meters devised by Sir. W. Thomson for signalling through submarine cables, are admirable examples of this class In Fig. 91 the general arrangements of

of instrument.

this instrument are shown. meter is supported on three be adjusted. The magnet

The body of the galvanoscrew feet by which it can consists of one or more

small pieces of steel watch-spring attached to the back

of a light concave silvered glass mirror about as large as a threepenny piece. This mirror is hung by a single fibre of cocoon silk within the coil, and a curved magnet, which serves to counteract the magnetism of the earth, or to direct the needle, is carried upon a vertical support above. Opposite the galvanometer is placed the scale. A beam of light from a paraffin lamp passes through a narrow aperture under the scale and falls on the mirror, which reflects it back on to the scale. The mirror is slightly concave, and gives a well defined spot of light if the scale is adjusted to suit the focus of the

mirror.1 The adjusting magnet enables the operator to bring the reflected spot of light to the zero point at the middle of the scale. The feeblest current passing through the galvanometer will cause the spot of light to shift to right or left. The tiny current generated by dipping into a drop of salt water the tip of a brass pin and a steel needle (connected by wires to the terminals of the galvanometer) will send the spot of light swinging right across the scale. movement of the persons at once.

If a powerful lime-light is used, the needle can be shown to a thousand For still more delicate work an astatic pair of needles can be used, each being surrounded by its coil, and having the mirror rigidly attached to one of the needles.

Strong currents must not be passed through very sensitive galvanometers, for, even if they are not spoiled, the deflections of the needle will be too large to give accurate measurements. In such cases the galvanometer is used with a shunt, or coil of wire arranged so that the greater part of the current shall flow through it, and pass the galvanometer by, only a small portion of the current actually traversing the coils of the instrument. The resistance of the shunt must bear a known ratio to the resistance of the instrument, according to the principle laid down in Art. 353 about branched circuits.

203. Differential Galvanometer.-For the purpose of comparing two currents a galvanometer is sometimes employed, in which the coil consists of two separate wires wound side by side. If two equal currents are sent in opposite directions through these wires, the needle will not move. If the currents are, however, unequal, then the needle will be moved by the stronger

1 As concave mirrors are expensive, a plain mirror behind a lens of suitable focus may be substituted. The thin discs of glass used in mounting objects for the microscope form, when silvered, excellent light mirrors. Where great accuracy is desired a fine wire is placed in the aperture traversed by the beam of light, and the image of this appears when focused on the screen as a dark line crossing the spot of light.

of them, with an intensity corresponding to the difference of the strengths of the two currents

204. Ballistic Galvanometer.-In order to measure the strength of currents which last only a very short time, galvanometers are employed in which the needle takes a relatively long time to swing. This is the case with long or heavy needles; or the needles may be weighted by enclosing them in leaden cases. As the needle swings slowly round, it adds up, as it were, the varying impulses received during the passage of a transient current. The sine of half the angle of the first swing is proportional to the quantity of electricity that has flowed through the coil. The charge of a condenser may thus be measured by discharging it through a ballistic galvanometer.

LESSON XVIII.-Chemical Actions of the Current :

Voltameters.

205. In addition to the chemical actions inside the cells of the battery, which always accompany the produc tion of a current, there are also chemical actions produced outside the battery when the current is caused to pass through certain liquids. Liquids may be divided into three classes (1) those which do not conduct at all, such as turpentine and many oils, particularly petroleum; (2) those which conduct without decomposition, viz. mercury and other molten metals, which conduct just as solid metals do; (3) those which are decomposed when they conduct a current, viz. the dilute acids, solutions of metallic salts, and certain fused solid compounds.

206. Decomposition of Water.-In the year 1800 Carlisle and Nicholson discovered that the voltaic current could be passed through water, and that in passing through it decomposed a portion of the liquid into its constituent gases. These gases appeared in bubbles on the ends of the wires which led the current into and out of the liquid; bubbles of oxygen gas appearing at the point

where the current entered the liquid, and hydrogen bubbles where it left the liquid. It was soon found that a great many other liquids, particularly dilute acids and solutions of metallic salts, could be similarly decomposed by passing a current through them.

207. Electrolysis. To this process of decomposing a liquid by means of an electric current Faraday gave the name of electrolysis (¿.e. electric analysis); and those substances which are capable of being thus decomposed or "electrolysed" he termed electrolytes.

The ends of the wires leading from and to the battery are called electrodes; and to distinguish them, that by which the current enters is called the anode, that by which it leaves the kathode. The vessel in which a liquid is placed for electrolysis is termed an electrolytic cell.

208. Electrolysis of Water.-Returning to the decomposition of water, we may remark that perfectly pure water appears not to conduct, but its resistance is greatly reduced by the addition of a few drops of sulphuric or of hydrochloric acid. The apparatus shown in Fig. 92 is suitable for this purpose. Here a battery of two cells (those shown are circular Bunsen's batteries) is seen with its poles connected to two strips of metallic platinum as electrodes, which project up into a vessel containing the acidulated water. Two tubes closed at one end, which have been previously filled with water and inverted, receive the gases evolved at the electrodes. Platinum is preferred to other metals such as copper or iron for electrodes, since it is less oxidisable and resists every acid. It is found that there is almost exactly twice as much hydrogen gas (by volume) evolved at the kathode as there is of oxygen at the anode. This fact corresponds with the known chemical composition of water, which is produced by combining together these two gases in the proportion of two volumes of the former to one of the latter. The proportions of gases evolved, however, are not exactly two to one, for at first a

very small quantity of the hydrogen is absorbed or "occluded" by the platinum surface, while a more considerable proportion of the oxygen-about 1 per cent

is given off in the denser allotropic form of ozone, which occupies less space and is also slightly soluble in the water. When a sufficient amount of the gases has been evolved and collected they may be tested; the hydrogen by showing that it will burn, the oxygen by its causing a glowing spark on the end of a splinter of wood to burst into flame. If the two gases are collected together in a common receiver, the mixed gas will be found to possess the well known explosive property of mixed hydrogen and oxygen gases. The chemical decomposition is ex

pressed in the following equation:

209. Electrolysis of Sulphate of Copper.-We will take as another case the electrolysis of a solution of the well-known "blue vitriol" or sulphate of copper.

If