A Simple Lecture Experiment with Radium Rays

Abstract

WHILST preparing some experiments for a lecture on this matter, I found a very simple device to demonstrate the important fact that radium rays are very easily transmitted through a high vacuum; and I am not aware that it has been published before in this way. I had at my disposal the strongly acting compound of radium bromide which is prepared at Brunswick, in Germany; 10 mgr. were enclosed in a small box of ebonite with a mica cover having a diameter of 20 mm. This was put down in a Dewar's tube with vacuum jacket, as is commonly used in experiments with liquid air, and held in place by a stopper of cotton wool. The tube was then turned upside down in a little dish with some mercury, so as to obtain a perfectly enclosed space, and the radium rays could only get out by the vacuum walls or through a thick layer of mercury; by taking enough of this dense liquid the escape may be stopped altogether. Putting now a charged sensitive gold leaf electroscope at a distance of 5 cm. from the tube, a leakage instantly sets in, so as to cause the instrument to be wholly discharged in fifteen seconds. I also tried a vacuum jacketed tube with silvered walls, but though this affords much better protection against the heat rays, I did not detect any considerable difference with regard to the former experiment; the discharge was almost as quick, demonstrating that radium rays are not reflected to an appreciable amount. Even when the radium bromide was put into a large Dewar's silvered balloon of 5 litres capacity, wrapped in cotton wool, and enclosed in a wooden case, in which liquid air would be preserved during more than a fortnight, the charged electroscope came to zero in half a minute when it was placed very near to it. The experiments are effective and easily arranged.