A commutator or pole-changing switch is often added to change the polarity of the battery current. The diagram of connection is shown in Fig. 10. When the levers are as in the figure, the circuit is broken and no current flows through the coil.
Coils in Series.
Ruhmkorff coils can be connected in series, but it is not to be recommended. When it becomes necessary, however, the cores should be removed, and one long core inserted, extending through each primary. This will bring the time constants of each primary coil together and prevent the interference otherwise present. The primary coils and secondary coils are connected in series by assuming that they are but adjacent sections of one complete instrument. Of course, as the resistance of the primary is raised, the electromotive force of the battery must be raised also.
Oil Immersed Coil.
A highly satisfactory induction coil can be made without much labor and few tools, and will prove useful in many experiments which would not warrant a more expensive instrument.
Make a bundle of soft iron wires, No. 22 B W G, for the core, ten inches in length and one inch or more in diameter. Wrap this with insulating tape or even ordinary tape to prevent the primary coil from coming in contact with the iron. Now, wind on a primary of two layers No. 14 B & S gauge cotton-covered copper wire, and insert the coil into a hard rubber (or glass preferred) tube large enough to hold the coil tight and to project an inch or so beyond the core ends.
A secondary coil of about one pound No. 36 cotton-covered magnet wire should now be made on a hard rubber spool, the hole through centre of this spool must be at least one inch larger in diameter than the diameter of the primary cover. This spool should not exceed four inches in length, and is to be slipped over the primary coil and held suspended by blocks of wood in such a manner that it does not touch the primary coil or cover. The whole outfit is now immersed in an earthenware or glass vessel filled with linseed or heavy paraffin oil. The contact breaker and condenser will be mounted independently; the condenser for the two-inch spark coil will be suitable (see Table on [page—7]).
"Tesla" Coil.
The coil just described, without contact breaker or iron core, can be connected up and used in place of a "Tesla coil," which it resembles. The coils used by Nikola Tesla are so many and varied that it becomes a difficult task to describe a mode of construction which will meet the wants of those who ask for "Tesla" coils. The American Electrician gives a description of one wherein a glass battery jar, 6 inches × 8 inches, is wound with 60 to 80 turns of No. 18 B & S magnet wire. Into this is slipped a primary, consisting of 8 to 10 turns of No. 6 B & S wire, and the whole combination immersed in a vessel containing linseed or mineral oil.