Show I 81 i f iti Just Just What hat Occurs Inside Voltmeter Happenings Differ in De- De Detail De Detail tail in Various Types of Meters Cy JOSEPH L MCGRATH I In Boston Globe Gobe I When hen testing A B D or C batter batter- batterIes batteries ies leg with a 0 voltmeter the terminals ot of otI I the meter are connected to the post post- positive I tive and negative e of the battery hattery and the pointer of ot the meter lies flies over o to tos toi s 1 certain point on a R calibrated scale What Shat happens Inside the meter lIt dif differs fers In detail In different meters but n a all common common types current flows In at one terminal by force of the volt olt volt ne age performs Its duty In the meter and flows out nt at the other oilier terminal In n a perfect voltmeter there would be no current flowing through h the meter meter but we do not need such n a do- do de degree gree of ot perfection By ny allowing a avery very ery small current to flow magnetism Is set up whIch causes onuses the needle to register The force acting on the needle Is figured b by multiplying the number of turns of wire In a n coil coll In Inthe Inthe Inthe the meter by hy the amount of current flowing through It As example A coil coll having turns with n a current of or OJ ampere would give a product of or 75 and anti ft l colt coli coltof ot of 50 turns with a n current of ot IGO am- am ampere am ampere pere would also give ghe a II product of 7 Vow ow ow let us assume that the force rep rep- f ff f N Te SPRINGS SPRING yi AXIS TERM NALS Ply Fl j J f D t ABC A B Fly ji f 4 Fig 1 Shows Principal Parts Inside of One Type of Voltmeter resented by 70 75 J will cause the needle to rend read 45 volts on the scale Though both meters read 45 volts on the scale one Is a good meter and the other Is not so good because e It Is too far for from our theoretical standard and has ex- ex excessive excessive ex excessive losses Other I Important Factors It If we had perfect voltmeters we could leave ea e them connected In the clr- clr cir cult at all times but because even en c good meter will take whIch would be a large ar e percentage ot of the normal current of ot a B battery it II itIs ItIs Is not well to leave lease the meter In cir cir- circuit cir circuit cult The bearing In which the needle swIngs Is another Important factor In Ina Ina Ina a meter for a poor bearing means menns that extra current will wilt be bo required In the coil coli to swing the needle The better hetter meters therefore have ha more turns In their coils colls and better hetter bearing than the cheaper chen per type nil all for forthe forthe forthe the purpose of keeping the current at ata ata nta a minimum and the efficiency high high efficiency also niso means correct In- In IndIcation indication In Indication use and that Is what we use me me- meters meters meters to get The Voltmeter Fig 1 shows the lie principal 1 parts Inv In- InsIde In Inside side of one type of ot voltmeter ThIs m meter ter may be connected with either terminal positive Other meters of ot dif dlf different ferent type must have ha the terminal marked Positive or Plus Pius connect connected ed to the positive side of the battery An example of connections for testing a 11 volt 22 volt B n batter battery Is shown Con Con- Connection to minus and 22 would give gl the voltage of the other 2 22 volt battery Connection to minus and 15 would give I e the voltage ot othe of the two 1 batteries together Fig 2 A represents the normal maxImum current which may be from for tor Instance n a B n batter battery The amount of current required to operate a good voltmeter Is represented by bythe bythe bythe the flow through B n The rhe amount of current required to operate a poor meter would Oil fill pipe C If It the inn max roa- maximum mum current available a through A Is passed Into C It Is seen that the me- me meter meter me meter ter connected to C will not ot operate correctly due to the Insufficient sup sup- supply supply IY ply ply- |