compared to the external resistance, the gain of effect from enlarging the battery plates is but small. To determine the relative value of R, as compared with r, a battery was constructed so as to collect and measure the gas evolved by its action. The plates were placed in contact with each other, and the gas evolved in thirty minutes taken as a unit of effect. As in this case the current did not pass through anything but the battery, there is no resistance to be represented by r, or r in the formula will be equal to 0 E and Q=R=1. The battery was then attached to a pair of electrodes, in a certain solution of sulphate of copper and sulphuric acid, especially recommended by all the writers on electro-metallurgy, the arrangement being such as to produce good metal. The gas now evolved in thirty minutes was found only one-twentieth of the former amount; hence the introduction of the resistance, r, had diminished Q twenty times, and 1 E whence r is equal to 19 R. E R+r = Q 20 R, battery enlargement, we now have Q= 1 +19. To exhibit the effect of If m=1, then Q=.05; if m=2, Q=.0512; if m=3, Q=.0518; if m=4, Q=.0524, &c., &c. This shows a gain of only a fortieth from doubling the size of the battery, &c.—an advantage too small to repay for the enlargement. These calculations are in accordance with experimental results from small batteries, but in large ones the necessity of further separating the plates, in increasing their size, makes the resistance increase, instead of diminish, and there is consequently a loss from enlargement. It is not, therefore, by merely increasing the battery surface that the time for electrotyping can be shortened. Mr. Smee, the distinguished writer on electro-metallurgy, by covering the negative plate of the battery with pulverulent platinum, produced a very energetic form of the instrument. When the plate is freshly platinized, it acts violently, and throws off the hydrogen in torrents. But this increased energy of the plate is gradually lost, from the electrive current depositing upon it impurities from the zinc. As this deposite has a strong attraction for the hydrogen, it is retained on the plate. The plate, being thus encased in air, is virtually excluded from the liquid of the battery. The ordinary solvents of the metals do not readily remove this coating of impurity. The plate can be renewed by replatinization; but, as this is both tedious and expensive, I was urged to find a menstruum which would restore the original platinum to its energy. This I attained, at length, by immersing the plate in a solution of per-chloride of iron, which almost immediately restores the action of the plate. The plates are now daily immersed in the chloride of iron, by which the tone of the battery is constantly maintained. By this last discovery, together with obtaining better solutions for the decomposing cell, the time for making a casting was reduced; but still the time required for making a plate was too long when only one electrical equivalent was employed. The effective force of one battery may be added to another. This is increasing E in the formula, and this will sometimes increase Q. We unite the effective force of many batteries by joining their dissimilar ends in consecutive order. As the current in such an arrangement has to traverse every battery in the chain, R will be multiplied as many times as we multiply E. The formula then becomes Q = n E n R+r When the value of r and R are nearly equal, and we have batteries of definite construction to work with, it becomes a matter of some importance to determine whether we shall use the whole galvanic apparatus, as a single electrical equivalent, by connecting all the similar parts of all the battery cells, or whether we shall convert it into a battery of two pairs, in consecutive order, by joining dissimilar ends. As doubling the battery is doubling R, and to double the electrical equivalents is also to double R, we shall increase R fourfold by the double arrangement. E Instead of Q = R+r Q = .50 in the single arrangement, and Q = .40 in the double-showing that we may double the expense, and yet make the casting more slowly than before. Conditions as above are of frequent occurrence, and a knowledge of them without experimenting is of very great importance. we have Q = 2 E 4 R+r Taking R=r we have For R 10r, with a single equivalent of battery, Q= For two batteries in series Q = 2 0.166. = The use of two batte ries in consecutive order, as thus exhibited, doubles the expense, but does not double the effect. A regard for economy prohibits us from further E increasing the series. To represent an effect double of we have R+r surface, we may now make Q = .183. The gain per cent., now indicated by doubling the surface, makes it advantageous to make this increase when two consecutive batteries are used. The difficulty of obtaining large flat plates of silver proved a serious obstacle in effecting an increase of battery surface, for the irregularity of the surface requires the plate to be placed at an increased distance from the zinc, thereby augmenting R, the very thing sought to be diminished. Plates could be made flat by the planishing hammer; but the operation being expensive, and the plates continually liable to accidents in use, economy prohibited this mode of forming flat plates. Though the plating of metallic bodies with silver had been well executed, it had not yet been determined that electro-casting of silver could be executed in a desirable manner, and at a moderate expense and trouble. At first, every attempt to make plates weighing 2,500 grains to the square foot failed, on account of the impossibility of observing Mr. Smee's laws relative to E for the time required. But after modifying the solutions of silver, and using a register battery, a plate could be made in thirty hours, perfectly flat, and possessing the mechanical qualities of hardness, elasticity, and malleability, in an eminent degree, and not costing over 16 cents per ounce for the making. The perfectly flat plates admit of a very close approximation to the zincs. Their size may therefore be increased to more than twice their former surface. As in the double arrangement, r is relatively smaller to R. Important changes have also been made in the modes of operating, and in the arrangement of the apparatus. It had early been noticed that changes of temperature influenced the rate of working; and every electro-metallurgist knows the importance of keeping the laboratory warm. To determine where and how the effect of temperature took place, a battery, at 60 degrees of Fahrenheit, was connected with a wire 120 feet long, and enclosing a galvanometer. The deflection was 40 degrees; the battery was then cooled until the temperature was 48°; the needle was still deflected nearly 40 degrees. This experiment indicated that the batteries were not greatly affected by ordinary variations of temperature. Advantage was then taken of this development to secure a more perfect ventilation. Accordingly, a small room, to contain the battery, was partitioned off from the general apartment by a glass partition, and large outward openings made at the top and at the bottom of the room, to give a circulation of air for carrying off the battery fumes. At