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��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
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��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
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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
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