Open main menu

Page:Popular Science Monthly Volume 89.djvu/132

This page needs to be proofread.


118 Popular Science Monilih/

An Easily Made Mercurial Barometer ^HE accompanying diagram, Fig. i,

��T'

��shows a useful barometer of simple construction. The baseboard A may be of mahogany, 38 ins. long, 2^^ ins. wide in the central part and 4 ins. at the ends. A straight glass tube B is needed, 36 ins. long and about }4 in- in bore. After filling by the simple method about to be described, it may l)e fastened to the center of the board by means of

���A mahogany base, a glass tube and a mer- cury cistern comprise this barometer

neat brass saddles. The cistern (', to be described in connection with the method of filling, is provided with a wider saddle at the lower end of the board. This may be hidden, for the sake of appearance, by means of the [xilishcd mahogany disk M. The appear- and' will be impro\cd by lurning the disk, or providing a small beveled mirror in the center. The upper end of the tube may also be concealed by means of an ornamental disk of turned wood. It is in filling the lube with mercury that most amateur barometer makers

��experience the principal difficulty, owing to the necessity for exc-luding air. The regulation method is fairly easy in experienced hands, but the following will be found much simpler. The tube

B, Fig. 2, is of soft glass with walls of medium thickness. It is therefore a simple matter to make a constriction at X, 3 ins. from one end, by softening the tube in the flame of an alcohol lamp and drawing the ends gently apart. To the open end above the constriction should be attached a short length of India rubber tube D, capable of being closed with a brass clip or pinchcock E. The other end of the tube should be passed through an India rubber cork, fitting tightly into a wide glass tube

C, forming the cistern. A bent tube F, previously passed through the cork towards the side, serves for connection with a cycle-pump and valve, with thick- walled rubber tube.

Let the tube be supported vertically, the cistern being rather more than half filled with mercury — before inserting the cork, of course — and force a little air in with the pump so as to drive mercury into the tube to about two-thirds of its length. Without removing the cycle- valve attached to F, that is, without letting any air escape, lower the tube gradually in a slanting direction. The mercury will rise still higher until it passes the constriction and fills the India rubber tube. Then close the pinch- cock, remove the valve from F, and replace in the vertical position. The mercury will come to rest somewhere near the end of the tube, as shown in Fig. 2. The space above it is a vacuum siiu^e the air has been dri\en out and prevented from re-entering. The upper end of the tube above the constriction is no longer required and may be removed by directing the alcohol lamp flame against the narrowest portion with a mouth blowi>iiH', (or even an odd piece of thin lube), the same operation serving to close the top of the barometer tube with ,1 neat and pcrfeclK- airtight seal.

A scale of inches ami tenths must be made on glazed cardboard or imitation ivory and attached to the top of the board, a sinu'lar scale being fixed on the other side of the tube to show the words

�� �