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Page:Popular Science Monthly Volume 90.djvu/323

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�e AmatGur • Electrician

��^d Wii'dess Operator

��Easily Constructed Step-Down Transformer

THE greater part of the work of building a small step-down transformer is in the cutting and assembling of the iron. By securing the reactance core of a dismantled alternating current arc light there will be had a well proportioned and efficient core ready for taping and winding. These cores

���A laminated iron core of a discarded arc lamp used for a step-down txansformer

can be easily secured as, in a great many places, the new high powered Mazda and nitrogen lamps are replacing the old style arcs. The core I used was of the dimensions shown in the accompanying illustration.

File the sides of air gap smooth and parallel; clean the iron, tape and then shellac it. When the shellac is dry wind on No. i8-gage double cotton-covered copper wire in even turns. For a maximum secondary voltage of 12 volts wind 120 turns, starting i in. from the opening on the inside circumference. Tape may be taken off at any turn, figuring ten turns for each volt. Wrap the secondary coil with linen tape, and cover with shellac. Over this wind iioo turns of No. 26-gage double cotton-covered copper wire as evenly as possible. Cover the completed primary and core to the opening with black friction tape and then coat it with shellac. Cut a disk of J^in. fiber and bore holes for binding posts as shown. Slip a piece of J^-in. cotton tubing or lacing over

��each lead and connect it to the binding posts. Fasten the fiber to the transformer by t>ing it with cloth tape.

The proper and efficient way to fill the air gap would be to cut small pieces of transformer iron and wedge them in place in line with the other laminations. Another way is to cut pieces of the metal i^ in. long by I in. wide, and drive them in at right angles to the core iron. This latter method causes the wedge to heat up, showing there is considerable loss through eddy currents. For most experimenters the transformer will operate efficiently enough with gap left open. — O. J. Hurlbut.

One Telephone Used on Two Lines

WHERE it is desired to use one tele phone on two lines so that both lines will be kept separate and yet sound the rings from each line, the following rnethod may be used. Connect an. exten- sion bell and a double-pole double-throw switch to the telephone and the lines, as shown in diagram.

If one of the lines is a grounded circuit, the ground should be connected to one side of the switch and not to the telephone or


Connections of telephone wires and bells for using one telephone on two lines

bells; but if both are grounded circuits the common wire should be connected to the ground.

In the diagram, if the switch is thrown up, the telephone is connected to the line i


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