In the history of aerosol science the measurement of the mobility of ions has played an important part.  This paper recalls several of the advances made as a consequence of changing needs and improved equipment, particularly those associated with the name of McClelland. For users of sophisticated modern equipment, such as differential mobility analysers, it is worth recalling some of the steps that led to their development.

John A. McClelland was born in Coleraine in 1870.  He graduated with an MA in Physics from Queen’s College Galway in 1894 and spent four years on research in the Cavendish Laboratory with J.J. Thomson from1896 to 1900.  Here he did pioneering work on the measurement of the mobility of ions, particularly those from flames and incandescent bodies. In 1900 he was appointed professor of physics in University College Dublin (UCD) where he established a flourishing school of aerosol science that had a profound influence on the development of Physics in Ireland.  Among his students were the brothers John J. and Patrick J. Nolan who carried on this line of research in Dublin after McClelland’s death in 1920.  Through other graduates this line of research spread for a while to all the constituent colleges of the National University of Ireland at Cork, Maynooth and Galway, where it still continues. [1]