This process consists, so to speak, in draining the electric charge out of the rod, and it takes the form of an electric current in the rod, which has a zero value at the top insulated end, and has its maximum value at the spark-ball end. Also, when the oscillations take place, we have variations of electric pressure, or potential, which are at a maximum at the upper or insulated end, and have a zero value at the spark-ball end. From the rod we have a hemispherical electric wave radiated. In the language of wireless telegraphists, such a simple insulated rod is called an insulated _aerial_, or an insulated _antenna_. A simple insulated aerial has, however, a very small electrical capacity, and it can store up so little electric energy that the whole of it is radiated in the first oscillation. Hence, strictly speaking, we have no train of electric waves radiated, but merely a solitary wave or electric impulse. The effect on the æther thus produced corresponds to the effect on the air caused by the crack of a whip or an explosion, and not to a musical note or tone as produced by an organ-pipe. We can, however, make an arrangement which is superior in electric wave-making power, as follows:— [Illustration: FIG. 82.—Transmitter for wireless telegraphy.] The vertical rod, or antenna, A, is not insulated, but is connected by its lower end with one end of a coil of insulated wire, S, wound on a wooden frame (see Fig. 82). The other end of this last coil is connected to a metal plate, _e_, buried in the earth. Around the wooden frame is wound a second insulated wire, P, one end of which is connected to one spark-ball of the induction coil, and the other end to the outside of a Leyden jar, L, or collection of jars. This double coil on a frame is called an oscillation transformer. The inside of this condenser is connected to the second spark-ball of the induction coil I. When these spark-balls S are placed a short distance apart, and the coil set in action, we have a torrent of oscillatory electric sparks between these balls, and powerful oscillations set up in one circuit of the oscillation-transformer. These oscillations induce other oscillations in the second circuit of the oscillation-transformer, viz. in the one connected to the aerial. The oscillations produced in the air-wire, or aerial, are therefore induced, or secondary oscillations. The aerial wire, or antenna, has therefore a much larger store of electric energy to draw upon, viz. that stored up in the Leyden jars, than if it was itself directly charged by the coil. In order, however, to obtain the best results certain adjustments have to be made. It has already been explained that every open electrical circuit has a certain natural time-period for the electrical oscillations which can be set up in it. This is technically called its _tune_.