FIG. 35.—Induction coil for wireless telegraph purposes.
FIG. 36.—Induction coil, primary and secondary.
The voltage of the currents in the secondary is high enough to leap across an air gap in a torrent of sparks. The spark of an induction coil intended for wireless work should be thick and heavy. It should be sufficiently hot and flaming to ignite a piece of paper. A rapid vibrator giving a high pitched spark is better than a slow one not only because it causes a more intense and powerful spark but because the human ear is the most sensitive to high pitched sounds and such a spark is more easily read at the receiving station.
FIG. 37.—Interrupter for induction coil.
FIG. 38.—Electrolytic interrupter.
When the coil is a very large one and operated on the 110 volt current an electrolytic interrupter is substituted for the mechanical type. One pole of the current is connected to a lead plate placed in a jar containing a mixture of sulphuric acid and water. The other side of the current is connected to a platinum wire placed in a porcelain tube so that only a small part of the lower end is in contact with the solution. When the current passes a bubble forms at the end of the wire shielding it from the liquid, and thus interrupting the current. The bubble is almost immediately discharged however and the current allowed to flow an instant before a new one forms. This operation is repeated continuously at a frequency sometimes as high as a thousand per second. An electrolytic interrupter is both an expensive and a troublesome device. There are other types of interrupters of value in wireless service but the limitations of space prohibit any account.