Report on the Examinations in Magnetism and

Electricity.

EVENING EXAMINATION.

STAGE 1.

Results 1st Class, 709; 2nd Class, 708; Failed, 793; Total, 2,210.

This paper was, on the whole, rather better done than the corresponding paper last year, and this remark applies to each of the three divisions of the paper. Comments on the answers to individual questions are given below.

Magnetism.

Q. 1. A wooden ball contains a bar magnet imbedded so that the axis of the magnet lies along a diameter, but the ends do not reach the surface. Explain carefully how you would mark on the surface of the ball the points where the axis of the magnet prolonged would cut the surface.

Few of the answers were more than partially correct. It was a common mistake to float the ball in water, and then to assume the magnet to lie N. and S., and to be horizontal. Various tests (by compass, dip needle, bar magnet, etc.) were suggested, which would only give correct results if the earth's field could be neglected.

Q. 2. What is meant by the statement that the declination at a place is 18° west? At such a place how must a boat be steered by compass so that its course may be due east?

Well done on the whole. A curious and not infrequent blunder was to direct the helmsman to steer so that the compass pointed 18° S. of E.

Q. 3. A horse-shoe magnet is brought due south of a small compass needle, the line joining the poles of the magnet being east and west, with the north pole to the west. Describe the manner in which the compass is deflected.

Describe and explain what will happen if the keeper is placed on the magnet.

The drawings were very bad as regards proportions. In the first part the influence of the earth's field was generally ignored. In the second part all the lines of force due to the magnet were regarded as passing through the keeper.

Q. 4. How would you hold a rod of soft iron so that the influence of the earth's magnetic field upon it may be (1) as great as possible, (2) as small as possible?

Frequently well done. For the least magnetic effect almost all placed the rod horizontal, E. and W. Extremely few gave the more general answer, viz., that the rod might be placed anywhere in a plane at right angles to the line of dip.

Frictional Electricity.

Q. 5. Describe and explain the action of a gold-leaf electroscope.

Having charged the instrument positively, how would you test the sign of the charge on the inner coating of a Leyden jar without discharging or moving either the jar or the electroscope? Answers to the first part were usually reasonable; there were scarcely any correct answers to the second. Nearly all the candidates adopted a method by which the jar was partially discharged.

Q. 6. What is meant by the statement that a given body is at a higher potential than the earth? Illustrate your answer by means of analogous phenomena in heat and hydrostatics.

The main part was badly answered. There were two types of answer; the first omitted to state that it is a flow of positive electricity from a body to the earth that is a test of higher potential in the former; the second, whether expressed in terms of two fluids, one fluid or Faraday tubes, merely amounted to saying that a body has higher potential than the earth if it is positively electrified.

Q. 7. A hollow metal can is placed on an insulated stand and electrified Describe the distribution of the electrification on the can and explain how you would examine the distribution experimentally. Many knew that there is no electrification inside; not so many stated that the density is greatest at the edges, etc. The statement that the absence of electrification can be proved by connecting the inside to an electroscope was made a good many times; but a fair proportion knew all about the question.

Q. 8. A metal plate A, thickly varnished on the top, is insulated and connected to a gold-leaf electroscope, the whole being charged with positive electricity until the leaves diverge widely. When another insulated plate B (for instance, the cover of an electrophorus) is placed on the top of A, the divergence of the leaves is very little altered, but if B is touched the leaves fall nearly vertical. How do you explain these effects?

There was a greater number of unsatisfactory answers to this than to any other question in the paper. The usual mistake was to assume that the touching of B connected the whole apparatus to earth, and allowed the electricity to escape. The common explanation in terms of "free" and "bound" electricity was sometimes given, but the number of incorrect explanations in terms of capacity and potential changes was great.

Q. 9. Describe and explain the action of a plate-glass frictional machine How would you use it to charge a Leyden jar?

The machine was well described, but the action of the collecting combs seemed to be imperfectly understood. Some attempted to describe the Wimshurst machine, and knew very little about it.

Voltaic Electricity.

Q. 10. Describe the Daniell cell, and explain the functions of each part of the cell and the action that takes place when the poles are connected by a conducting wire.

What advantages does this form of cell possess over a simple voltaic cell consisting of plates of copper and zinc immersed in dilute acid?

