Ruhmkorff Coil.
Fig. 44 shows a diagram of a Ruhmkorff coil, the letters referring as follows:
C the iron core, P the primary coil wires, I the insulating tube between primary P and the secondary coil S. In small coils this may be dispensed with, and a heavy layer of paraffin wax laid over the primary coil. D D are the ends of the secondary, showing sparking taking place between a pair of balls (or between the electrodes of a gas burner); R is a stiff spring fastened to the coil base and carrying a soft iron hammer, which is attracted toward the iron core, C, when current passes through the primary coil and magnetizes it. L is a battery, J, a condenser, to be more fully described later on. When the spring R touches the adjustment screw A at B, as they are insulated from each other, contact is made and reference to circuit will show that the current from battery L flows through primary coil, magnetizing the core and attracting soft iron hammer on R. As this bends forward, it breaks contact at B, the core loses its magnetism and the spring flies back, to again make contact. This is repeated many times per second.
Fig. 44.
As a heavy spark occurs at B on the break of contact, the condenser, J, is attached at M K. This is a series of insulated tinfoil sheets, which has the property of nullifying the spark at B, and so preventing the waste of platinum with which both adjustment screw A and spring R are equipped.
A Ruhmkorff coil differs from a simple primary coil in three main points. Two separate coils instead of one; high insulation, and a primary coil of few turns. In the simple coil we desired self-induction; here, we desire to avoid it as much as possible.
The average size Ruhmkorff coil, for jump spark work, would be one giving a 2-inch spark, specifications for which are as follows:
Spool—Nine inches long by one inch in diameter. End cheeks 4 inches high by 3 inches wide.
Core—Sufficient soft iron wires, 9 inches long by No. 22 B. W. gauge as will fill the spool tube.
Primary—Two layers No. 14 B. & S. gauge cotton-covered copper wire.
Secondary—Two and one-half pounds No. 36 B. & S. gauge double cotton or silk-covered magnet wire wound in four sections (or more than four sections, if feasible).
Condenser—Seventy sheets tinfoil 4 by 7-1/2 inches; 80 sheets condenser paper 5 by 8 inches.