the stage of improvement now described, one of our medium plates, having 8 square feet of surface, could be readily made in from 8 to 10 days. But wishing to still further quicken the process, or attain my first desire to deposite one pound per day on the square foot, with a single equivalent of battery-improvements were again sought after. As the E of the formula has been increased to the greatest extent the cost would permit, and r had been diminished, or the plates increased in size to the greatest useful extent, it was sought to increase Q by diminishing r, or the electrolytic resistance. It was sought to increase the conducting power of the electrolyte by adding easily decomposable salts to it; but with no success. The accelerating effect of temperature being found, as above stated, to be confined chiefly to the decomposition cell, it was evident that by using the electrolyte alone, at a high temperature, a considerable advantage might ensue. To determine the most advantageous working temperature, and the resulting gain of effect, a voltameter battery was connected to a pair of electrodes, in the solution formerly described as being generally recommended. Each electrode had five square inches of face, and was coated on the back to prevent radiation. They were placed one inch apart, and had thin plates of wood bound against their edges, to prevent any lateral spread of the current in passing between them. The following was then obtained: do. Battery plate in contact gave 300 cubic inches gas per hour. Do. Do. Do. 23.15 do. 60° do. 20 do. do. 18.15 The last column of figures shows the value of the resistance of the solution, as compared with R of the formula. This column was obtained by first uniting the battery plates, and afterwards the electrodes. From the above table it appears that heat may be made to diminish the resistance in the decomposition cell in the proportion of 2.58 to 1; and the whole resistance by 2.25. And as 2 E R+r E =R+r; therefore, by 2 heating the electrolyte, we may with a single electrical equivalent make a plate as rapidly as by working at atmospheric temperatures with two batteries in consecutive order, with double surfaces, (four times the battery and twice the expense.) But as Smee's laws require that, in forming a plate, certain mutual conditions of apparatus be maintained, it follows that alterations in one element or condition must be attended by corresponding changes in the others. Hence, if the temperature of the electrolyte be raised to a certain point, and the apparatus correspondingly adjusted, it is evident that, to avoid incessant adjustment, the original temperature must be maintained. Thus, to avail ourselves of the advantages experimentally found from heating the solutions, an apparatus for steadily maintaining a high temperature in the electrolyte through several successive days becomes indispensable. As the electrotype operations are not suspended at night, it is important that the heating apparatus should perform its office for at least twelve hours without supervision or replenishing its fuel; and its action should be sensibly uniform, during all the time, between successive replenishings. Such an apparatus I have devised, and is now in use. A peck of charcoal furnishes fuel for twelve hours, and maintains 100 gallons of copper solutions steadily, at any required point between 100° and 200°. With the above arrangement in use, I have made a large reverse or alto, and returned the original to the engraving department, in 55 hours from its being placed in my hands. This time included trimming the edges and the preparations to prevent adhesion. Again recurring to Ohm's formula, the relative value of R to r was once more experimentally found. This gave Rr :: 1:4 or Q: =0.20, a great improvement as compared with the first deter 1 1+4 mination of Rr::1: 19, or Q small compared with R, the size until the result is about 0.24. 1 1+19 =0.05. Having now made r so of the battery can be profitably increased Moreover, using a double arrangement of cells with double surfaces, for a double effect, we now have 2 (1+4)= 2 =0.40. As the relative resistance of the electrolyte becomes now still smaller, we may yet more increase the battery surface until the result is nearly 0.5. The electrotype has now ceased to be a mere experiment, uncertain, expensive, and slow. I have lately formed plates of most excellent quality, at the rate of 3 lbs. to the square foot, in 24 hours. This rate will require but two days to form one of our largest plates, having ten square feet surface, and one-eighth of an inch thick. Actions in the electrolytic solution. The quantity of the deposited metal is governed solely by the relations between the quantity of electricity passing through any solution and the amount of metal the solution contains. The usual supposition is, that the acid of the salt goes to one electrode and the metal to the other. It is now ascertained that no such mutual transfer takes place; for, while the acid is carried to the positive electrode, the metal is not carried to the negative electrode. Hence, however strong the solution on commencing the process, the negative electrode, by abstracting the metal in its vicinity, is soon surrounded with a weak solution. With a simple wire electrode, the exhausted solution surrounding the electrode is readily renewed by mere difference of specific gravity producing a flow. But, with large parallel plate electrodes, this rapid renewal of dense solution becomes impossible, and the electrode is soon surrounded with a weak solution. This state of things must be recognised in adjusting our battery arrangements. Electrotypists not aware of this fact find themselves much plexed by failing to accomplish with large plates what is so easily done with medals or small plates. per It would, at first sight, appear that, by strengthening the solution of sulphate of copper, a more rapid supply of metal to the electrode would be obtained. Unfortunately, the effect of this is to diminish the solvent capacity of the water in the solution for the sulphate formed on the positive electrode by the action of the transferred acid. The grand essential in electrolysis is liquidity in the solution. Thus, if the quantity of free water surrounding the positive electrode be small, this electrode is soon enveloped in a saturated solution, and the newly-formed salt remains undissolved upon it. This salt, being a non-conductor, virtually excludes the electrode from the solution, and thus arrests the current, except when the efflux of saturated solution permits the salt to dissolve, and so reopens the passage for the current in irregular quantities. From this spasmodic action result plates of copper-sand, or sometimes copper as soft as lead. By applying heat to the solution when this state of things exists, the solvent capacity of the water for the salt is increased, rapid diffusion takes place, the salt is carried to the negative electrode, and the exhausted water to the positive electrode; the dormant batteries rush into uninterrupted action, and in a short time a plate is deposited, having all the hardness and elasticity of hammered or rolled copper. Smee's conditions, then, seem to maintain themselves. The electrotypists' |