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G. The symbol for Gravity.

Gab Lever.-A lever having a hook or notch at one end, to be readily detachable.

Gage. See Gauge.

G

Gaggers, or Prods.-Blunt-pointed projections cast on core plates and loam plates to retain the loam securely. They are from in. to 1 in. long, by about in. or 3 in. in diameter at the base.

Gallon. A measure of capacity equal to 4 quarts. It contains 10 lb. of distilled water, or 277-274 cubic inches. The United States gallon is smaller, being but 231 cubic inches, the measure legalised by Queen Anne. Hence 59 imperial gallons equal about 71 U.S. gallons. One gallon also equals 4.54 litres.

The

Galloway Boiler.-A type of boiler, Fig. 63, modelled on the Lancashire, but having a special design of flues, named after the Manchester firm of that name. Messrs Galloway have made several changes in its design since the year of its introduction (1849). earlier boilers had furnaces of elliptical cross section, and the cone tubes were set vertically. These were soon discarded for the present form, with the furnace section having the top and bottom plates struck from a common centre, and the conical tubes set with their axes radially therewith. This, with the taper given, standardised the tubes, and rendered their insertion easy. See Conical Tubes. When this alteration was made, the well-known pockets A, at each side of the flue beyond the furnaces were substituted for a narrow neck previously made behind the furnace, and between it and the rows of Galloway tubes. The number of tubes has also been increased. The first boiler made had seven only. The present largest standard boiler has thirty-five, in a boiler 30 ft. x 8 ft. 6 in. Pressures as high as 200 lb. per square inch are employed. More than twelve thousand of these boilers had been constructed up to the end of 1902. The Galloway boiler has greater

evaporative capacity than the ordinary Lancashire, because the heating surface of the cone tubes lies directly in the course of the flame and hot gases.

Galloway Tubes.-See Conical Tubes. Galvanic Action. The production of electric currents by chemical means in a battery having zinc-copper elements.

If, in a vessel containing a dilute solution of water and sulphuric acid, a plate of zinc and a plate of copper are placed, upon their ends being connected by a thin copper wire, it will soon be observed that the wire becomes heated. This is caused by the passing of an electric current which is generated by the chemical action taking place in the "Galvanic Battery" or "Voltaic Cell" thus constructed, and is dissipated as heat in the wire.

During the working of the battery the zinc is consumed. The galvanic battery is thus a means of converting the element zinc into electricity, and is therefore more in the nature of a generator than the dynamo electric machine. The latter being driven by mechanical energy, supplied by an engine and boiler which utilise the potential energy of coal, the combination converts the coal to electricity through several stages, whilst the galvanic battery acts direct upon its equivalent zinc. The battery is therefore the most direct and simple, and is also the most efficient converter or generator. Considering a piece of machinery to be electrically driven, and working back in the one case through belting, motor, cables, dynamo, engine, and boiler to the coal, there will be a loss of 90 per cent. of the power actually in the fuel used, whilst in the case of the belting, motor, and a sufficient number of battery cells, the inefficiency losses will be only 30 per cent.

Thus the chemical is three times as efficient as the mechanical plant. But the cost of the zinc consumed in the battery would be at least

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Steel sheets coated with zinc to prevent corrosion. The term derived its name from the fact that zinc being electro-positive to iron, the two form a galvanic couple, the zinc being attacked first, and so preserving the iron.

In present practice the sheets, thoroughly cleaned, are first pickled in a bath of hydrochloric, or sulphuric acid diluted in water, for about twenty minutes, and at a temperature of about 90° Fahr. If the latter acid is used, the sheets are subsequently soaked in a tank of cold water, which is not necessary if hydrochloric is employed. They are then transferred to the bath of molten zinc-spelter-which is maintained at a temperature of 20° or more above the melting point. The regulation of temperature is most important, as the process may be delayed by too low, or the zinc oxidised by too high a temperature.

The bath-of steel plate-is surrounded by, and rests above a brick-work body, within which a coke fire is kept burning, with provision for regulating the temperature.

The flux used is sal-ammoniac, placed on the surface of the molten metal, within partitions, one at each end. The sheets are guided by rolls in the bath, down into it at one end, through the flux horizontally along, and up out at the other end. The sheet must attain the same temperature as the bath before the zinc will adhere, so that the time of its transmission has to be varied with the thickness. The sheets are then dropped into a tank of water, followed by immersion in a second tank. The difference in the dull and the crystallised appearance of sheets is due to the immediate immersion, or to delay in the dipping respectively. The dipped hets are then rubbed over with sawdust, and

dried by coke fires, or in ovens. 1 lb. of zinc will coat a surface of 15 sq. ft.

Castings and forgings are frequently galvanised. The same treatment is adopted, except that the articles must be dipped singly instead of being passed through by rolls. If very small, a number can be dipped at once in wire baskets. The cleaner and smoother the castings or forgings, the better will they take the zinc.

