An account is given of the use of a thin foil one-dimensional sonic spark chamber as a focal plane detector for a double focussing magnetic spectrometer. Using a continous flow filling of argon-ethanol and 0.0025 cm aluminium electrodes with a separation of 0.86 cm, spatial resolutions of 9 mm fwhm have been attained with particles in oblique incidence, at angles of 57° to the electric field. The chamber is used in coincidence with a double scintillator telescope, enabling particle identification and selection to be performed before triggering, on the basis of range estimation in a set of variable absorbers before the scintillators. The resulting backgrounds, with a time resolution of 1.2μs, are negligible. The system operates reliably at 50 c/s at efficiencies differing from 100% by less than 1 part in 10 4, along the whole lenght of the focal plane. Line spectra, with good mass resolution, have been obtained for charge one particles resulting from reactions induced by 30 MeV protons. Overall energy resolutions of 60 keV have been attained for these particles.