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Six significant items from the period of early radio, 1900 - 1914

Ship's radio room before 1920

Image: Ship's radio room before 1920

Source: Museum Victoria

Museum Victoria's Collection includes six significant items from the period of early radio; a period often referred to as the 'Spark era'.

 

Souvenir cigarette lighter, 1912

(HT 16883)

 

These lighters were presented to all male delegates of the 1912 International Radiotelegraphic Conference as souvenirs. It is signed 'Marconi's Wireless Telegraph Co. Ltd.'

The lighter, silver on a marble base, was designed in the form of a Rotary Spark Gap. A spark-gap transmitter generated radio frequency electromagnetic waves. These devices served as the transmitters for most wireless telegraphy systems for the first years of radio (1900 -1915).

 

Female delegates were presented with silver perfume bottles.

 

Display demonstrating spark wireless

(ST 038448)

 

This display reproduced the experiment by Sir Oliver Lodge, (circa 1898) demonstrating  the production and detection of an electromagnetic field and the effect of tuning. It was probably in an exhibition in the first half of the 20th Century at the Science Museum of Victoria or its predecessors.

 

The following four items are representative of earliest radios that were in use in ship to shore and ship to ship communication. They are all part of early receiving equipment. At the time of writing, there is no equally significant early transmitting equipment in Museum Victoria's Collection.


Discharger - Marconi, Fixed Gap, Radio Transmitter, circa 1912

(ST 015462)

* used mainly with small radio transmitters on ships.

Discharger is the term used to describe the device in which a spark jumps a gap.

The fixed gap was the first type of discharger used by Marconi; others introduced the quenched gap. Marconi replaced the fixed gap with the rotary gap discharger. These improved the efficiency of spark transmitters and permitted the use of higher power.

The spark gap is enclosed in a lead lined wooden box to reduce the considerable noise produced during operation; the spark gap was often kept in a ship's radio room. The box is also lined with asbestos to protect the wood from the heat of the spark.

Magnetic Detector

 (ST 015464)

* used mainly in ships for detecting spark radio transmissions in radio receivers.

The magnetic detector was developed by Marconi in 1902 to provide a more sensitive detector than the coherer. It was based on earlier work by Ernest Rutherford. This type of detector became the standard on all ships using Marconi apparatus until 1914. It was particularly well adapted to the detection of damped wave-trains of the type produced by spark transmitters.

The magnetic detector produced a sound in the operator's headphones (called 'head-telephones' or just 'telephones' at the time) which could be interpreted as the dots and dashes of the signal being received. The overall sensitivity of the system was dependent on the good hearing of the operator. Unlike the coherer based receiver, the magnetic detector was unable to automatically produce a permanent record of the received signal; the only record was what the operator wrote down as the signal was received.

 

Coherer Based Detector Assembly

 (ST 18546)

* an early detector of radio waves, largely supplanted by magnetic and crystal detectors.

One advantage that the coherer had over other early forms of detector was that it could be connected to a morse inker (ST 018547) which would produce a permanent record of a received signal without operator intervention.


Multiple Tuner

(ST 015459):

* used to select required radio wavelength (or frequency) for reception.

 

The practical realisation of a tuned transmitter resulted in an arrangement which contained two tuned circuits: the main oscillatory circuit where a capacitor is charged from a high voltage and then discharged by a spark into an inductor creating a damped oscillation and a second tuned circuit designed to maximise energy transfer to the aerial. The adjustable inductors used were called 'jiggers'.

The tuned receiver developed by Marconi used a fairly complex device called a multiple tuner. This device was patented in 1907 and was designed to provide maximum energy transfer from the aerial to the detector at the required frequency but to reject all other frequencies. This device could be tuned over a range of wavelengths from 2000 m to 50 m corresponding to frequencies from 150 kHz to 6 MHz.

 

Background: the period of early radio


This period contained the key events in the development of radio.

 

During this time it moved from a scientific curiosity, only experienced in the laboratory or in large-scale scale experiments and demonstrations to the stage where wireless equipment was becoming common on ships. The main use of radio at that time was for ship to shore and ship to ship communication.

During this period radio communication was telegraphic and was referred to as 'wireless telegraphy'. Although the Fleming diode and DeForest audion were invented during this period they were not widely used.

 

The main components of a wireless telegraph station were a spark transmitter and a receiver which could be based on a crystal detector, a magnetic detector or a coherer. None of these detectors were particularly sensitive and no means of amplifying the radio frequency signal existed so to achieve a useful range, aerials had to be as large as possible and the transmitter power had to be as high as was practicable. A typical ship's spark transmitter was rated at 1.5 kW. Frequencies used tended to be low as it was believed at that time that the longer the wavelength the better the range. Wavelengths from 6000 m to 600 m (50 kHz to 500 kHz) were where most of the activity took place.


 

 

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