Hertz experiments with electromagnetic waves

Hertz experiments with electromagnetic waves
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Hertz originally conducted this experiment in 1887.

Google translation of the YouTube blurb (the original is in Italian):

Hertz's Experiments with Electromagnetic Waves

The experimental setup consists of two metal parabolic mirrors. At the focus of the transmitter mirror is a spark oscillator connected to an induction coil.

In the receiver is a metal filings coherer connected to a Grenet battery and an electric bell.

The oscillator's spark plug is filled with Vaseline oil, which allows for higher-energy discharges.

The trains of electromagnetic waves produced by the oscillating sparks are focused on the coherer. Its resistance suddenly decreases, and the battery current activates the bell. A light tap on the coherer restores its non-conductive state, and the bell stops ringing.

A copper plate is inserted between the oscillator and the receiver to reflect the electromagnetic waves. The coherer, shielded by the plate, remains in its high-resistance state, and the bell does not ring.

The transmitter and receiver are oriented so that the electromagnetic waves produced by the former are reflected back to the latter by the copper plate. The coherer responds to the waves, its resistance decreases, and the bell rings.

I understand the parabolic mirrors, but what is a coherer? YouTube knows:

Vintage Technology: The Coherer
Ludic Science