POTASSIUM. 
 
 
 SULPHUR. 
 (S, O 2 ) 
 (S, 2 ,aq) 
 (S0 2 , 0) 
 (S0 2 , 0, aq) 
 (SO 2 aq, O) 
 
 71070 
 
 78770 
 32160 
 71330 
 63630 
 
 (K,0, H) 
 (K,0, H,aq) 
 (K, S, H, aq) 
 (K 3 ,0,aq) 
 
 IK, ci) 
 
 (K, Cl, aq) 
 (K 2 , O, SO 3 aq) 
 
 I 04000 
 116460 
 65100 
 164560 
 105610 
 101170 
 195850 
 
 ( O. O ) 
 
 103230 
 
 
 
 (S0 3 , aq) 
 
 39170 
 
 SODIUM. 
 
 
 (S,0 4 ,H 2 ) 
 (S, 4 ,H 2 ,aq) 
 
 192910 
 210760 
 
 (Na, O, H) 
 (Na, O, H, aq) 
 
 102030 
 111810 
 
 (S, H 2 ) 
 
 4510 
 
 (Na, S, H, aq) 
 
 60450 
 
 (S, H 2 ,aq) 
 
 9260 
 
 (Na 2 , 0, aq) 
 
 155260 
 
 IODINE. 
 (H, I) 
 (H,I,aq) 
 
 6040 
 
 (Na 2 , O, SO 3 aq) 
 (Na, O, Cl, aq) 
 (Na, Cl) 
 (Na, Cl, aq) 
 
 186640 
 83310 
 97690 
 96510 
 
 BROMINE. 
 (Br, H) 
 (Br, H, aq) 
 
 8440 
 28 3 80 
 
 CALCIUM. 
 (Ca, 0) 
 (Ca, 0, aq) 
 
 131360 
 
 149460 
 
 NITROGEN. 
 
 
 (Ca, C1 2 ) 
 
 170230 
 
 (N,H 3 ) 
 
 11890 
 
 (Ca, C1 2 , aq) 
 
 187640 
 
 (N, H 3 , aq) 
 (N 2 ,0) 
 (N,0) 
 (N 2 ,0 3 ,aq) 
 (N,0 2 ) 
 (N 2 ,0 5 ,aq) 
 
 20330 
 - 18320 
 
 21575 
 6820 
 2OO5 
 29820 
 
 STRONTIUM. 
 (Sr, 0) 
 (Sr, O, aq) 
 (Sr, C1 2 ) 
 (Sr, C1 2 , aq) 
 
 i 30980 
 157780 
 
 18455 
 195690 
 
 (N,0 3 ,H) 
 
 4I5IO 
 
 BARIUM. 
 
 
 (N, 3 ,H,aq) 
 
 49090 
 
 (Ba, O) 
 
 130380 
 
 
 
 (Ba,0, aq) 
 
 158260 
 
 PHOSPHORUS 
 
 
 (Ba, C1 2 ) 
 
 194250 
 
 (P,0 4 ,H 3 ,aq) 
 
 305290 
 
 (Ba, C1 2 , aq) 
 
 196320 
 
TABLES. 
 
 MAGNESIUM. 
 
 
 LEAD. 
 
 
 (Mg, 0) 
 
 145860 (Pb, O) 
 
 50300 
 
 (Mg, 0, H 2 0) 
 
 148960 
 
 (Pb, C1 2 ) 
 
 82770 
 
 (Mg,0 2 ,H 2 ) 
 
 217320 
 
 (Pb, Cl 2 ,aq) 
 
 75970 
 
 (Mg, C1 2 ) 
 
 151010 
 
 (Pb, 0, S0 3 aq) 
 
 73800 
 
 (Mg, Cl 2 ,aq) 
 
 186930 
 
 (Pb, 0, N 2 5 aq) 
 
 68070 
 
 (Mg, 0, S0 3 aq) 
 
 180180 
 
 (Pb, S) 
 
 20400 
 
 ALUMINIUM. 
 