Answered by nearly all, and the best answers were very good. A large number knew nearly all about the Daniell cell. The function of the porous pot was seldom stated explicitly; a good many stated that bubbles of hydrogen passed through, and then interacted with the sulphate. Some had very vague notions of polarisation. A few had apparently been taught that copper coated with hydrogen is more electropositive than zirc. Local action was frequently stated to be absent from the Daniell cell, without any mention of amalgamation of the zinc.

Q. 11. State Ohm's law, and explain the terms used.

An incandescent electric lamp takes a current of 5 ampère when connected to a circuit of 100 volts.

of the lamp?

The answers to this question were too indefinite

What is the resistance

Nearly all the candi

dates stated that current equalled electromotive force divided by resistance, but did not explain themselves by considering any particular case of the

application of the law. The arithmetical mistakes in the second part of the question were shockingly bad. An extraordinarily large number failed to divide 200 by 5 correctly.

Q. 12. Explain how you would wind a U-shaped piece of soft iron with wire and how you would connect the wire to a battery so as to form an electro-magnet with two north poles at the tips of the U. Carefully describe the magnetic state of the iron when a current is passed through the winding.

A few good answers; the majority were poor. Many diagrams were such that it was impossible to tell how the winding was supposed to run. The expressions "right and left handed," "clockwise and anti-clockwise,” were often used ambiguously. Many failed to state the position of the consequent S. pole, or else said there would be two such poles.

Q. 13. Describe some simple form of galvanometer, and explain the method of using it.

There were a fair number of good answers, and a large number of answers of varying degrees of badness. Many candidates appeared to be acquainted with the galvanometer only as an indicator of current. Some knew that there was a suspended needle, and thought that the current was sent into it. Others described the virtues of the astatic pair, and then placed them at the centre of a large coil.

Q. 14. Assuming that the rate of production of heat by a current in a wire varies as the product of the resistance and the square of the current, compare the amount of heat developed by a current of 2 ampères in 3 minutes in a wire 3 feet long, with that produced by a current of 3 ampères in 2 minutes in 2 feet of the same wire.

Correct answers were frequently obtained, though few of the candidates made explicit reference to CRt, and the arithmetic was often lamentably

wrong.

STAGE 2.

Results 1st Class, 297; 2nd Class, 486; Failed, 450; Total, 1,233.

There were few defects of a general kind to be noticed, and the papers, as a whole, showed improvement in some respects on last year, though there was a larger proportion of failures, but not so many as in 1904. It was evident that there were two very different types of students: (1) the student with a fair general knowledge of theory derived from books, but deficient in laboratory work and in details of practical applications; (2) the technical electrician, familiar with details of construction and use, but weak in theoretical knowledge. It was difficult to frame questions to suit both types of student.

Magnetism.

Q. 1. A thin uniform magnet, 1 metre long, is suspended from the north end so that it can turn freely about a horizontal axis which lies magnetic east and west. The magnet is found to be deflected from the perpendicular through an angle (sin ·1; cos 0 = 995). If the weight of the magnet is 10 grams, the horizontal component of the earth's field is 2 C.G.S. units, and the vertical component 4 C.G.S., find the moment of the magnet.

=

A fair percentage of candidates were successful with this question. The most common mistake was taking the vertical component of the earth's force on the S end of the magnet as downwards instead of upwards. Many had insufficient knowledge of mechanics.

Q. 2. What is a isogonal line?

Describe the general form of the isogonals over the surface of the earth. How are the observations made which are used in determining the isognals?

A good deal of uncertainty was shown as to the nature of isogonal lines. Only a few knew their general form over the earth's surface, and many confused them with isoclinic lines.

Q. 3. Two exactly equal magnets are attached together at their mid

points so that their axes are at right angles, and the combination is pivoted so that the axes of the magnets are horizonal and they can turn freely about a vertical axis. How will the system set itself under the influence of the horizontal component of the earth's field? If the moment of each magnet is M, and the moment of inertia about the axis round which it can turn is K, what will be the period of vibration of the system?

The first part was often answered correctly. The second part scarcely ever. Very few knew how to find the magnetic moment of the compound magnet.

Q. 4. Prove that the magnetic field due to a short bar magnet at a given distance from its centre is twice as great at a point in the direction of the axis of the magnet as it is at an equi-distant point in the plane at right angles to the axis of the magnet and passing through its centre.

