The appliance consists of a large bundle of fine iron wires, which are wound over with a long coil of insulated wire. This forms the _primary coil_. It is enclosed entirely in a tube of ebonite. One end of this coil is a _contact-breaker_, which automatically interrupts an electric current flowing from a battery through the primary coil (see Fig. 81). A _hand-key_ is also placed in the circuit to stop and start the primary current as desired. Over the primary coil is a very long coil of much finer silk-covered copper wire, called the _secondary coil_. The length of this coil is very considerable, and may amount to many miles. The secondary coil is divided into sections all carefully insulated from each other. Another important part is the _condenser_. This consists of sheets of tinfoil laid between sheets of waxed paper, alternate tinfoil sheets being connected. The arrangement forms virtually a Leyden jar, and one set of tinfoils is connected to one side of the automatic break, and the other to the adjacent side. When, therefore, the primary circuit is interrupted by the break, the condenser is at that moment thrown into series with the primary coil. The rapid interruption of the primary current causes a secondary current in the fine-wire coil. The automatic contact-breaker makes from ten to fifty such interruptions per second. At every “break” of the primary a very high electromotive force is generated in the secondary circuit, which may amount to many hundreds of thousands of volts. This very high secondary electromotive force is able to cause an electric discharge in the form of a spark between brass balls connected to the secondary circuit terminals. Coils are generally rated by the length (in inches) of the spark they can produce between brass balls about ¹⁄₂ inch in diameter. The coil most commonly used in wireless telegraphy is thus technically termed a “10-inch induction coil,” from the length of the spark this particular type of coil can produce. If the insulated brass balls, called the spark-balls, connected to the secondary terminals, are placed an inch or so apart, and the hand-key in the primary circuit is closed, a battery connected to the primary circuit will send a rapidly interrupted current through the primary coil, and a torrent of sparks will pass between the spark-balls. The primary current of the 10-inch coil is usually a current of 10 ampères, supplied at a pressure of 10 volts. If the hand-key is raised or pressed, it is possible to make long or short torrents of secondary sparks.