Popular Science MonthJy 



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•-.-Supply moins- 



shell-type polyphase transformers is 

 shown in Fig. 5 and 6. The three-phase 

 type is equivalent to three single-phase 

 transformers placed against one another. 

 There are three distinct 

 sets of coils, but the 

 core forms a part com- 

 mon to all three. By 

 combining the core in 



UilMMilMlKMiMMMMj 



tp 



A diagram for the winding of an autotrans- 

 former arranged to step up the voltage 



this way, a saving of iron is effected and 

 the core loss slightly reduced. 



A two-phase transformer gives but 

 little saving over two single-phase ones, 

 as is seen by reference to Fig. 6. 



Another type of transformer often 

 employed is the autotransformer. They 

 differ from the ordinary type, in that 

 they only transform part of the total 

 power supplied to them. In addition, the 

 primary and secondary coils are con- 

 nected in series, instead of being entirely 

 separated. In the autotransformer, power 

 is partly transferred by direct electrical 

 conduction from primary to secondary, 

 and partly by means of the alternating 

 Supply mains 



Ep 



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A diagram for a 

 transformer that 

 lessons or steps 

 down the voltage 



f.g. 



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field. In the ordinary transformer, the 

 two coils are electrically separate, and 

 neglecting losses, the total power supplied 

 to primary is transferred to secondary 

 through the medium of the alternating 

 flux. 



In Fig. 7 is shown a diagram of the 

 winding for an autotransformer, arranged 

 to step up the voltage, and Fig. 8, a dia- 



gram of one for stepping down the voltage. 

 In the former the high-voltage mains are 

 attached to secondary, while in the latter 

 (hey are connected to primary. 



Referring to Fig. 7 and 8, Ep denotes 

 the primary E. M. F.; Ip, primary cur- 

 rent; i/X secondary voltage; and In, 

 secondary current. Whatever the wind- 

 ing arrangement may be, the rule of 

 voltage transformation is the same as for 

 transformers having two insulated wind- 

 ings. Thus, if Tp denotes the number of 

 primary turns, and Ts of the secondary, 

 then, Ep : Es = Tp : Ts. 



Neglecting losses, primary input equals 

 secondary output, or Ep Ip = Es Is. 

 Whence, Is = Tp Ip -^ Ts. 



Thus, the secondary current in the 

 autotransformer is equal to primary 

 current multiplied by the ratio of primary 

 to secondary turns. 



Here Is an Interesting and Artistic 

 Electric Battery Tester 



THE battery tester shown is designed 

 to show the condition of electric 

 batteries and will indicate roughly the 

 amount of energy left in any cell. It is 

 very easily constructed, and if the work 

 is carefully done, the instrument will 

 make a handsome and useful addition to 

 any amateur's set of electrical appliances. 



Procure a 4-in. test tube. These tubes 

 come in different lengths and the longer 

 the tube the higher the wire coil will be 

 and the more wire will have to be used. 



Having selected the tube, measure its 

 outside diameter. Then get a piece of 

 pine board ^ in. thick and about 5 in. 

 wide. Mark with dividers, a small circle 

 in the center of the board. The circle 

 should be ^ in. larger than the outside 

 diameter of the test tube. Also mark a 

 43/2-in. circle concentric to the small one. 

 Bore a hole through the center circle, 

 then cut the wood with a saw or knife as 

 close to the line of the outside circle as 

 possible and finish with sandpaper tacked 

 on a block of wood. Get a small stove 

 bolt or a machine screw and a cork. 

 Punch a hole lengthwise through the 

 center of the cork with a bradawl. Then 

 put the stove bolt through the cork, using 

 a screw driver for the purpose. Allow 

 the bolt to extend more on the bottom 

 than on the top of the cork. The bolt 