A large number of candidates answered this question well.

Frictional Electricity.

Q. 5. An insulated sphere having a diameter of 20 centimetres is charged. It is then connected to an electrometer by a fine wire, the deflection being 50 divisions. An insulated and uncharged sphere of 16 centimetres diameter is then joined to the first by a long wire, and the electrometer deflection falls to 32. Calculate the capacity of the electrometer.

Many candidates found no difficulty with this question. Arithmetic often bad.

Q. 6. What is meant by the term the capacity of a condenser ?

Calculate the capacity of a parallel plate air condenser of which each plate has an area of 400 square centimetres, the distance between the plates being half a millimetre. Be careful to state the unit in which you express your answer.

Only about half the candidates gave a satisfactory definition of the capacity of a condenser. The numerical part of the question was very fairly answered by many.

Q. 7. Give a careful drawing of the lines of force due to a charged point placed near to an uncharged insulated sphere.

Usually carelessly and badly answered. The drawings were poor, and little or no explanation was given.

Q. 8. A small pith ball weighing one decigram suspended by a silk fibre and charged with positive electricity is repelled when a charged glass rod is brought near it. If the direction of the electric field of the glass rod near the ball is horizonal and its magnitude equal to 20 C.G.S. electrostatic units, when the deflection of the fibre is 45°, what is the charge on the ball?

Often answered satisfactorily.

Voltaic and Technical Electricity.

Q. 9. Describe, and explain the mode of action, of some form of sensitive galvanometer suitable for use in a place where the earth's field is much disturbed by the presence of variable electric currents. The astatic galvanometer was given more frequently than one would have expected. As a rule, the suspended coil galvanometer was well described, but too often the mode of suspension of the coil was not given,

or was stated to be glass, or quartz, or silk. Few explained why this type was to be preferred.

Q. 10. A copper disc having a diameter of 40 centimetres is rotated about a horizontal axis perpendicular to the disc and parallel to the magnetic meridian. Two brushes make contact with the disc, one at the centre and the other at the edge. If the value of the horizonal component of the earth's field is 2 C.G.S., find the potential difference in volts between the two brushes when the disc makes 3,000 revolutions per minute.

Not often attempted, and then only by good candidates, who were usually successful with it.

Q. 11. Describe and illustrate by figures some form of self-feeding arc lamp.

Q. 12. Describe a drum armature. What are the advantages of this form of armature over the Gramme armature?

Some very good answers were sent in by the comparatively few candidates who attempted these two technical questions. The most common fault was to dilate on small details to the omission of essential features.

Q. 13. Describe carefully, stating the precautions necessary, how you would test the accuracy of an ammeter reading to about 15 amperes.

The voltameter method was commonly employed. Only a few used a potentiometer method.

Q. 14. Explain how the mechanical equivalent of heat may be determined by measuring the electric energy spent in heating a resistance. What instruments would you require, and how would you perform the experiment?

Many candidates gave good answers, and the majority showed good acquaintance with the principles involved. Weakness in experimental details is the chief criticism, especially with regard to the calorimetry.

Q. 15. How would you connect two equal constant cells of internal resistance 5 ohms each, if you wished to deposit copper as rapidly as possible in a voltameter of 7 ohms resistance?

A good proportion answered this question well. With the data supplied the actual relative currents should have been worked out.

Q. 16. Explain the terms specific resistance, and temperature-co-efficient of resistance of a material. What material would you employ for constructing a standard resistance, and how would you wind the wire ?

A good definition of specific resistance was often given. Many candidates were unfamiliar with term temperature-co-efficient of resistance. The suitability of manganin and other alloys was put forward by most; and the non-inductive method of winding was given by almost everyone.

STAGE 3.

Results 1st Class, 28; 2nd Class, 74; Failed, 39; Total, 141.

The paper was on the whole very fairly done. The results compare favourably with those of last year. Subjoined are detailed remarks on the answers to some of the questions.

Q. 1. Describe some accurate method of comparing the capacities of two condensers. Enumerate the chief precautions required?

Fairly done. General principles and methods correctly given, but details (e.g., as to character of galvanometer, magnitude of resistances, errors due to absorption, etc.) often neglected.