The best method of finding the most sensitive adjustment for the apparatus is to place the transmitting and receiving stations only a few feet apart from each other in the same room. After you learn how to adjust the coherer and relay properly you will then find that you can move the transmitter quite a distance away from the coherer and it will still operate. Of course a coherer outfit is not very satisfactory and reliable for the regular reception of wireless signals even over short distances because of its irregular working. It is, however, very useful for sort of a calling or signal outfit. It may be connected to the tuning coil by means of a double-pole, double-throw switch and left in the circuit so as to indicate by the working of the tapper when any of the other stations in the near neighborhood are sending. The detector can then be quickly inserted in the circuit in place of the coherer by throwing the switch in the opposite direction and the signals read in the telephone receiver. CHAPTER XVII. HOW TO BUILD A TESLA HIGH FREQUENCY COIL. IF the discharge from a Leyden jar or a condenser is passed through a coil of wire acting as a primary and the primary is provided with a secondary coil of a larger number of turns, a peculiar current known as high frequency electricity is generated in the secondary. Such a device is known as a Tesla Coil or Transformer. [Illustration: FIG. 170.—Tesla Coil Circuits.] When a Leyden jar or a condenser discharges through a coil of wire, the spark which takes place does not consist simply of a single spark passing in one direction only, but is really made of a number of separate sparks passing alternately in opposite directions at the tremendous rate of from one hundred thousand to one million times per second. A Tesla high frequency coil opens a field of wonderful possibilities to the amateur experimenter and brings innumerable weird and fascinating experiments within his reach. The Tesla coil described in the following pages will give sparks of high frequency electricity two or three inches long from the secondary, when used in connection with a two inch spark induction coil. If used in connection with a small high potential wireless transformer, the high frequency discharge can be increased to six or eight inches. A Tesla coil, suitable for use with a smaller spark coil, say one capable of giving sparks from one-half to one inch in length can be made by following the same plans but cutting all of the dimensions in half. Make the secondary six inches long and one and one-half inches in diameter instead of twelve inches long and three inches in diameter, etc. [Illustration: FIG. 171.—Secondary Tube.]