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378 THE BOY ELECTRICIAN

thick coat of shellac, both inside and out, before it is used.
This treatment will prevent it from shrinkage and avoid
the possibility of having to rewind the tube in case the

' wire should become loose.

The secondary is fitted with two circular wooden heads

  

   

   

 

 

 
 
  

 

 

.l.
7

 

—
D

 

 

7
3 fi
FIG. 320. — The Secondary Head.

 

fl

just large enough to fit
tightly into the tube, hav-
ing a half-inch flange, and
an outside diameter of three
and seven-eighths inches.

The' Base of the coil is
fifteen inches long and six
inches wide and is made of
wood.

The coil is assembled by
placing the primary across
the base and exactly in the
center. Two long wood-
screws passing through the
base and into the primary
heads will hold it firmly in
position.

The secondary is passed
through the center of the

primary and supported in that position by two hard rubber
supports, four inches high, seven-eighths of an inch wide and
one-half of an inch thick. A brass wood-screw is passed
through the top part of each of the supports into the sec-

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