The first cost of the magneto-electric machine is certainly greater than that of a battery neces sary to produce the same amount of power ; but the working expense is considerably less, being limited to a trifling motive power ; it appears destined to supersede every other apparatus in the deposition of metals of low equivalent proportions, as the expense of the battery in such precipita tions renders the process an expensive one. Hence the expense of depositing, compared with the value of the metal deposited, is very great ; in the magnetic machine, on the contrary, the expense of depositing is limited to the power required to produce the rotation of the armatures.
The uniformity of the current developed in the magneto-electric machine is not the least of its many advantages. In the best constant battery, the quantity varies during the course of several hours; and even good operators find it necessary to give close attention to the state of the instru ment and the progress of the deposition. With the magneto machine, the deposition goes on with extreme regularity, and, when once adjusted, may be left for any length of time without fear of derangement. So accurately is the deposition by this machine proportioned to the time of working, that, in an establishment in Birmingham where this process is extensively employed, the quantity of metal deposited is estimated by the time during which the machine works ; repeated weighings having demonstrated that the relation between the time of working and quantity of deposited metal is sufficiently accurate.
Another generator of electricity, which has been practically employed in plating, is the thermo electric pile, whioh consists of a number of couples of different kinds of metal, which are caused to generate electricity by their action upon each other, through the instrumentality of heat ; the two best kuown of these are Noes' and Clamond's. It is said that Clamond's thermoelectric pile, con suming 150 litres of gas an hour, is capable of depositing a kilogram of copper at a cost of about 2s.
Every article upon which an adherent electro-metallic deposit is required must be chemically clean, and is therefore submitted to processes, differing according to its character, which have for their object removal from the surface of every trace of oxide, grease, dirt, or other foreign sub stances. Without this preparation the coating will not adhere firmly to the receiving surface; it is usual also with articles other than those of copper to smooth, as well as cleanse the surface, by the aid of mechanical appliances. For the latter purpose the usual means of abrasion—files, emery
wheels and blocks, polishing powders, and scratch-brushes—are utilized. The latter, consisting of bundles of thin wire, applied either by hand or arranged on a spindle to be rotated, are universally used, and scratch-brushing is resorted to at all stages of deposition : a little stale beer is commonly allowed to fall on the goods whilst being scratch-brushed. Where chemical methods for cleaning can be employed, they are to be preferred, as they give very perfect results, but unfor tunately they can seldom be utilized except with copper and its various alloys; in most cases chemicals may be employed to commence the cleaning, which must nearly always be completed mechanically. Copper and its alloys, blase, German silver, and the like, are usually "dipped," that is, passed through a series of chemical baths. The first are to remove all greasy substances from the surface of the metal, as these are invariably to be found there, being acquired either during the process of manufacture, or from contact with the hands. Those objects having no soldered joints, and whose size and construction do not render them liable to injury by beat, are submitted to a temperature producing red heat, whilst more delicate articles, and also those in which it is necessary to retain both rigidity and sonorousness, are immersed in an alkaline solution, composed of caustic potash ; for this a lb. of caustic lime, and 1 lb. of pearlash, may be allowed to each gallon of water, the lime being first made into a cream by stirring with sufficient of the water, and then added to the pearlash, which has been previously dissolved and boiled.
The mixture is kept at boiling point until it is clear, and does not effervesce under the action of dilute hydrochloric acid, when it is ready for use ; hut as it ie liable to be effected by the carbonio acid contained in the air, it should be kept covered when not in use ; to keep it in condition, a little cream of lime may be added at intervals. These mixtures are usually made in iron boilers. Some times exceedingly strong solutions are required, which must then be made with stick caustic potash.