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| CONTENTS | ix |
| Art. | Page |
|---|---|
| 462. J. A. Broun's method of correction | 115 |
| 463. Joule's suspension | 115 |
| 464. Balance vertical force magnetometer | 117 |
Chapter VIII.
Terrestial Magnetism.
| 465. Elements of the magnetic force | 120 |
| 466. Combination of the results of the magnetic survey of a country | 121 |
| 467. Deduction of the expansion of the magnetic potential of the earth in spherical harmonics | 123 |
| 468. Definition of the earth's magnetic poles. They are not at the extremities of the magnetic axis. False poles. They do not exist on the earth s surface | 123 |
| 469. Gauss' calculation of the 24 coefficients of the first four harmonics | 124 |
| 470. Separation of external from internal causes of magnetic force | 124 |
| 471. The solar and lunar variations | 125 |
| 472. The periodic variations | 125 |
| 473. The disturbances and their period of 11 years | 126 |
| 474. Reflexions on magnetic investigations | 126 |
Part IV.
Electromagnetism.
Chapter I.
Electromagnetic Force.
| 475. Örsted's discovery of the action of an electric current on a magnet | 128 |
| 476. The space near an electric current is a magnetic field | 128 |
| 477. Action of a vertical current on a magnet | 129 |
| 478. Proof that the force due to a straight current of indefinitely great length varies inversely as the distance | 129 |
| 479. Electromagnetic measure of the current | 130 |
