In Hertz'* experiment, oscillating electric charge on radiating electromagnetic waves carries energy which is given at the cot of kinetic energy of oscillating charges.
Hertz Apparatus : The experimental arrangement used by Hertz for the production and detection of electromagnetic waves in the laboratory, is shown in the figure. His experimental arrangement consists of two metal sheets P1 and P2. These sheets are connected to a source of very high voltage (i.e. an induction coil, which can supply a potential difference of several thousand volts). S1 and S1 are the two metal spheres connected to the metal sheets P1 and P2. The distance between the metal sheets is kept nearly 60 cm and that between the sphere is normally from 2 cm to 2.5 cm.
The two plates P1 and P2 form a capacitor of very low capacitance (C). The circuit containing P1 and P2 (being completed by conducting wire), has also some low value of inductance L. It thus forms an LC-circuit. Detector (D) consisting of a coil to the ends of which two other small metal spheres S1'and S2' are connected.
Working of Hertz apparatus : Due to the existence of very high voltage, air present in the gap across the plates or spheres S1 and S2 gets ionised. Due to the presence of the ions or charged particles, the path between the spheres S1 and S2 become conducting. As a result of this, very high time varying current flows across the gap between S1 and S2 (as plates P1 and P2 form an LC circuit). Due to this, a spark is produced. Since, sheets P1 P2 form an LC-circuit, hence, electromagnetic waves of
Function of the detector D :Hertz detected the electromagnetic waves by means of a detector D, kept at suitable distance from the conducting spheres S1 and S2. Detector D is made of two similar conducting spheres and joined to the ends of a coil to form another LC circuit. The frequency of this LC circuit is made equal to the frequency of electromagnetic waves reaching it. The frequency can be adjusted by changing the diameter of the coil of the detector and by changing the distance between S1' and S2' . Hertz placed the detector in such a way that the magnetic lines of force produced by the oscillating electric field across the gap between S1' and S2' are normal to the plane of coil (C). When the magnetic lines of force cut the detector coil, an emf is induced in it. Hence, air in between the gap gets ionised. A conducting path becomes available for the induced current to flow across the gap. Thus, the spark is produced between S1' and S2' . Hertz also observed that the spark across the gap was the greatest when S1' and S2' were parallel to each other. This clearly established that electromagnetic waves produced,were polarised i.e. 
and 
always lie in one plane.
