
—— ee 
= ==se * 

Radium in the Electric Field, 305 
field. Thus (a) we might ascribe it to the presence of elec- 
trified particles or ions s of both signs in the region between 
the radium and the fixed electrified body. We may suppose 
these ions are not strongly attracted to the vane carrying the 
radium, and more abundant in front than at the rear of the 
vane. Then if a plus charge is presented opposite the radium 
the positive particles are “repelled against it, the negative 
withdrawn. The first effect produces mechanical repulsion 
of the vanes, the second a relatively small force in the oppo- 
site sense. A minus charge acts in the same manner, the 
repulsive force being now caused by the negative ions. If 
the charged body is behind the disix, a mechanical force acting 
in the same sense as before is occasioned by the attractions, 
the repulsions being now ineffective. 
(b) The presence of the charged body induces a charge of 
opposite sign upon the radium, and repels a charge of like 
sign to the remote side of the disk. The first ( attractive) 
charge rapidly dissipates in virtue, chiefly, of the ionizing 
influence of the a-rays, the second (r epellent) charge remains 
to exert a mechanical effect upon the vane. Too intense a 
charge may cause the attractive effect to predominate. 
Some experiments were made to decide among these 
possible sources of undirectional motion. Five milligrammes 
of radium bromide were divided between two thin metal 
disks, 12 mms. in diameter, which were then attached 
in a vertical plane at the extremities of the beam of the 
Coulomb’s balance and in the plane of the beam. Fine 
aluminium wires were brought from the vanes nearly to the 
centre of the beam. The beam was a drawn glass tube, and 
the suspension was a fine quartz fibre. A U-shaped rider 
with two hooks, suspended from the beam at its centre, 
served to place the disks in metallic connexion. Removal of 
the rider disconnected them. A second fixed metal sphere 
was added to the balance diametrically opposite the usual 
one, and all was adjusted so that when the torsion-head was 
turned the disks anki be brought up to the spheres, each pre- 
senting its radium-coated surface to one of the spheres. The 
spheres could be charged from without either singly or con- 
nected together. Drying material was placed in the balance. 
If explanation (a) is correct, 7. e. that the effect is due to 
the pressure of electrified particles, then whether the vanes 
are electrically connected one with another or not should 
make no difference in the repulsion of the vanes. It was 
found, however, that when one of the spheres is electrified 
and the vanes are connected there was no repulsion, but, on 
the contrary, a brisk attraction, whatever the sign of the 
