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II. , -




 

THE RUSTING OF METALS

 

It has long been known that metals can pass from a metallic to an earthy form. One must be careful not to expose unprotected iron to the open air, as under the influence of atmospheric conditions the outer layer of the iron quickly changes into a reddish crust called the rust. One can easily find that this new material is a different substance from iron. The rust is very brittle, it is much lighter, bulk for bulk, than iron and it is not attracted by a magnet.

It was early observed that, although the specific gravity of the earthy product was less than that of the metal, yet there was a much greater bulk of it, and that, in fact, the earthy material weighed more than the original specimen of the metal.

It was Lomonosov who first proved that the extra material came from the air. He placed some tin in a flask and sealed up the mouth of the vessel. The tin was heated and converted into the white powder. Upon weighing the apparatus it was found that no change in weight occurred. It turned out, however, that when the mouth of flask was opened, a good deal of air rushed in and the total weight was then greater. A portion of the original air had joined itself with the tin to form the powder.

Eighteen years later the same experiment was made and the same conclusion drawn from it by Lavoisier. He named the gas taken from the air, oxygen.

It is possible to confirm these conclusions in various ways.

For example, when the air is pumped out of the flask before it is sealed, the metal can be heated in the vacuum indefinitely without rusting. Thus it can be seen that fundamentally rusting is the process in which metals oxidize and form compounds.

It has been estimated that more than 25 per cent of the annual production of iron in the USA is consumed in replacing other iron that has been rendered unserviceable by rusting.

It is necessary, therefore, to wage a constant battle against this chemical process and our immediate concern is how to take care of metallic devices and structures once they enter our possession.

We try to prevent or reduce rusting by various means, such as electroplating, painting or otherwise treating the metals that are susceptible to attack by the agencies of this harmful action.

 

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1. Atomic nuclei can interact with neutrons, electrons, and gamma radiation. 2. As we should infer from the vigour with which its constituents combine, water is a very stable substance. 3. The current can flow through a wire only when the circuit is closed. 4. The magnitude of electrical current may vary from a minute amount to a very large quantity. 5. In the ammeter the current which is to be measured flows between terminals A and B. 6. Speaking about the structure of the atom one should remember that the proton is smaller but heavier than the electron. 7. Only very little current should flow through the voltmeter to operate it, and its mechanism must be very delicate and fine. 8. To tear away from the liquid the molecule which leaves it should have a large amount of kinetic energy. 9. Elastic limit is the point beyond which one should not attempt deforming the body if it is to return to its original condition. 10. The convocation of international scientific conferences, which are to be attended by scientists from different countries and belonging to different schools, can undoubtedly promote the development of science. 11. The investment in an atomic reactor must be high, but the running cost is so low that something like a large tanker can be run as cheaply on atomic energy as on conventional fuel. 12. Working with picric acid one must remember of its explosive nature. 13. Our scientists had to solve the problem of controlling chain reactions. 14. The nature of the raw material must be taken into consideration in the selection of a method of its analysis. 16. A voltmeter may be connected between any two points whose voltage difference is to be measured. 17. You ought to know the properties of the ingredients. 18. There must be a greater number of turns of wire on the field magnets to produce a magnetic field of adequate strength. 19. If water gas is to be used as a source of pure hydrogen, carbon monoxide must be removed. 20. One must remember that any reaction is theoretically reversible. 21. It we are to convert a gas into a liquid, we must increase the attraction of the molecules for one another. 22. The unit of heat should not be confused with the degree of temperature. 23. This explosive may be either a liquid or a solid.

 

II. , :

EQUILIBRIUM

 

With what can we associate the word equilibrium? We ought to know that the term applies equally well to a motor-car moving along a straight road at a constant speed. From a scientific point of view an object can be in equilibrium not only when it is standing motionless, but also when it is moving in a straight line at constant speed. On the other hand it would not be in equilibrium if it were increasing speed or slowing down or provided it were going around a corner.

Evidently, when there are no forces pulling or pushing a body, that body must always be in equilibrium. However, objects with forces acting on them may also be in equilibrium. But, then, the forces must balance; that is, their vector sum is to equal zero. Thus, in the case of the speeding automobile there are forces acting, but they balance each other in pairs. The pull of gravity downward is counteracted by the push of the road upward. The driving force supplied by the motor is balanced by air resistance and other frictional forces. However, as the driving force supplied by the motor is greater than the frictional resistance, the car is accelerated.

But sometimes an object may not be in equilibrium even though the vector sum of all the forces is zero. It may have a tendency to rotate unless all the forces are applied at a single point, or unless the tendency to rotate in one direction balances the tendency to rotate in the opposite.

Thus the second condition for equilibrium is that the tendency to rotate should be zero. There are always two requirements for equilibrium: first, the vector sum of all the forces acting on a body should be zero; and, second, there must be no tendency to rotate, that is, the tendency to rotate the body in one direction must be balanced by the tendency to rotate it in the opposite direction.

We have to point out the importance of equilibrium in any engineering design and construction. For example, if a house is to stand, each of its parts must evidently be in equilibrium.

 

 

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Measuring voltage we use voltmeters. , .

 

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The measuring of voltage is effected by means of voltmeters. .

 

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