When a magazine is placed in a bookcase the outer pages are liable to turn back if it is inserted with the back on the outside. To overcome this difficulty I made clips for each magazine to hold the open pages together. Each clip was made of wire, about 8 in. long, shaped as shown. The width of the clip is made equal to the thickness of the magazine and the extending ends are slightly pressed together so that they will spring and grip the pages.—Contributed by W. A. Saul, E. Lexington, Massachusetts.
Slide-Opening Cover for a Plate Holder
The length of time required for the slide of a plate holder to be removed on a reflecting camera spoiled many of my plates, because strong light would enter the unprotected slot when the camera was in certain positions. To protect this slot so that the slide could be left out indefinitely, I made a cover of a piece of sheet metal having three slots, to admit screws turned into the camera. A knob was attached at the center. The illustration shows the application of this cover. In Fig. 1 the plate holder is shown slipped in with the cover back, and Fig. 2 shows the slide drawn and the cover over the slot opening.—Contributed by B. J. Weeber, New York City.
The Two Positions Occupied by the Slide-Opening Cover as It is Used on a Camera (Fig. 1, Fig. 2)
Magnetic-Suspension Pendulum
When a pendulum is not periodically supplied with energy its amplitude grows smaller and finally the motion ceases, due to the resistance of the air and the friction at the point of suspension. Usually the suspension is in the form of a knife edge bearing against plates of agate; sometimes the pendulum rod is simply attached to a very slender and flexible spring without any bearings. But the minimum of friction is obtained by means of magnetic suspension, as the following experiment will prove.
If the rod of a pendulum about 12 in. long, beating half seconds, is sharpened to a needle point and suspended from one of the poles of a magnet, it will be found that, if set into motion, it will continue to swing 15 times as long as the ordinary knife-edge suspended pendulum, and it will not stop until after about 16 hours, while one working on agate plates will stop in from 50 to 60 minutes. Similarly a top, provided with a fine-pointed axis of iron, will spin much longer when suspended from a magnet.
Magnetic suspension is used in precision instruments; for example, the minute mirrors which are used in certain telegraph systems to register writing photographically at the receiving end.