The secondary terminals are led out to two binding-posts to which are connected two electrodes or handles by means of flexible wires. The electrode may be made of two ordinary flat strips of sheet-metal or a piece of tubing. In the latter case, the wires may be connected by wedging them in with a cork. If the handles are grasped while the battery is connected to the primary posts and the interrupter is in operation a powerful shock will be felt. The shock may be regulated from a weak current that can hardly be felt to a very powerful one by providing the coil with a piece of iron tubing of about seven-eighths of an inch inner diameter and two inches long which will slip on and oh the coil. When the tube is all the way on, the shock is very mild, and when all the way off, the shock is very strong. Of course any intermediate strength may be secured at stages between the two extremes. The current from medical coils is often prescribed by physicians for rheumatism and nervous disorders, but must be properly applied. The coil just described is harmless. It will give a strong shock, but the only result is to make the person receiving it drop the handles and not be anxious to try it again. Spark-Coils A "spark-coil" is one of the most interesting pieces of apparatus an experimenter can possess. The experiments that may be performed with its aid are varied and many. The purpose of a "spark-coil" is to generate enormously high voltages which are able to send sparks across an air space that ordinary currents of low voltage could not possibly pierce. The spark-coil is the same in principle as the small induction coils used as medical or shocking coils, but is made on a larger scale and is provided with a condenser connected across the terminals of the interrupter. [Illustration: Fig. 157.—Diagram showing Essential Parts of Induction Coil.] It consists of a central iron core surrounded by a coil of heavy wire called the "primary," and by a second outside winding of wire known as the "secondary." The primary is connected to a few cells of battery in series with an interrupter. The interrupter makes and breaks the circuit, i. e., shuts the current on and off repeatedly. Every time that the current is "made" or broken, a high voltage is induced in the secondary. By means of the condenser connected across the interrupter terminals, the current at "make" is caused to take a considerable fraction of time to grow, while at "break," the cessation is instantaneous. The currents induced in the secondary at break are so powerful that they leap across the space in a brilliant torrent of sparks. Building a Spark-Coil