Given that transmitters at the time were dependant on the aerial length to determine the wavelength, swaying of the aerial had the effect of varying the height of it & hence its capacity to "ground". This was problem enough on the shore, but on a ship it was difficult.
This experimental cruise was to evaluate three experimental concepts under sea-going conditions. One of these was to control the transmitter frequency by a "separately excited grid circuit".
The second was to load a long aerial with inductance to allow operation on 4,000 metres.
The third was to use a spark gap attachment. Judging by the picture this cumbersome apparatus was taken from a Condenser Transmitter Large Power Shore Station No.14.
Although this might seem to be a retrograde step it was to provide a note or at least a noise for operators at receiving stations. The normal transmitter was a continuous wave (CW) that was keyed. In those days there were no BFOs to provide a tone in the receiver nor MCW (Modulated CW) to send a tone out on the transmission.
The cruise of HMS Yarmouth from 4th Sept to 26th Sept 1924 was from Portsmouth to Madeira, Las Palmas, Gibraltar & return. The schedule of transmissions on the various wavelengths with the different accessories was monitored & recorded by a number of listening stations.