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6. Find an expression for the entropy of a perfect gas, and deduce the equation of saturation pressure for an ideal

vapour.

7. Describe one form of electrical thermometer and explain how its indications may be reduced to the absolute scale.

PRACTICAL EXAMINATION.

The time allowed for this examination is six hours.

1. Determine the vapour density of the given liquid.

2. Determine the fixed points of the given mercury thermometer and determine the calibration correction at four points between 0 and 100°.

3. Determine the specific heat of the given substance.

4. Construct a weight thermometer and determine the coefficient of expansion of the glass, being given that of mercury.

SUBJECT IX. MAGNETISM AND

ELECTRICITY.

INSTRUCTIONS.

Read the General Instructions on page 3.

Stage 1.

You are not permitted to answer more than eight questions; these may be selected from any part of the paper in this stage.

The examination in this subject lasts for three hours.

Magnetism.

1. When a magnet is plunged into iron filings, the filings cling to the ends but not to the middle. Does this mean that there is no magnetism at the middle of the magnet? State clearly the reasons for your answer.

(12)

2. What is meant by the earth's north magnetic pole? Where is it situated? How would a dip needle behave if carried from the north to the south magnetic pole, the axle of the needle being always at right angles to the magnetic meridian?

(12)

3. A bar of soft iron is laid on a table with its length in the direction of the magnetic meridian. Give a sketch of the magnetic field in its neighbourhood in the plane of the table and explain how you would investigate it with a compass needle. (13)

Frictional Electricity.

4. Find the position at which there is no resultant force due to two small spheres at a distance apart of 10 centimetres, one sphere having a charge of +16 units and the other a charge of 4 units.

(13) 5. Describe experiments to show that equal quantities of electricity of opposite sign are generated (a) by friction and (b) by induction.

(12) 6. One side of a plate air-condenser is connected to earth and the other side is charged positively and connected to an electroscope. Describe and explain the effect on the deflection of moving the plates nearer together.

(12)

7. What is meant by the term difference of potential? Illustrate your answer by reference to analogous phenomena. (12)

Voltaic Electricity.

8. What do you understand by the electromotive force and by the internal resistance of a voltaic cell?

The electromotive force of a cell A is 2 volts and its resistance 10 ohms; of a cell B, 1 volt and 1 ohm. If the two cells were connected in turn with a length of copper wire, which cell would cause the larger current to pass (a) if the wire has a resistance of 10 ohms and (b) if the wire has a resistance of ohm? Give reasons for your answer.

(13)

9. A dip needle with its axle perpendicular to the magnetic meridian dips 67°. A vertical wire carrying an upward directed current is moved round the needle at a distance of 6 inches. How will the inclination of the needle be affected when the wire is successively north, east, south, and west of the dip needle?

(13)

10. Two coils of platinum wire, A and B, are included in the same circuit, and a current is passed such that they glow visibly in the dark. When A is heated white hot in a Bunsen burner B ceases to glow, while when A is put in a freezing mixture B becomes very bright. Explain these results. (12)

11. What noticeable effects are produced (1) when a plate of zinc and a plate of copper are dipped into dilute sulphuric acid, (2) when the plates are connected by a wire ?

(12)

12. A layer of insulated copper wire is wound on a glass tube and a current is passed through the wire. Make a sketch showing the lines of force produced, and compare their form with that of the lines of force of a hollow magnetised steel tube.

(12)

but

Stage 2, Stage 3, and Honours.

You may take Stage 2, or Stage 3, or, if eligible, Honours, you must confine yourself to one of them.

The examination in this subject lasts for three hours.

Stage 2.

You are not permitted to answer more than eight questions; these may be selected from any part of the paper in this stage.

Magnetism.

1. Two bar magnets, respectively 10 and 12 centimetres long, are placed at right angles to the magnetic meridian; the first 10 centimetres north and the other 12 centimetres south of a small compass needle. Compare the moments of the magnets if the needle is not deflected.

