Page:Popular Science Monthly Volume 67.djvu/14

Observer.	Value of Velocity.	Value of e/m.
Rutherford	$\scriptstyle 2.5\times 10^{9}$ cms. per sec.	$\scriptstyle 6\times 10^{3}$ electromagnetic units.
Des Coudres	$\scriptstyle 1.6\times 10^{9}$ cms. per sec.	$\scriptstyle 6\times 10^{3}$ electromagnetic units.

Now the value $e/m$ for the hydrogen atom is 10⁴. On the assumption that the $\alpha$ particle carries the same charge as the hydrogen atom, this result shows that the apparent mass of the a particle is about twice that of the hydrogen atom. If the $\alpha$ particle consists of any known kind of matter, this result indicates that it is the atom either of hydrogen or of helium. The $\alpha$ particles thus consist of heavy bodies projected with great velocity, whose mass is of the same order of magnitude as the helium atom and at least 2,000 times as great as the apparent mass of the $\beta$ particle or electron.

If the a particles carry a positive charge, it is to be expected that the particles, falling on a body of sufficient thickness to absorb them will, under suitable conditions, give it a positive charge, while the substance from which they are projected acquires a negative charge. The corresponding effect has been observed for the $\beta$ rays. The $\beta$ particles from radium communicate a negative charge to the body on which they fall, while the radium from which they are emitted acquires a positive charge. This effect has been very strikingly shown by a simple experiment of Strutt. The radium compound, sealed in a small glass tube, the outer surface of which is made conducting, is insulated by a quartz rod. A simple gold leaf electroscope is attached to the bottom of the glass tube, in order to indicate the presence of a charge. The whole apparatus is enclosed in a glass vessel, which is exhausted to a high vacuum, in order to reduce the loss of charge in consequence of the ionization of the gas by the rays. Using a few milligrams of radium bromide, the gold leaf diverges to its full extent in a few minutes and shows a positive charge. The explanation is simple. A large proportion of the negatively charged particles are projected through the glass tube containing the radium and a positive charge is left behind. By allowing the gold leaf, when extended, to touch a conductor connected to earth, the gradual divergence of the leaves and their collapse becomes automatic and will continue, if not indefinitely, at any rate for as long a time as the radium lasts.

When the radium is exposed under similar conditions, but unscreened in order to allow the a particles to escape, no such charging action is observed. This is not due to the equality between the number of positively and negatively charged particles expelled from the radium, for no effect is observed when the radium is temporarily freed from its power of emitting $\beta$ rays by driving off the emanation by heat. The writer recently attempted to detect the charge carried by the a rays from radium by allowing them to fall on an insulated