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 



