There are a few observations to be made respecting electrolysis. One rule is, that it “never takes place unless the electrolyte is in a liquid state.” The liquid state is essential. It is also observed that the components go to the different electrodes; such elements as go to the negative electrode are termed electro-positive, the others electro-negative; or, as Faraday termed them, “anions” or “kations:” The chemical power or electrolytic action of the current is the same at all parts of the circuit; the quantity of the substance decomposed is in exact proportion to the strength of the current. Faraday measured the strength of the electric current, and invented for the purpose an instrument called the Voltameter. We have mentioned the Galvanometer more than once, and will proceed to describe it. There are several forms of this instrument: the Tangent, the Marine, and the Reflecting Galvanometers, and the Astatic, or “Multiplier.” In the first-named the direction of the current is determined by Ampère’s rule, which is as follows:—
“Imagine an observer placed in the wire so that the current shall pass through him from his feet to his head; let him turn his face to the needle: its north pole is always deflected to his left side.”
The “Tangent” Galvanometer consists of a vertical circle like an upright ring, across which is a support in the centre holding a copper wire, through which the electric current passes. On this point (where the wire is) a needle is very lightly supported, and when the instrument is to be used it is placed so that the plane of the circle is parallel to the line in which the needle points. The current passes, and the needle is deviated. By noting which side the north end of the needle goes the direction of the current is ascertained, and the length of the needle being small in comparison with the diameter of the circle through which the current passes, the strength of the current in the vertical circle is in proportion to the tangent of the angle through which the needle turns. Hence the term “Tangent” Galvanometer.
The “Reflecting” instrument is the invention of Sir William Thomson, in which a mirror is attached to the needle, and reflects a small focus of light upon a scale. The movements, however minute, are easily read. Sir W. Thomson’s Galvanometers are extremely sensitive. We need not mention any other varieties, as full descriptions can easily be obtained. We only need to indicate the mode of working.
Fig. 231.—Galvanometer.
The accompanying illustration (fig. 231) shows an Astatic Galvanometer which may be used in two ways—either to measure strength of current, or to find out a current; in the latter case it would be termed a Galvanoscope. It is a compound needle instrument, and consists of two needles placed in parallel directions with opposite poles above each other in a coil. The wire coil is wound round a bobbin, and the astatic needle is placed therein and suspended freely, as in the illustration, by a cocoon thread. The upper needle moves upon a scale, O O, and the instrument is enclosed in a glass shade. The screw, V, communicates with the upper needle, and fixes it at zero point when ready for use. The wires are fastened to the binding-screws, and the current is then sent. The needle is deflected accordingly, and the number of degrees on the scale can be read off.
The uses of the galvanic current are many. Amongst them Electroplating is perhaps the most generally useful, though Electrotyping is also a very important process in art. A visitor to Birmingham may view the process carried on there by some enterprising firms, who have succeeded wonderfully in producing electro-plate. The principle is very simple and easy to understand, but the greatest care and watchfulness are required on the part of the men employed. The principle, as we have said, is simple, and consists in the fact that if a plate of metal be suspended and attached to the positive pole of a galvanic battery and immersed in a solution of the same metal, the conducting substance hung opposite at the negative pole becomes coated with the metal immersed in the solution.
Fig. 232.—Trough for electro-deposition.