Page:Popular Science Monthly Volume 90.djvu/804

From Wikisource
Jump to navigation Jump to search
This page needs to be proofread.


788

��Popular Science Monthly

��the static. Further experiments may be conducted by "bottling" this wonderful agency in a Leyden jar, which is nothing more than a wide-mouthed glass jar with a wood cover, through which a brass chain is depended, and whose outside surface is partly covered with tinfoil. To gather the static, place the transmitting rod against the ball that tops the chain. A powerful discharge is released when contact is formed between the ball and the tinfoil. A glass tumbler filled with water offers a substitute for the Leyden jar and is charged by im- mersing the rod. Offer the glass to a friend — and watch the fun. — Joseph J. Steedle.

��INSULATOR

��SHALL BRftDS

��Making a Hot Wire Ammeter for Electric Currents

A HOT wire ammeter is nothing more than a piece of wire which expands when a current is sent through it. This expansion is taken up by a balance wheel which moves a pointer along a scale calibrated to read in amperes. The sensitive or movable parts of this hot wire ammeter are a balance wheel and hair-spring which are taken from the works of an old alarm clock. When taking out the balance wheel and hair-spring the framework with the bearings should also be taken out and the other part of the works cut away with a pair of tinner's snips.

The balance wheel and its bearings are mounted on a piece of wood or fiber, which is nailed to the alarm clock casing with small brads, as shown at A. The end of the spring is attached to the post B. After the balance wheel is put into tension by rotating the wheel a few times a piece of silk thread should be tied to the balance wheel spoke and wound five times around the axle of the wheel. The thread should be wound in the direction which will hold the tension of the wound-up spring.

The hot wire is a piece of No. 36-gage platinum or German silver wire. This is stretched between C and D, which should be well insulated from the casing E. The thumbscrew D is used to adjust the pointer by tightening the hot wire. The pointer should be made either from a piece of

��sheet aluminum or a piece of light, stiff wire. It is painted black and tied to one of the spokes of the balance wheel after the thread has been pulled so that the spring is under tension. Adding a drop of wax or glue will make a stronger joint.

The dial is made from a piece of card- board and is glued to the sides of the alarm clock. It is placed in back of the pointer and in front of the hot wire, and may be divided off according to the range of the instrument. The hot wire ammeter is provided with a- glass which is painted black. In putting the parts together the balance wheel and parts are put in the clock casing first; then the dial is glued in, the hot wire is placed in position and the thread from the balance wheel is tied to it.

When the antenna- current passes through C and D the hot wire expands, which tends

��POINTER

���HWR SPRING

���BALANCE WHEEL

6EARIN6

END Of5PRlN6 FASTENED HERE

��FINISHED AMMETER FRONT PAINTED BLACK

��ALARM CLOCK BALANCE WHEEL AND FRAME

��The casing anci works of a discarded alarm clock used for making an especially delicate hot wire ammeter

��to leave a slack in the silk thread. This is taken up by the tension of the spring, and causes the axle to wind up, making the pointer move along the scale. When the current stops flowing through C and D the hot wire shortens it up and the pointer returns to zero. The hot wire ammeter will make a useful addition to any wireless station, and is particularly handy for tuning. The success of the instrument depends largely on the care taken in its construction, for although simple it is delicate and requires more accuracy in

�� �