FIG. 23. Open and Closed Core Transformers.
Transformers are of two general types, the "open" core and the "closed" core. Closed core transformers are the most efficient. The open core transformer is similar in construction to an induction coil, the core being a straight bar, while that of the closed core machine is usually in the form of a hollow square or rectangle.
In practice, the cores of transformers are built up of laminations, usually of thin, soft sheet iron strips piled together and shaped so as to constitute a closed magnetic circuit of rectangular shape in order to avoid constructional difficulties incurred in making a ring.
LESSON NINE. THE LEYDEN JAR AND CONDENSER.
The Leyden Jar, called after the city of Leyden, Holland, where it was invented, is a form of condenser consisting of a glass jar coated up to a certain height inside and out with tinfoil.
FIG. 24. Leyden Jar.
A Leyden jar may be charged by holding the rod to the prime conductor of an electric machine, the outer coating being held in the hand. If a piece of wire connected to the outer coating is then brought near the rod a brilliant snapping spark will pass across the space.
Any two conductors, separated by an insulating medium termed the dielectric, constitute a condenser and possesses the property of receiving and retaining an electric charge.
If a charged condenser or Leyden jar is discharged slowly by allowing the electricity to pass through a high resistance conductor the flow of current increases in strength at first and then gradually dies away.