Unipolar Induction. 189- 



solenoidal. But free aether has no such means of self- 

 defence, and consequently all forces acting on it must them- 

 selves be solenoidal. 



The moving-line theory seems therefore to be definitely 

 disposed of by the results obtained by Barnett, Fehrle, and 

 the author. 



§ 8. Conclusion. 



The practical bearing of these experiments is small, yet 

 they do necessitate a correction of certain statements that are 

 common in the textbooks. For instance, it is not correct to 

 say that the effect of rotating the armature of a dynamo is 

 the same as that of rotating the field-magnets in the opposite 

 direction. The total E.M.F. is the same, but in the first case 

 it is developed almost entirely in the longitudinal parts of 

 the winding, while in the second case a large fraction of it is 

 developed in the radial parts, and the distribution of electri- 

 fication on the armature will be different. 



The most interesting case theoretically is that where the 

 solenoid and condenser rotate together at the same speed. 

 The charging up of the condenser cannot be conditioned by 

 rotation relative to the connecting wires and electrometer. 

 For suppose that the sliding contacts had been exactly on the 

 axis, which was nearly true, and that they were connected 

 by an axial wire extending through the condenser so as to 

 form a closed circuit. Then it is certain, by Faraday's law,, 

 that rotation of the connecting wires and electrometer would 

 cause no deflexion of the latter ; and this negative result can 

 hardly be due to an E.M.F. in the axial wire which was 

 added. Rotation relative to the earth might in some unknown 

 way be responsible for the observed effect, but this seems 

 improbable. If we reject this assumption, then the effect is 

 due to an absolute rotation in the same sense in which the 

 operation of a gyrocompass is due to an absolute rotation of 

 the earth. 



But rotation of the whole is essentially only translation of 

 the parts. We must suppose each electron in the condenser 

 to experience a radial force proportional to its distance from 

 the axis ; and it is interesting to inquire what can cause such 

 a force. It cannot be due either to the radial acceleration, 

 for that is proportional to the square of the distance, nor to 

 a possible rotation of the electron about its axis, for that 

 should be the same for all electrons, nor to their motion 

 relative to other essential parts of the apparatus, for there is 

 no such motion. Apparently the only remaining alternative 

 is to ascribe the force to motion of the electrons relative to 



