One or two cells of battery should be sufficient to operate the coil. Connect the battery to the two binding posts. Adjust the thumbscrew until the contact on the end just touches the contact point on the vibrator spring. The current will then flow through the primary winding and magnetize the core. The magnetism of the core will draw the armature towards it, thus pulling the contacts apart and interrupting the current. The process will repeat itself very rapidly at a speed depending upon the adjustment of the thumbscrew and as long as the battery remains connected.
[Illustration: FIGS. 107 and 108.—Two Types of Handles.]
The handles for the coil are very easily made out of a piece of brass tubing about two and one-half inches long and five-eighths of an inch in diameter. They can also be made by rolling a piece of sheet brass or tin into a tube and soldering the edges together. The illustration shows two methods of making the handles and attaching the cords. The handle in the upper part of the illustration is made from a piece of tubing. A wooden plug or cork is fitted into one end. The wire passes through a hole in the centre of the plug and is turned back over the edge so that when the plug is forced into the tube, the wire will make contact with it. The handle in the lower part of the illustration is made from one of the brass cylinders in which the indelible ink for rubber stamps is supplied. A hole is bored in the bottom through which the wire passes. The end of the wire is soldered to a piece of stiff brass wire bent into the shape of a hairpin. The ends of the wire are spread apart so that they will spring against the inside of the tube. The connector is then pulled up inside of the handle.
The flexible wires or "cords" as they are sometimes called, attached to the handles, should be about eighteen inches long. The opposite ends to the handles should be led through two holes in the base, in front of the bobbin. The left hand cord should be connected to the vibrator post and the right hand cord to the outside secondary terminal.
The coil is now complete, ready for operation. Connect the battery to the binding posts. Start the interrupter and grasp one of the handles in each hand. Draw the regulator tube out of the bobbin gradually and the current will gradually grow stronger and stronger until it is impossible to hold the handles. When the regulator tube is all the way in the bobbin it is practically impossible to feel the current. Of course any intermediate strength between the two extremes can easily be secured. The shock from the coil is harmless. When the regulator is all the way out, the shock is very strong. The only result, however, is to make the person receiving it drop the handles. The current from medical coils is often prescribed by physicians for rheumatism and nervous disorders, but it must be properly applied in order to be of benefit.
CHAPTER XI. THE CONSTRUCTION OF SPARK COILS.
A Spark Coil is one of the most interesting pieces of apparatus that an experimenter can possess. Not only can it be used to transmit wireless messages, but numerous and various experiments may be performed with its aid.
The purpose of a spark coil is to generate enormously high voltages which are able to send sparks across an air space which ordinary battery currents of low voltage could not possibly pierce.
[Illustration: FIG. 109.—Induction or Spark Coil.]
It consists of an iron core surrounded by a coil of heavy wire known as the "primary" and a second outside winding of finer wire known as the "secondary." The primary is connected to a few cells of dry battery in series with an "interrupter."