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5OO
Radio Broadcast
FIG. 4
The receiver slides into the middle compartment, the side sections being provided
for the A and B batteries. If the constructor of the set prefers not to make the
cabinet, it may be bought for a reasonable sum
on ordinary cotton covered wire is thus reduced. It is interesting to note that when coils, especially cotton covered, are not treated with some form of moisture-resisting material, a relatively great amount of moisture will be absorbed, the insulation between turns is materially reduced, and this fairly low-resistance shunt across the coil is extremely detrimental to sharp tuning. The particular size wire is chosen because with it a relatively small length of wire is required for any given inductance and in addition the value of capacity between turns and therefore the total value of distributed capacity is lower than would be the case with heavier wire.
MAKING TRANSFORMER TI
ONE of the two Formica forms (see list of materials) should be provided with four terminals and two mounting screw holes, made with a No. 27 drill. The terminals are situated f" apart, J" from one edge. The mounting holes are \" from each edge on a line parallel to the axis and between the two center terminals. The terminals may consist of switch points with the heads outside and hexagon brass nuts clamping them to the form
inside. The projecting pieces of the screws are cut off and solder flowed over the nut to prevent loosening. Small holes to pass the wire should be drilled near terminals i and 3 (Fig. i).
The secondary coil is wound on the form first; starting at terminal No. 3 to which the wire is soldered, 60 turns are placed evenly and tightly; the end of the wire is brought through the form at a point opposite terminal No. 4 and soldered to that terminal.
A one-inch strip of cambric cloth or other flexible sheet insulating material is wrapped over the secondary and held in place with glue. The primary, of 15 turns, is wound on the insulating material, spaced in the center and in the same direction as the secondary.
The beginning is soldered to terminal No. i and the end of the coil is brought through the form at a point opposite No. 2 and soldered to that terminal.
The entire form may be given a light coat of thin shellac, collodion or airplane "dope" leaving only the terminal heads untouched for soldering. When throughly dry the transformer is mounted on its condenser — one method of accomplishing this is shown in Fig. 2. Two holes \\" apart may be drilled and tapped for -^ thread in the end plate of the variable condenser. Two -fa machine screws and small brass pillars are used to support the transformer away from the condenser. The arrangement should be similiar to the illustration in order to retain short leads.
T
MAKING TRANSFORMER T2
HIS transformer is constructed in a manner similiar to Ti with the difference that the primary (top coil) has 35 turns.
Referring to the diagram, Fig. i, it will be seen that there are five connections to T2; the fifth connection is a center tap on the secondary and should be used only if the receiver is to be operated in the vicinity of an interfering