Galvanometer.—An electrical instrument for indicating the presence, and measuring the quantity and pressure of electric currents.

A simple form of galvanometer is shown in the diagram, Fig. 64, which illustrates the

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Fig. 64. Galvanometer.

action of the instrument. The coil c is wound with insulated copper wire. In some instruments a double winding is used, one part indicating quantity, the other intensity of electricity passing. The armature A is pivoted to balance the indicating needle N at zero when the circuit is open. When a current from the battery B or other source flows through the coil, the armature is attracted at one end, and repulsed at the other by the north and south poles in the magnetic field created within the hollow coil, with the result that the needle is deflected over the scale. If the current is so applied as to produce a north pole at the righthand end of the armature, the needle is deflected to the right, whilst if the flow is reversed it is deflected to the left hand. Thus the direction of current in any circuit may be tested after

having once determined which end of the winding gives a right-hand deflection when connected to the terminal of a testing battery. As the tangent of the angle of deflection is proportionate to the strength of the field acting on the balanced armature, the scale may be calibrated, and the ampères passing deduced from the value of ampère-turns.

The turns being fixed, the deflection varies according to the different field strengths produced by a variation of ampères passing in the "Ampère Winding" of low resistance. If the high resistance winding be connected as a shunt (shown by dotted lines a, b) and the other winding disconnected at the terminal c, the E.M.F. may be measured.

For by Ohm's Law

E

R

=

= C, and R being constant, C will vary as E; E may therefore be deduced by the current indicated, and the instrument be calibrated as a voltmeter. Thus, referring again to the diagram, the direction, quantity, and intensity of the electricity supplied by the battery to the resistance x (which may be any electrical apparatus) can be indicated and measured.

A more delicately adjusted galvanometer has its moving part suspended by an extremely fine silk thread-this is called a "Suspensory Galvanometer." In this an astatic pair of magnetic needles is used, that is, two magnetised needles of equal strength and size are secured one over another in reverse positions as regards their polarity, so that the force which urges one to set itself in the magnetic meridian is counterbalanced by the force which acts on the other. Such a pair being independent of the earth's magnetism will remain in any position in which they are set, and will be readily deflected by a current flowing through a wire coiled round one of the needles. In another type the moving part carries a screen which reflects from a mirror a beam of light upon the scale-this is the "Mirror Galvanometer." Both these latter types are extremely sensitive and delicate instruments suitable only for laboratory work.

Gang Mill. A term applied to a gang or set of milling cutters all arranged on one arbor, to tool intricate or irregular surfaces. In such cases the work could not be done with a single

cutter, either owing to the difficulty or impossibility of making it.

Gang Saw. See Log Frame Saw.

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Ganister. A highly silicious lining used for Bessemer converters, furnaces, steel moulds, &c., and containing from 92 to 95 per cent. of silica.

Gantry. The elevated strutted framing, of timber or steel, supporting the rails upon which an overhead traveller runs.

Gantry Crane.-An overhead crane which has a Goliath type of travelling framing, but carries a jib crane instead of a crab. The crane is generally of fixed type, pivoted in a bed at one end, or at the centre of the gantry; but travelling cranes are also employed, running along the gantry beams on rails. The term generally denotes a crane mounted on a gantry of considerable length, corresponding to that of a Goliath. When short, the term portal crane is commonly applied. Gantry cranes are rarely operated by hand, and only in some few cases hydraulically. They are chiefly of steam, and electric types, the latter being the newer agency.

The value of the gantry crane lies chiefly in docks and harbours, for loading and unloading vessels. The tall gantry spanning the lines of rails does not interfere with the movements of the trucks. They are travelling cranes, and they are travelled generally by similar devices to the Goliaths, the framings of which they closely resemble. See Goliath Crane.

Fig. 65, Plate IV., illustrates a 4-ton steam travelling gantry crane of 7 ft. gauge, and of 19 ft. radius. The span of the gantry is 39 ft. to the centres of rails, the height of lift is 25 ft. The head room to the underside of the gantry girders is 13 ft. The speeds are: lifting, 80 ft. per minute: the crane traverses at a rate of 120 ft. per minute; and the travel of the gantry is 80 ft. per minute. The travel is by means of bevel gears, one set of which is seen at the left hand of the crane.

Gap Gauge-See Caliper Gauge.

Gap Lathe.-The value of this old type of lathe, Fig. 66, Plate V., has been somewhat discounted by the increasing specialisation of tasks in m n shops. The face lathes have taken muc its work, and work between often appropriated by lathes the general shop it remains

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Fig. 72.-360 B.H.P. VERTICAL GAS ENGINE, DIRECT COUPLED TO ALTERNATOR. (Campbell Gas Engine Co., Ltd.)

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Fig. 75.-GAS EXHAUSTER, DRIVEN BY STEAM ENGINE. (The Bryan Donkin Co., Ltd.)

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