English: Pierre and Maria Skłodowska-Curie in the laboratory, demonstrating the experimental apparatus used to detect the ionsation of air, and hence the radioactivity, of samples of purified ore which enabled their discovery of radium. Maria is operating the apparatus. With her right hand she is adding/subtracting known weights from a pan hanging from a strip of piezo-electric material which generates a very small elecrical charge (of the order of picoamperes) according to the weight hung on it. This is nulled against the charge accumulated on an ion chamber due to radioactivity. In her left hand she has a stopwatch to measure the rate of change of charge using a quadrant electrometer. When the weight is changed, the time elapsed for the charge to be nulled is measured by the stopwatch. The charge is indicated by a light spot on the scale in front of her projected by the quadrant electrometer, which is off the left of the picture.