Show aks H brae 6 b how make and use ua it sa by to tn ai anuw 10 1 l io C by hb H b ewh 77 XIII AMMETERS Amm ammeters oters are instruments designed to inc measure asure the flow of electricity through the wires and are often very essential parts of a radio outfit they consist as far ns as exterior app appearances entrances go of a dial marked with figures and a hand or needle although they are not cot high priced and it rs is not advisable to try to make them still there Is nothing mysterious or complicated about them and the amateur who likes to experiment with homemade home made instruments can readily construct an ammeter which will work and Is fairly reliable this instrument la Is known as a hot wire ammeter mg fig 20 and consists of a fino fico platinum atre A secured between t two w 0 fixed s supports 13 13 a thread C fastened atoned fa to the center of the wire andol and passed around a spool or spindle D a spring at accod to the end of tho the thread 14 and a pointer or hand fastened immovably to the top of the spindle P F tho the electrical connections aro are made at D B U D had and ns as soon ns a current passes through tb too platinum wire TIM A the wire becomes heated and expands thus allowing the thread to slacken the slack is instantly taken up lip by the ici spring iring E thus revolving the spindle and swinging the needle to one side the greater tho the current the more the pointer swings nod find so by arranging a i dial with marks under the needle 4 and testing the device with currents of known za r 47 29 force orce a fairl fairly T accurate instrument can be made needless heedless to say the parts must be small and bently and accurate atif ly made and the spring must be adjusted to merely hold bold the thread tightly without pulling polling or bending the wire appreciably alsot also the length of th the wire Is a fy jt groat great factor actor for the longer the wire the greater will be the e amount it expands and f con consequently the greater the movement oatho of the needle ne cille but on the other hand it will bo be more difficult to adjust a long wire to remain tight than a shorter one and the only way to determine the proportion of the various parts Is to experiment 4 AS peri perl ment |