 
 COPPER. 
 
 
 (A1 2 , C1 6 ) 
 
 321870 
 
 (Cu 2 , 0) 
 
 40810 
 
 (A1 2 , C1 6 , aq) 
 (A1 2 , 3 , 3S0 3 aq). 
 
 47556o 
 45 '770 
 
 (Cu, 0) 
 (Cu 2 ,Cl 2 ) 
 
 37160 
 
 65750 
 
 MANGANESE. 
 
 
 (Cu, C1 2 ) 
 (Cu, C1 2 , aq) 
 
 51630 
 62710 
 
 (Mn, C1 2 ) 
 (Mn, C1 2 , aq) 
 (Mn, O, H 2 0) 
 (Mn, 2 , H 2 0) 
 (Mn, 0, S0 3 aq) 
 
 111990 
 128000 
 94770 
 116280 
 121250 
 
 (Cu, O, SO 3 aq) 
 (Cu, O, N 2 O 5 aq) 
 (Cu 2 ,S) ' 
 CADMIUM. 
 (Cd, C1 2 ) 
 
 5596o 
 52410 
 20240 
 
 93240 
 
 ZINC. 
 
 
 (Cd,Cl 2 ,aq) 
 
 96250 
 
 (Zn, O) 
 
 85430 
 
 (Cd, O, SO 3 aq) 
 
 89500 
 
 (Zn, O, H 2 O) 
 
 82680 
 
 SILVER 
 
 
 (Zn, C1 2 ) 
 (Zn, C1 2 , aq) 
 (Zn, O, SO 3 aq) 
 (Zn, S) 
 
 N^ICKKI 
 
 97210 
 112840 
 106090 
 41989 
 
 (Ag 2 , 0) 
 (Ag, Cl) 
 (Ag, Br) 
 (Ag, I) 
 
 5900 
 29380 
 22700 
 13800 
 
 (Ni,0, H 2 0) 
 (Ni, C1 2 ) 
 (Ni,Cl 2 , aq) 
 
 60840 
 74530 
 93700 
 
 (Ag 2 , 0, N 2 5 aq) 
 (Ag 2 , 0, S0 3 aq) 
 (Ag 2 , S) 
 
 16780 
 20390 
 53io 
 
 (Ni, 0, S0 3 aq) 
 
 86950 
 
 MKRCURY. 
 
 
 OoBALX 
 
 
 (Hg 2 , 0) 
 
 42200 
 
 (Co,0, H 2 0) 
 (Co,Cl 2 ) 
 (Co, Cl,, aq) 
 (Co, 0, S0 3 aq) 
 
 6^400 
 76480 
 94820 
 88070 
 
 (Hg, 0) 
 (Hg 2 ,Cl 2 ) 
 (Hg, C1 2 ) 
 (Hg, C1 2 , aq) 
 
 30660 
 82550 
 63160 
 59860 
 
 IRON. 
 (Fe,Cl 2 ) 
 (Fe,Cl 2 , aq) 
 (Fe 2 ,Cl 6 ) 
 (Fe 2 ,Cl 6 ,aq) 
 
 82050 
 
 9995 
 192060 
 255420 
 
 TIN. 
 (Sn, C1 2 ) 
 (Sn, C1 2 , aq) 
 (Sn, C1 4 ) 
 (Sn, C1 4 , aq) 
 
 80790 
 81140 
 127240 
 157160 
 
 (2Fe, C1 2 aq, C1 2 ) 
 
 55520 
 
 GOLD. 
 
 
 (Fe, 0, S0 3 aq) 
 (Fe 2 , 3 , 3 S0 3 aq) 
 
 93200 
 224880 
 
 (Au, C1 3 ) 
 (Au, C1 3 , aq) 
 
 22820 
 27270 
 
 (Fe, S) 
 
 3554 
 
 (Au, C1 3 , HC1 aq^ 
 
 31800 
 
III. WIRE RESISTANCES. 
 
 DIAMETER. 
 
