This is in the form of a powder which contains a combination of fluxes and finely powdered tin. It is non-acid, and will tin any metal except aluminium. It avoids the need of cleaning any corroded, rusty, greasy or dirty surface before the operation of tinning. No previous preparation of the surface is necessary, all that is required is that the metal article be heated in any convenient manner to somewhat above the ordinary soldering temperature, and some of the compound be then sprinkled over the part to be soldered. A rag or cloth is then used to wipe off the flux residue, and any dirt that was on the surface, a well tinned surface will then be disclosed. Ordinarv sol-dering can then proceed. An ordinary iron poker will make an effective soldering iron by heating it and rolling it in some of the compound, which will well " tin " it, an ordinary soldering iron can be tinned at once by heating it, and without cleaning, dipping into the compound, and wiping with a cloth.

To tin cast-iron, heat the metal hot enough to melt the powder, and work in with a soldering iron, it may be found necessary to wipe away the residue thrown off, and apply a little more compound, which is sold under the name of " Soldo." It is claimed that metals tinned by "Soldo " will resist rust and corrosion. The National Physical Laboratory report states that "effective tinning of severely rusted steel and rusted cast-iron has been accomplished by means of ' Soldo' the rust being removed, and interpenetration between tin and steel or iron obtained, illustrating in a marked manner the cleaning properties of the material." There are many repair jobs where the use of such a compound should be of real value.

Springs.

These in most frequent use are generally made of metal, and are helical in shape (sometimes called spiral) and made of circular section wire. When the stresses are within the safe limits of the metal, the extension or the compression of the spring requires equal force to produce such movement per unit of length. Hence a helical spring used to determine the weight of parcels, etc., will extend an equal distance or amount per each additional one lb. weight supported, and this will continue up to the safe limits of the metal. Should a helical spring be desired to do certain work, but the correct size and length be unknown, it may be arrived at from the following considerations-the work done in either stretching or compressing such a spring up to its safe limit will varv

(1) ;As the square of the stress in the metal.

(2) ;As the square of the diameter of the wire.

(3) ;Directly as the number of turns.

(4) ;Directly as the mean diameter of the coil.

If, therefore, any spring of approximately the correct size is available to test on the work, it is easy to see from its behaviour how to modify it to obtain the correct dimensions.

Should it be desired or necessary to use a wire of a certain diameter, it should be remembered that

(1) ;Increasing the mean diameter of the coil will increase the flexibility of the spring.

(2) ;Decreasing the mean diameter of the coil will increase its stiffness.

(3) ;Decreasing the number of turns will increase the stiffness.

To join a bolt, or hook, or a form of eye to a helical spring is best done by selecting such bolt, etc., with a screwed end which will lightly screw into the coiled spring. To secure it still more firmly, a tube preferably of metal should be slipped over the coil end, and held there, so preventing the screwed bolt pulling out, although it might still be possible to screw it out.

Springs of rubber are used sometimes, generally for absorbing shock as in some railway waggon buffers, carriage spring buffers, etc. As rubber is not compressible, space must be allowed for the rubber when under compression to expand in a direction at right angles to the line of compression. A piece of rubber extended to, say twice its length, will be seen to reduce in cross section, the volume of the rubber being (usually) not altered : it has merely altered its shape.

Many have the opinion that rubber, or indiarubber, when pressed or stretched appreciably, alters its volume, that is, it gets smaller or bigger, but this is not the case. In consequence of this alteration of shape, a spring of rubber is not so " lively " as a metal spring, i.e., if equal weights be suspended on the end of a helical spring and also on the end of a suitable size piece of rubber, and the weights be set moving up and down vertically, then the one on the rubber will come to rest well before the one on the metal spring, showing that the alteration of shape in the rubber sets up internal friction as it were, and soon brings it to rest.

If it is required to maintain a weight or object vibrating vertically due to any impulse for as long a time as possible, the best way to suspend it is on to the centre of a flat coiled spring (such as a clock or watch spring), the coiled spring being held in a horizontal position. Such a spring may require too much space, in which case the next best method is to use a flat strip of spring material, set it horizontally when loaded with the object, and fix the other end firmly. The strip must be wide compared with its thickness, and the width must be in a horizontal position.

Energy Stored In Springs

One frequently hears the view expressed that energy can be stored up in a spring for use at a later date, quite regardless of the fact that such stored energy may put a spring of any practicable size into a state of stress far beyond its limits of resistance. All work stored in any spring puts the spring into a stressed condition and assuming that it is a helical (commonly called spiral) spring in question, it is possible to define the amount of energy or work that can safely be stored in it, and this can be given in terms of the height to which the stored energy would be capable of lifting the spring, while not working over the safe limit of stress in the spring metal. Naturally the amount of the stored energy will vary depending upon what metal, brass or steel, etc., is used in the spring, and also whether the metal has a round or square section, also the stress in lbs. per square inch that the metal is subjected to. Prof. Goodman has calculated out and given the following figures for a spring of circular section steel at different stress values, and showing the height to which the stored energy could lift the spring, this height being given in fact Stress 30,000 60,000 90,000 h (ft.) 5-56 224 50-3

The figures for a square section steel wire are :-

Stress 30,000 60,000 90,000 h (ft.) 3-17 12-7 28-6 The above figures show clearly that the amount of energy that can be stored in a metal spring is really very small, and also show that a circular section spring is very much more economical than a square section spring. • Experiments by Mr. Wilson Hartncll are stated bv Prof. Good-man to show that for steel wire the following are the maxima consistent with safety :- W ire diameter. ;Safe stress.

1/4inch 70,000 lb. per sq. inch 8 " 60,000 „ „ „ 1/2 " 50,000 , „ ,. It is obvious that while the commercial use of metal springs may be possible in watches, clocks, air guns, gramophone motors, toys, etc., it can never be of real commercial value for heavy duty work, so far as the question of storage of energy goes.