Изображения страниц
PDF
EPUB
[graphic]

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.

[blocks in formation]

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.

The attraction of B on the silver balls must draw the oscillating system out of plumb; but as this effect amounts to only about a millionth of a millimeter, it is entirely negligible.

Prior to using the apparatus the cylinders are swung into a position at right angles to that shown, all lights are extinguished except those to be used in making observations, and the room is closed against all draft. After two or three hours of repose, to equalize temperatures, one of the cylinders is moved into operative position a minute or two only, to start a definite movement of the oscillating balls. Then the diminishing oscillation limits are read 6 or 8 times to establish the zero point. Next, one of the cylinders is moved into operative position and left there until 6 or 8 oscillation limits have been read; then the cylinders are exchanged by reversing the carrier, and 6 or 8 more deflection readings are made. Twice again the cylinders are exchanged and similar readings taken, so that two sets of readings are had for each cylinder. Usually they agree very well indeed, and their mean is taken as the true value. Finally, both cylinders are swung out of position as at first, and another set of readings taken to redetermine the zero point.

Although such a series of readings occupies about four hours, the zero drift rarely exceeds two scale divisions (2 mm.) and, assumed to have been progressive, is apportioned among the several sets of readings.

In the above manner many comparisons have been made of lead and bismuth; lead and zinc; bismuth and tin; bismuth and zinc; silver and zinc; and lastly of aluminum and bismuth and aluminum and zinc.

The interlacing observations support each other very satisfactorily.

In every case the observed deflection is divided by the weight of metal causing it, so as to reduce all to a common standard of attraction per kilogram.

Table I embodies the results thus far obtained, taking zinc for a standard and calling its attraction per kg. 100.

« ПредыдущаяПродолжить »