A resistively heated metal wire is shown to be a source of charged atomic clusters consisting of only a few atoms. They are size classified with a differential mobility analyzer, and their relative abundance is determined as a function of size. AgnK+ clusters are obtained from wires containing silver with traces of potassium to provide the electric charge. First principles calculations reveal that the abundance observed can be fully explained by the energetic and chemical stability of the neutral cluster and K+ attachment energy. K+ attachment is a non-invasive way of charging, as the observed AgnK+ cluster properties are similar with respect to the pure Agn clusters in terms of energetic and electronic stability and cluster structure. The equally sized clusters are basically available for reactivity, coalescence and deposition studies, and the method is extendable to other materials.