Rotary Spark Gap Transmitter #1

This shipboard transmitter, built by Marconi  Wireless Telegraph Company of Canada (MWTCC), has no model so its being assigned designator  Rotary Spark Gap Transmitter #1. An article written about it, refers to this transmitter as the " All Canadian Ship Set".


Frequency range: Three wave - 300, 600 or 800 metres wavelength
Type: Rotary Spark Gap transmitter
Power output: Low, medium and full .
Vintage:  This transmitter is described in the April 1922 issue of Canadian Wireless Magaziine. The full article is here.


The Canadian Wireless  article said that this transmitter was new  when, in fact, it was  already obsolete in 1922. Tube transmitters transmitted twice as far and with a much narrower band width. By international agreement, spark transmission was to be outlawed starting in 1930 but the transition from spark spanned several years.

At the International Radio Telegraph Convention of Washington, held in 1927, an agreement was reached in the following general terms:

(a) The use of damped wave trains (Type B waves) employing frequencies below 375 kc/s were forbidden from 1st January, 1930, onwards.

(b) No new spark transmitting installation could be fitted in a land or fixed station or in shore station from the 1st January, 1935.

(c) No new installations for the emission of spark wave trains could be fitted in ships or in aircraft from 1st January, 1930. The only exception to this was the situation where the transmitters used 300 watts or less of power as measured at the input of the supply transformer.

(d) Use of spark transmission on all frequencies were forbidden from the 1st January, 1940, except for ship installations. In those cases, the transmitter, would have to meet the power criteria of article (c). In the Royal Navy, by the end of the 1930s, a radio installation employed a spark gap transmitter for emergency purposes only under the following conditions:

(e) A spark attachment, for use as a stand-by transmitter in the event of a complete breakdown of the tubes or essential components of the main tube transmitter(s).

(f) An emergency coil, designed for use when power from the ship's mains also fails. It consisted of an induction coil and an associated oscillatory circuit. It derived its power supply entirely from batteries. These emergency transmitters, while rarely ever used, were fitted only to provide a last line of "defence".

Some ship owners converted spark transmitters  to tubes. Unfortunately the article does not tell us the model name or number , power, how many were sold , or name of the ship in the photo. Numerous photos of this transmitter and its components are in the Marconi Collection in the National Archives. This was probably the last of the ship spark transmitters.

/rotary_sg_xmitter_01_103_1290b.jpg The rotary spark gap transmitter installation aboard a ship. The big wheel was used to change the wavelength  (LAC photo) 
On the table , second shelf up (L-R) is the model 2846 B and the 3986 detector amplifier/ .The two headphones were made by S.G. Brown. Under the lower table is an unidentified device.  (LAC Photo) 
Transmitter with rotary unit extended. (LAC photo) 
Rear view of transmitter.(LAC photo) 
This variant of the rotary spark gap transmitter is all buttoned up.(LAC photo) 
All photos in this table provided by Lewis Bodkin, 



This is an under-the-table key used with the rotary spark gap transmitter. Only the stem and the knob protruded above the table surface for the safety of the operator. (LAC photo)
Rotary unit by itself. (LAC photo) 
RF coil sample. (LAC photo) 
Starting and dynamic resistor assembly. (LAC photo) 
High voltage capacitor sample. Made by Dubilier Condenser Co.(LAC photo) 
 Three band coils on a rotor. This assembly is (called an oscillation transformer in the article (LAC photo) 
 All photos in this table are from the Museum Of Science and Technology, Ottawa. 
Here is a comparison between the prevailing transmitter technologies of the post WWI era:
Advantages Disadvantages
(1) Robust and durable. 
(2) Faults easily cleared. 
(3) Emits a wave, which forces its 
way well through interference. 
(1) Wasteful of power. 
(2) Short range as compared with continuous wave 
(3) Interferes badly. 
(4) Requires high insulation on account of initial peak 
Advantages Disadvantages
(1) Robust and durable. 
(2) Faults easily cleared. 
(3) Can be easily constructed to handle large powers. 
(1) Slow in starting up. 
(2) Presents certain "keying" difficulties. 
(3) High-power sets radiate harmonics badly. 
(4) Unsuitable for use in a fleet, as it is not possible to " listen through" for Admiral's signals, messages of distress, etc. 
(5) High frequencies cannot be produced. The upper value is 250 KHz. 
Advantages Disadvantages
(1) Radiates a wave which is very pure 
(2) Easy to key
(3) Very suitable for radiotelephony
(4) Suitable for high power work. 
(1) Requires expert supervision and maintenance
(2)Iys frequency cannpt be varied as in other systems. 
(3) Its initial cost is high.
(4) Only suitable for low frequencies. 
Advantages Disadvantages
(1) Radiates a very pure wave.
(2) Easy to key.
(3) Very suitable for radiotelephony
(4) Transmits damped or undamped waves at will.
(5) Quick in starting up.
(1) Valves are fragile and require periodic replacement.
(2) If faults develop, they are not easy to trace as in other systems.

Contributors and Credits

1)  Lewis Bodkin  <05bodkin555(at)gmail,com>
2) Jerry Proc

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Mar 2618