It is a curious fact that the kind of insulator between the charged conductors of a condenser affects its capacity. Thus if glass, paraffine, or beeswax is between the plates instead of air, the plates will "hold" more electricity at the same potential. For this reason condenser plates are often separated by sheets of glass, paraffined paper, or mica. [Illustration: FIG. 211.--A Leyden jar and a discharger.] =232. The Leyden Jar.=--A convenient form of condenser, used as long ago as 1745, is the Leyden jar. It consists of a glass jar (Fig. 211) coated part way up, inside and out, with tinfoil. The inner coating is connected by a chain to a knob at the top. The Leyden jar is charged by connecting the outer coating to the earth while to the inner coating is given a charge of either kind of electricity. The other kind of charge is developed by induction upon the outer coating, and each charge binds the other. To discharge a jar, a conductor, as a wire, is connected first to the outer coating and held there while the other end is brought to the knob at the top. A bright spark is produced when the two charges combine. It is best not to let the discharge from the jar pass through the body unless one is certain that only a very small charge is present. =233. Oscillatory Discharge.=--The discharge from a Leyden jar is an interesting phenomenon. The rush of electricity from one coat to the other does not stop when the two coats are exactly neutralized but continues until the two plates are charged just oppositely to their condition at first, then a rush of electricity in the opposite direction occurs. This alternation continues several times and constitutes what is called the _oscillatory discharge_. (See Fig. 414.) This oscillatory discharge sets up waves in the ether. These are called _Hertzian_ waves in honor of their discoverer, Heinrich Hertz. They are the ether waves used in wireless telegraphy. A _lightning flash_ has been shown by photographs and by other means to be oscillatory. This fact supports the idea that the electrical conditions just preceding the stroke of lightning reproduce a condenser on a large scale. The charged cloud is the upper charged plate, the earth beneath, charged by induction from the cloud, is the lower charged plate, while the air between is the insulator or _dielectric_ as it is sometimes called. Important Topics 1. Potential: high, low, zero, positive, negative, similar to temperature and air pressure. 2. _Capacity_ affected by (1) area, (2) induction. 3. Condensers, Leyden jar, parallel plate. 4. Oscillatory discharge, conditions, results. Exercises 1. Is the air a conductor? Explain. 2. Can the Leyden jar be strongly charged if the outer coat is insulated? Explain. 3. Upon what two conditions does the capacity of a body depend? How in each case? 4. Would a lightning discharge produce wireless waves? Explain.