desired, so as to be able to attack the question above, numbered 4, without additional expense. Accordingly, I ordered from Messrs. Mather and Platt an oblate spheroid of best Swedish iron, a yard in diameter and half a foot thick, with a deep channel or groove half an inch wide cut into its rim to a depth of one foot all round. It was not found practicable to make the iron all in one piece, and accordingly it was constructed of two pieces bolted together, and its section is shown in fig. 1.
The bottom of the groove was wound with wire to a depth of 4½ inches, the wire used being No. 20 B.W.G. double silk-covered copper; and of it 14 lbs. 10 ozs. was wound on, in 94 layers of 9 convolutions per layer, the central iron core being 1 foot thick. The ends of the wire come out through small holes drilled for the purpose, with balancing holes drilled at equal opposite radii so as to leave the centre of gravity undisturbed, and the wire was there tightly bound with tape and steel to resist centrifugal force.
The free ends of the covered wire were clamped to the surface of the disk and led to a set of insulated brass rings on the upper part of the axle, so that am electric current either steady or commutated could conveniently be supplied whenever desired.
The resistance of the wire coil was measured by one of my students as 29.9 ohms, end its insulation resistance was just short of 2 megohms. The length of the wire is about 1 kilometre or two-thirds of a mile.
The magnetising current was usually supplied from the town mains, at 110 volts nominal, which gave a current of 3.8 amperes through the coil of 846 turns, and accordingly developed a magneto-motive force of 4000 egs.
The lines of force so generated streamed across the half inch gap from the one half of the oblate spheroid to the other, being rather more plentiful in the deep parts of the channel. But the course of the beam of light only partially penetrated into the most intense region, and its mean track was situated about 4.6 inches from the periphery; so at this place I asked a student to measure the magnetic field excited by various strengths of current (by the common method of suddenly snatching out a small exploring coil and comparing the galvamometer throw with that caused by an
