P. H. G . ALLEN

Acknowledgments CARTER, F. W., 1901, Electrical World and Eneineer, 38,

and Mr K. L, Eatwell in preparing this article. He HARRAP, M. J., 1966, Edrtcation, 46. The author acknowledges the help of Dr R. Hawley 884.

wishes to his colleaguey Mr H’ Kayser3 with KIRCHHOFF, G., 1845, Annalen der Physik und Chenlie, the development of the magnetic face plate technique No. 4, 497. and to thank Associated Electrical Industries Ltd for RABY, K. F., 1966, Field Ana/?,$js (London: Van permission to reproduce figures 10 and 1 1 . Nostrand), Chap. 5.

SUROWIAK, S . , 1965, A.E.I. Engng, 5, 127.

References VITKOVITCH, D. (Ed.), 1966, Field Analysis (London: Van

ARCHENHOLD, W. F., 1966, Physics Education, 1, 171. WRIGHT, E. G., 1966, Field Analysis (London: Van BARKER, J. R., 1960, Amer. J. Phys., 28, 139. Nostrand), Chap. 4.

HOEFNAGEL, L., 1964, Electrotechnik, 42, 267.

Nostrand), Chap. 5, 6 and 7.

A permanent magnet Gouy balance

A. SAUNDERSON Kilburn Polytechnic, London

A simplified form of a classical magnetic balance is described. This makes it possible to observe the essential difference in behaviour between paramagnetic and dia- magnetic substances, and to make determinations of paramagnetic constants.

The Gouy balance is a classical method of investi- gating susceptibility. It consists essentially of suspend- ing the specimen (in the form of a long cylinder) from a sensitive balance, applying a magnetic field gradient along its length and measuring the force due to the magnetization of the specimen. In its usual form the apparatus needs a permanent location because a large electromagnet is used. The arrangement described here uses a strong permanent magnet (Eclipse Major) and can be set up on a bench in a short time.

The most convenient balance to use is a ‘top-pan’ balance with digital readout, sensitive to 1 mg and having a suspension hook beneath. This is supported on a stout wooden box, open to the front, with a hole cut in its upper surface to allow suspension of the specimen. The size of the box is dictated by the dimensions of the base of the balance together with the need to manipulate the specimen and magnet inside it: a 40 cm cube is suitable.

The application and removal of the magnetic field is achieved by resting the magnet, with its plane hori- zontal and the gap forward, on a simple non-magnetic carriage which can be slid forward and backward. In the rear position, the gap should be at least 20 cm behind the specimen so that its field has negligible effect. The forward position is such that the specimen

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is in the gap with its lower end level with the centre of the poles: this position is achieved by using packing- pieces of wood or hardboard.

The specimen may be either solid or liquid. If solid (e.g. glass, ebonite) it should be in the form of a cylinder at least 15 cm long and about 1 cm in dia- meter, suspended by silk or nylon thread attached to its upper end. If liquid, the specimen may be contained in a thin glass test-tube of similar dimensions: the 6 in by in size is convenient. Suitable liquids are water, benzene and acetone as diamagnetics, and 40% by weight solutions of manganese sulphate, nickel sulphate and nickel chloride as paramagnetics.

The experimental procedure is simple and can be performed in a few minutes. The reading of the balance is noted with no field applied (wl), and then with the field applied (wz). Then (wz-wl) is the force exerted on the specimen: it is seen to be negative for some specimens and positive for others. This shows the difference between diamagnetics and paramagnetics, the former being repelled from the region of greater flux density, and vice versa, To obtain a value for the susceptibility of a paramagnetic substance two further measurements are made: the cross-sectional area ( A ) of the specimen, and the flux density (B) , by means of a fluxmeter, in the centre of the magnet gap. The

volume susceptibility is obtained from the simplified expression

No such determination can be made in this case for diamagnetic substances since the force observed is only 3 or 4 mg.

The following table gives a typical set of observa- tions, together with the volume susceptibility obtained from them.

B = 0.197 T A = 1.61 cm*

Specimen w 1 g w 2 g (wz-wl) K (SI units)

MnSO, s o h 37.553 37,623 0.070 2.8 x NiSO, s o h 37,713 37,728 0.015 5 . 9 ~ NiCI, s o h 38.250 38,285 0,035 1 . 4 ~

There are several precautions which must be taken: (i) Air currents must be excluded from the balance

and the box.

A PERMANENT MAGNET GOUY BALANCE

(ii) Unless bench and box are very rigid, small dis- tortions (even due to the observer leaning on the bench) can alter the balance reading. The forward movement of the magnet can introduce this error, but since it is systematic it can be determined and allowed for.

(iii) With the top-pan balance, a denser specimen will hang lower, so the height of the magnet must be adjusted to allow for this when the specimen is changed.

If the concentration and density of a liquid speci- men are determined, it is possible to find the mass susceptibility and the molar susceptibility. For the three specimens recorded here the values are within 20% of the book values. Further, by assuming that for an inorganic salt the diamagnetic contribution to susceptibility is small compared with the para- magnetic, the magnetic moment of a single Ni+- ion, for example, can be found. It turns out to be of the order of SI units.

Physics Education - subscription rates for 1969

The Council of the Institute and Society at its meeting of the 2nd July 1968 decided that the subscription rates for Physics Education for 1969 will be:

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