Vocal gas alarm tested . Test are being made at Hendon to discover the best warnings in the event of a gas attack from the air . Sirens are being sounded separately from Hendon Police station , which is in the centre of a ring of observers , policemen the stationed in pairs at distances of 100 , 200 , 300 , 400 yards away . They have sound recording instruments to determine the power and penertration of the various notes . The findings of the observers will be recorded on a special map , and the best siren discovered . Photo shows , a vocal gas alarm being tried at at Hendon . The microphone is worked from the vocal cords as the mouth is covered by a gas mask . There is an amplifier on the operator ' s back and he carries the loudspeaker in his hand . 10 March 1936