Electric current was passed through the Crookes tube. Next, the Geissler tube was replaced by a Crookes tube. The nature of the Crookes tube "cathode ray" matter was identified by Thomson in 1897. The electronic vacuum tubes invented later around 1904 superseded the Crookes tube. Thomson constructed a Crookes tube with a better vacuum. It was a cold-cathode diode, a modification of the Crookes tube with a phosphor-coated screen. It was a cold-cathode diode, a modification of the Crookes tube, with a phosphor-coated screen. The Crookes tube was improved by William Coolidge in 1913. He explained that the Crookes tube had "a more complete vacuum" and "other improvements made".