(25)

2. A short rod of nickel is inserted in a magnetising coil through which a current may be passed.

Describe the procedure you would adopt if you wished to investigate the effect on the magnetic moment of the rod of slowly increasing the current in the coil, and state what results you would be likely to obtain.

(25)

3. In a determination of magnetic dip it is usual, in order to set the instrument in the meridian, to turn the circle until the dip is 90°. How is this position accurately obtained in the case of an instrument in which the line joining the upper and lower 90° graduations is not quite vertical ?

State briefly any other possible errors that have to be provided against in obtaining an accurate value of the dip, explaining how they may be eliminated.

Frictional Electricity.

4. Describe some form of electrostatic voltmeter.

(25)

How would you find the value, in volts, of 1° divergence

of the gold leaves of an electroscope?

(25)

5. What is meant by the electrostatic unit of capacity?

The capacity of a spherical condenser is 0033 microfarad, the diameters of the inner and outer surfaces of the dielectric being 20 and 20 5 centimetres respectively. What is the specific inductive capacity of the dielectric? (1 microfarad 900.000 electrostatic units of capacity.)

(25) 6. What do you understand by the energy of a charged conductor? Find an expression for its magnitude. (25)

Voltaic and Technical Electricity.

7. Describe carefully the Wheatstone's bridge method for comparing the resistance of two coils.

If the two coils are of nearly equal resistance, how would you arrange the experiment so as to obtain great accuracy?

(25)

8. Two wires, one of copper, the other of iron, are twisted together at one end, the other ends being connected to a suitable galvanometer. Describe and explain the indications of the galvanometer as the iron-copper junction is gradually heated to bright redness. What becomes of the heat absorbed at the junction?

(25)

Deter

9. An electric current of one ampere flows round a circular metal ring the radius of which is 10 centimetres. mine the strength and direction of the magnetic field at a point on the line drawn through the centre of the ring perpendicular to its plane and 10 centimetres distant from the plane of the ring.

(25)

10. Describe the construction of a carbon filament glow lamp. What is meant by the efficiency of such a lamp and how would you determine this quantity?

(25)

11. Describe, illustrating your answer with a diagram of the winding of the armature, a motor of about 10-horse power. If 90 per cent. of the energy supplied is turned into useful work, what current would the above motor take at 100 volts?

(25)

12. Describe the ordinary form of Morse sender and receiver, and explain how a line may be used for sending signals simultaneously in opposite directions.

(25)

Stage 3.

You are not permitted to answer more than five questions.

1. An electric current oscillates in a circuit, the frequency being of the order 106. How would you determine the strength of the current?

(60) 2. How would you determine the specific inductive capacity of a solid substance, being given a slab of the material in question? (60) 3. Show that a uniformly magnetised sphere produces the same effect at external points as that produced by a small magnet at the centre of the sphere.

(60)

4. Give a short account of how it is possible to neutralise the disturbing action of the magnetised iron and steel of a ship on a compass needle.

5. What is a secohmmeter and what is its use?

(60)

Describe in detail a method (involving the use of a condenser and a secohmmeter) of finding the self-induction of a coil.

(60)

6. Distinguish between the mean value and the root mean square value of an alternating current, and find the relation between them.

Prove that the power absorbed by a coil traversed by an alternating current is EC cos , where E and C are the root mean square values of the E.M.F. and current respectively and is the difference in phase between these two quantities.

(60)

7. Discuss the phenomena of the direct current arc in relation to the length of arc, current and P. D. between the carbons.

State the conditions under which a direct current arc can be made to emit a musical note.

(60)

Honours.

PAPER I.

You are not permitted to answer more than five questions.

Candidates who do well in Paper I. will be summoned to South Kensington or some other centre to undergo a further examination, which will consist of (1) another paper, and (2) a practical examination in the laboratory.

NOTE. No candidate will be credited with a success in this examination who has not obtained a previous success in Stage 3, or in Honours, of the same subject.

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