Measurements of atmospheric electricity began at Kew Observatory, London (51°28'N, 0°19'W) in 1843, with recording apparatus installed by Lord Kelvin in 1861. The measured electric potential gradient (PG) at Kew has always been influenced by smoke pollution, causing a December PG maximum and July minimum. Theory links PG variations with aerosol concentrations, and the 20th century smoke measurements made at Kew permit smoke concentrations to be retrieved from 19th century PG data. Absolute calibration of the 1862-1864 PG is achieved by considering changes in the global electric circuit, for which the geomagnetic aa-index is used as a proxy. The mean annual PG in 1863 is estimated as 363±29 V m -1, and the mean smoke concentration found is 0.17±0.05 mg m -3. Diurnal variations in smoke pollution differ between the seasons, and change in their character after the advent of motor traffic.