The wire for the secondary winding consists of about No. 36 B and S gauge silk covered magnet wire, the amount used varying considerably, depending on the desired voltage of the secondary current.
When all the wire has been wound on, the ends are brought out to the binding posts, the coil is soaked in shellac dissolved in alcohol and baked, or in melted paraffin or a paraffin compound, and allowed to cool. It is then placed in either a cylindrical hard rubber shell or in a hard wood box.
The proportions of such coils vary greatly; for motor cycle use they are made long and of small diameter (10×21⁄2 inches for instance), while for some other purposes short and thick coils are found more convenient.
Ques. How may the coil just described be connected for demonstrating purposes?
Ans. Connect the ends of the secondary winding to fixed insulators and bend the ends so they are about 1⁄8 inch or less apart. Connect one end of the primary winding to a battery and brush the other end of the primary winding against the other terminal of the battery as indicated in fig. 137.
Ques. What happens when the primary circuit is made?
Ans. An electric pressure is induced in the secondary circuit, but of not enough intensity to cause a spark to jump across the air gap.
Ques. What happens when the primary circuit is suddenly broken?
Ans. A spark is produced both at the point of break in the primary circuit and at the air gap in the secondary circuit.