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ference between the gravitation field of a comparatively large mass of the metal bismuth, which is the most diamagnetic substance known, and the gravitation field of a similar mass of lead and of zinc, which are very much less diamagnetic than bismuth, and also of tin which is slightly paramagnetic. To this end it was proposed to measure the minute gravitational attraction between each of the above masses and a very much smaller nearby mass of some metal, the same small attracted mass to be used in all cases. In such a scheme the large masses would do nearly all the attracting, and their several gravitational pulls per unit of mass would be comparable.

To carry out this scheme Professor Dayton C. Miller very kindly provided, from his large collection of physical apparatus, a beautiful instrument designed for class-room demonstration of gravitational attraction between two small silver balls and two large lead spheres in the usual manner of such apparatus. It is a modification of the apparatus designed and used by Professor C. V. Boys for determining the gravitation constant and the mean density of the earth. Each small silver ball weighs three fourths of a gramme, and the pair are mounted at the ends of a horizontal small straight metal rod, with their centers 3.6 cm. apart. Rising from the center of this connecting rod is a small vertical rod carrying, at a distance of 6 cm. above the silver balls, the usual small mirror for scale reading, set at an angle of 45° with the ball-carrying rod. These parts constitute the oscillating system, and are suspended by a long quartz filament in a brass tube, the balls only projecting below the tube into a narrow glass-walled chamber, made shallow in order to minimize convection currents inside. Means are provided for leveling the whole apparatus, for orienting the free-hanging system and for clamping the balls when not in use. The apparatus is permanently grounded through one of its leveling screws.

The large lead spheres and their carriages were discarded and replaced by a light reversible wooden carriage.

The photograph, Plate V., shows the apparatus as set up in my basement laboratory. The delicate part first described is mounted on a heavy marble slab firmly bracketed in the angle of two twenty-inch brick walls. These are inside walls, and hence not liable to sudden

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temperature changes due to outside weather conditions. The whole lay-out is thirty feet from the nearest window, and the temperature of the laboratory is very uniform and steady. The room selected is an inside one and contains no heating apparatus. The floor is thick concrete. The reading telescope shown in front and the carriage referred to are mounted on a massive table with thick marble top, nowhere touching the bracketed slab or the walls to which it is attached. The illuminated millimeter scale is two meters to the right of the oscillating system and does not show in the picture.

It will be noted that the tall brass tube containing the quartz filament is loaded at the top with a hollow cone of metal. This is found to increase the stability of the suspension apparatus very greatly by so lengthening the period of free vibration of the upper end of the tube that it can not respond to vibrations of the building due to street traffic or other causes. Although the nearest street is 300 feet away, traffic vibrations often can be felt.

The whole apparatus is protected from radiant heat of the scale. lamp, and one other more distant lamp used to light the room, also from the heat and breath of the observer, by screens of cellular paper (not shown). All air drafts in the room are avoided as carefully as possible. The rheostat on the wall in the upper part of the picture has nothing to do with the apparatus, and never is used during observations.

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Fig. I is a plan diagram of the essential parts. The suspended silver balls are seen at A. B and C are cylinders of different metals,

such as bismuth and zinc for instance, whose attractions for the nearer silver ball are to be compared. The cylinders are carried on the ends of a thin strip of wood D, which is pivoted at its center to, and supported by, a thick disc of cast iron E whose upper face is dressed flat and leveled. The height of E is such that a horizontal plane midway between the upper and lower ends of cylinders B and C is in the center plane of the balls A. The carriage D is covered with tin-foil kept in metallic contact with E by a brass-wire spring. E is permanently grounded; thus B and C are always grounded.

The cylinders B and C are very carefully so placed on the carrier D that when the latter is revolved 180° and brought against a removable stop-pin F, C will occupy exactly the same position in respect. to the balls A as did B before the reversal.

All the metals experimented with are in the form of cylinders of the same size, 4.9 cm. high and 6.1 cm. diameter. When in position, the surface of a cylinder is 1.3 cm. from the center of the nearer silver ball.

The zinc cylinder weighs 1.014 kg., and the other cylinders weigh more or less than this according to their several specific gravities.

The zinc cylinder attracts the nearer silver ball with a force of about one three hundred thousandth part of a dyne, and as the oscillating system is very sensitive, having a free period of seven and a half minutes, the excess of this attraction over that for the more distant ball gives a scale deflection of about 4.2 cm., which is ample for observation, because deflections are easily read to 0.1 mm. As the mirror doubles the real deflection, the latter is 2.1 cm. at a distance of 2 meters. Hence the silver ball moves about 0.2 mm. toward the attracting cylinder, where the attraction is about I per cent. greater. This change in attracting force is approximately corrected by so locating the cylinder B that the angle a b c is slightly obtuse at the start, and becoming more so as the ball advances causes the attractive effort to be less effective. Hence the deflection as read by the telescope may be taken as a closely approximate measure of the attraction of the cylinder for the ball. Of course, the center of attraction in the cylinder does not lie in its axis, though near it. But this does not matter, because its location is the same in all the cylinders.

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