This sub-series consists of 19 files relating to the Second Polar Year observations, including material on the Tromsö expedition. Edward Appleton led a British party to Tromsö in northern Norway as part of the Second Polar Year observations of the ionosphere. The other members were G. Builder, W.C. Brown and R. Naismith. The party left for Tromsö in July 1932, observations began in August, and Appleton and Naismith returned to Britain later in August with the first records. Brown remained in Norway for a year to continue observations, and Appleton paid a further visit in the summer of 1933. The data thus obtained provided basic information for many research projects and publications, mainly in collaboration with Naismith and L.J. Ingram (see Coll-37/C.312 for her appointment as mathematical assistant). Almost all projects in Section C refer to or use Tromsö data in some way. The material contains Appleton’s journals of the expedition, photographs and data, background corres­pondence, and some documents relating to the writing-up of the research. These were all kept as such by Appleton, mostly in folders labelled ‘Tromsö’. It should be repeated, however, that many other topics in Section C involve data derived from the expedition.