The electrical conductivity of air results from the presence of molecular cluster ions, which are the smallest elements of the atmospheric aerosol size spectrum. Atmospheric cluster ions are produced by natural background radioactivity and cosmic rays, and then lost by self-recombination or attachment to aerosol particles. Cluster ion concentrations determine the charge on atmospheric aerosol, which strongly influences the removal rates of aerosol particles to cloud droplets. Little is known quantitatively about the detailed variability in molecular cluster ions in the atmosphere, and simultaneous measurements of the positive and negative conductivity are especially lacking. The atmospheric conductivity, ion mobility and ion number concentration can be measured using electrical measurement principles originally established by H. Gerdien in 1905. This paper describes the basis for an ion instrument employing two symmetrical Gerdien-type cylindrical sampling electrodes, allowing simultaneous measurements of positive and negative air conductivity using a voltage decay method. A unique feature is a semiconductor switch to initiate the voltage decay, as the mechanical reset switches previously employed have been found unreliable at low temperatures. Rather than carrying the sensors on a research aircraft, flying the instrument on a meteorological radiosonde provides a routine and inexpensive platform from which to obtain atmospheric ion measurements. The radiosonde provides the polarising battery voltage for the electrodes, as well as additional meteorological measurements, location determinations and data telemetry to the surface. As the instrumentation is unlikely ever to be recovered, the equipment must be disposable and inexpensive, whilst well-characterised.