The base forms the top of a flat box in which the condenser lies; but there are a few points worth considering right here. As the connections of the coil will probably be under the base, a sufficient space must intervene between the base and the top of the condenser. It is a good plan to lay the condenser at least one half inch below the top of this box, and fill up to, say, one eighth of an inch with melted paraffin, leaving the condenser wires projecting for attachment. The connections of the primary coil and contact breaker should by all means be soldered, not simply wires held under screw nuts. And, moreover, all wires under the base should be so run that they do not cross one another, which precaution only requires a little planning. Then, when the connections are all made and the base laid on top of the box, it can be pressed down if the paraffin be warm, so that the screw heads and wires mark out their own channels and cavities in which to lie.
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.
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.
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.