Note on Gravitation. 143 



equal and opposite total charge of uniform surface density 

 over the surface. If this rotates uniformly in a periodic 

 time t the magnetic field at external points is the same as 

 that of a small magnet along the axis of rotation having a 

 magnetic moment 



47rBM- DO 



//, = — Jtt-, 



n 15 CT 



where M is the mass and E the radius of the sphere and c is 

 the velocity of light. Putting B = 10" 6 E! and the values of 

 M and R for the earth, this gives the magnetic intensity at 

 the pole as 5*9 x 10 ~ 12 . 



One positive conclusion which may be drawn from these 

 notes is that the displacement of spectral lines as between 

 isotopes cannot be regarded as furnishing evidence of the 

 existence of cross terms in the law of force between electric 

 charges and mass elements. Its explanation is probably to 

 be sought in an electrostatic effect of some variation in the 

 geometrical structure of the elements of the nucleus. 



Appendix by Dr. L. Simons. 



The apparatus consisted of a heavy brass box of internal 

 •dimensions 34 X 11 x 2"5 cm., the lid, resting on three screws, 

 being adjustable. A silica framework consisting of two 

 squares of 10 cm. side were mounted coplanar with their 

 centres at a distance of 20 cm. apart, and a silica cross-piece 

 arranged to carry brass ferrules at its ends through which 

 there passed pointed steel screws which rested on steel 

 planes. Aluminium foil 0'002 cm. thick was fixed over the 

 two squares, and the whole balanced symmetrically within 

 the box which was lined with the same aluminium in order 

 to avoid irregularities arising from contact e.m.f. The total 

 mass of the moving parts of the balance was approximately 

 4 gm. The sensibility could be very finely adjusted either 

 by means of the weights (w) or by means of the screw 

 pivots. The motion of the balance could be observed by 

 means of a mirror attached to the cross-bar and a telescope 

 and millimetre scale at a distance of about 1 metre, the 

 -adjustment for zero being made w T ith the flag (/), which 

 was a short strand of silica movable by a lever from the 

 outside of the wooden box containing the apparatus. The 

 vanes could be charged from the outside via thin copper- 

 wires joining them to the brass ferrules, thence across the 

 steel pivots to the steel planes on which they rested, which 

 in turn were connected to terminals outside the box. 



