Hydraulic Microphones

Gallery opened 30 June 2018

Updated: 7 July 2018

Vanni hydraulic microphone added
Microphone history added

In the early days of radio, continuous-wave transmission suitable for voice communication could not be generated by valves as suitable types had not yet been developed. A continuous-wave carrier could be produced by Duddel's singing arc, or Fessenden's high-frequency (50 kHz) alternators. The only way to modulate audio frequencies onto the carrier was to pass the whole of the antenna current through a carbon microphone or equivalent. These naturally got very hot and water-cooled carbon microphones were used; given the heat and also the high voltages used, early broadcasters probably approached their microphones with some trepidation. A similiar approach was used in the Tannoy power microphone in 1937.
A lesser-known alternative was the application of hydraulics to microphones; at least two models of hydraulic microphone were produced in Italy in the 1900's.

A BRIEF HISTORY OF MICROPHONES
The first microphone was invented by the German Johann Philipp Reis. This had a flexible diaphragm connected to a metallic strip that made and broke contact. This was only capable of transmitting OFF or ON unless you could be sure of some sort of partial contact. This did not necessarily make it unworkable- the speech signal is very robust. For example, you can send perfectly understandable speech by just transmitting a click at each zero-crossing of the waveform. However the Reis device pushed this too far and it was not capable of reproducing understandable speech.
The first practical microphone was the carbon type invented by both David Edward Hughes and Edison in 1878, which was developed to give adequate audio quality.
There is some stuff on microphone history in Wikipedia.

THE MAJORANA HYDRAULIC MICROPHONE

Left: The Majorana hydraulic microphone: 1909
The first Italian hydraulic microphone was devised by Count Quirio Majorana (1871 - 1957) of the Italian Post Office.
In the Majorana microphone a fine column of water fell vertically from a jet; the pressure at this point was modulated by the microphone diaphragm, altering the diameter of the column and so varying its electrical resistance as it passed between the two contacts at the bottom of the case. Since the resistive element was water (or rather water with sulphuric acid or salts added to increase its conductivity) cooling was built in, though presumably if excessive power was used clouds of steam would start to issue from the microphone.
Majorana's high-power microphone was first used for long-distance telephony in Italy in 1906. Since amplification en route was not then possible, (see the Electro-mechanical amplifier page) the more powerful the signal from the microphone, the futher the distance you could communicate.
The label on the display says "Vibrating jet hydraulic microphone for radiotelephony". The mouthpiece is the short horn at the top, fixed by four screws. The spigot at the very top is the connection for the incoming water.
I wonder what sort of audio this machine produced. There appear to be lots of non-linear mechanisms. Firstly, the rate of flow through a nozzle
The label on the plinth says "Microfono idraulico MAJORANA".
Milan Science Museum. Author's photo, May 2018

Left: The Majorana hydraulic microphone: 1901
This shows the internal operation of an early version of the Majorana hydraulic microphone.
The microphone is modulating the current from a battery, and this passes through what is presumably an audio-frequency transformer. This seems to indicate it was being used for line telephony rather than radio work.
I put the german technical terms into Google translate, (ignoring the accents, as usual) and this is what came back:
sauerstrahl = jet.
achatscheibe = agate slice. (Presumably this describes an agate nozzle)
offnung = opening. (1mm diameter)
glasrohre = glass tube.

See Figure 8 above.
Image from Archiv fur Post und Telegraphie, No 24, 1901, p297.
The date indicates that Majorana's microphone was around long before its use in long-distance telephony in Italy in 1906.

THE VANNI HYDRAULIC MICROPHONE

Left: The Vanni hydraulic microphone: 1912
This hydraulic microphone for radiotelephony was introduced by Guiseppe Vanni. The microphone was connected in the aerial circuit of a transmitter, and providing a variable resistance to earth.
The configuration has changed from that of Majorana; now the liquid jet remains stationary, but the left electrode of the pair is moved by the microphone diaphragm.
Milan Science Museum. Author's photo, May 2018

Left: The Vanni hydraulic microphone: 1912
This diagram is displayed next to the Vanni hydraulic microphone in the Milan Science Museum. The microphone is again connected in the aerial circuit of a transmitter, and provides a variable resistance to earth.
See Figure 4 above.
Milan Science Museum. Author's photo, May 2018