 CROSS- 
 SECTION. 
 
 RESISTANCE PER i M. OF WIRE. 
 
 Nickelin. 
 li 
 
 Rheotan. 
 O 
 
 Copper. 
 
 O.IO 
 0.15 
 O.2O 
 0.25 
 
 0.008 
 0.018 
 0.031 
 0.049 
 
 51 
 
 22 
 
 13 
 
 8 
 
 60 
 26 
 15 
 
 9-5 
 
 2.2 3 
 0.99 
 0.56 
 0.36 
 
 0.30 
 
 0-35 
 O.4O 
 
 0-45 
 0.50 
 
 0.071 
 0.096 
 0.126 
 0.159 
 0.196 
 
 5-6 
 4-1 
 3-2 
 
 2-5 
 
 2 O 
 
 6-7 
 4-9 
 3-7 
 2.9 
 
 2.4 
 
 0.247 
 0.182 
 0.139 
 O.I 10 
 
 0.089 
 
 0-55 
 
 o 60 
 0.65 
 0.70 
 
 0.75 
 
 0.238 
 0.283 
 0.332 
 0-385 
 0.442 
 
 1.68 
 1.41 
 i. 20 
 1.04 
 0.90 
 
 1.99 
 1.67 
 1.42 
 
 1-23 
 1.07 
 
 0.074 
 0.062 
 
 0.053 
 0.045 
 
 0.040 
 
 0.80 
 0.85 
 
 0.90 
 o-95 
 
 I.O 
 
 0-503 
 0.568 
 0.636 
 0.709 
 0.785 
 
 0-79 
 0.70 
 
 0.63 
 0.56 
 0.51 
 
 0.94 
 0.83 
 0.74 
 0.66 
 0.60 
 
 0.035 
 
 0.031 
 0.028 
 0.025 
 
 O.O22 
 
 .2 
 
 3 
 4 
 
 5 
 
 0.950 
 I.I31 
 
 1.328 
 
 1-539 
 1.767 
 
 0.42 
 
 0-35 
 0.30 
 0.26 
 0.23 
 
 0.50 
 0.42 
 
 0-35 
 0.31 
 0.27 
 
 0.018 
 
 0.016 
 0.013 
 
 O.OII 
 O.OIO 
 
 .6 
 
 7 
 .8 
 
 9 
 
 2.O 
 
 2.009 
 2.270 
 2-545 
 2-835 
 3-!4i 
 
 0.199 
 0.176 
 
 0-157 
 0.141 
 0.127 
 
 0.235 
 0.208 
 o.i 86 
 o. 167 
 o. 150 
 
 0.009 
 0.008 
 0.007 
 0.0062 
 0.0056 
 
 2.1 
 
 2.2 
 2-3 
 2-4 
 
 2 -5 
 
 3-464 
 3.801 
 
 4.155 
 4-524 
 4.909 
 
 o. 1 15 
 
 0.105 
 0.096 
 0.088 
 0081 
 
 0.137 
 
 0.124 
 0.114 
 0.105 
 0.096 
 
 0.0051 
 0.0046 
 0.0043 
 0.0039 
 0.0036 
 
 2.6 
 
 2.7 
 
 2.8 
 
 2.9 
 
 3-o 
 
 5-309 
 5-725 
 6.158 
 6.605 
 7.069 
 
 0.075 
 0.070 
 0.065 
 0061 
 
 ^7- 
 
 0.089 
 0.082 
 
 0.077 
 
 0.072 
 
 _ 0^)67 
 
 0.0033 
 0.0031 
 
 0.0028 
 0.0026 
 0.0025 
 
I 
 
 UNIVERSITY OF CALIFORNIA LIBRARY, 
 . BERKELEY 
 
 THIS BOOK IS DUE ON THE LAST DATE 
 STAMPED BELOW 
 
 R^VC tint returned on time are subject to a fine of 
 
 ration of loan period. 
 
 20m-l,'22 
 
/67C2