Using a differential air calorimeter, Rutherford measured the heat given off from a sample of radium. The radiation heated the air in the right-hand flask, and displaced the xylene in the connecting tube. This change was measured by the current in the wire in the left-hand flask. Rutherford concluded that there was an enormous amount of energy latent in radium atoms. One historian has remarked that "It is no exaggeration to state that the innocent apparatus depicted by the two flasks … marks the beginning of the atomic age!" The instruments Rutherford used are noteworthy for their simplicity and their "home-made" quality. Rutherford designed the instruments himself, and they were constructed in the physics department’s machine-shop. It was the practice to return instruments to the machine shop so that the parts could be reused, but a colleague of Rutherford’s had the foresight to see that Rutherford’s apparatus were put aside. Today several pieces are preserved at the Rutherford Museum at McGill.