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." Every time that the interrupter shuts off the battery current in the primary, currents are induced in the secondary which are of sufficiently high voltage or pressure to leap across a space in the shape of sparks. A spark coil is not very hard to construct, but it requires careful workmanship and patience. It is not usually a job which can be finished in a day, but will take quite a little time, especially in winding the secondary. In describing the construction of a spark coil in this chapter, directions and dimensions have been given for a one-quarter inch coil. The general method of procedure is the same for all sizes of coils, the dimensions only, are different. The experimenter may therefore easily build a larger coil than the one-quarter inch by consulting the tables of dimensions. Parts for spark coils are for sale by many electrical houses and it is possible to purchase the complete parts for less than the separate materials would cost. Such parts are made by special machinery and therefore are produced quite cheaply.