Motion of the Earth and Ether, suggestedby the late Professor FitzGerald. 383 



relative motion of the Earth and the ether by means of the interference 

 of light. 



The explanation of this, which he had himself, simultaneously with Lorentz, 

 suggested, was that matter altered in linear dimensions according to the direction 

 of motion through the ether, and to an extent to just neutralize the calculated 

 optical effect. From some such cause a diminution of the electrostatic energy 

 might be brought about when the condenser was in the edge-wise position, just 

 sufficient in amount to provide the energy required for the magnetic field. 



II. — On the Possibility of obtaining a turning Force from the Motion op 

 THE Earth through Space. 



If we suppose the energy for the magnetic field between the plates of 

 a charged condenser when moving " edgewise " through the ether to be 

 supplied from the same source as that wliich charges the condenser, we are led to 

 the curious conclusion, that a charged condenser tends to set itself flatways 

 to the direction of motion though the ether. This may be easily understood from 

 the following considerations. 



A condenser charged when moving "■ edgewise " has more enei'gy to be 

 supplied it than when moving " flatwise." Now suppose a condenser which 

 is placed with its plates "flatwise" to the motion through the ether, to be first 

 charged and insulated, then turned round till it is standing with its plates 

 "edgewise" to the motion through the ether. The extra energy belonging 

 to the magnetic field, whicli we sujipose to now exist, must come from work done 

 in turning it round tlirough the 90^. That is to say, the condenser resists with, or 

 exerts, a couple in the opposite direction to the turning motion. 



Thus we see, on the assumption made above, that charged condensers must 



tend to set themselves at right angles to the direction of motion through 



the ether. When placed exactly " edgeways " to the motion, there is unstable 



equilibrium ; but when inclined to either side, rotation to the position at right 



angles tends to take place indiscriminately as to the sign of the charges on the 



plates. This follows at once from the energy of the magnetic field depending on 



the squares of the directed quantities in question. The value of the torque 



increases with the angle of inclination, till it reaches its maximum half way 



between the two positions of " edgeways " and right angles to the motion through 



the ether, at each of which it is zero. For if <^ is the angle of inclination of the 



plates of the condenser to the direction of motion through the ether, the 



velocity of flow parallel to the plates is u cos <^. Thus the energy of the 



magnetic field is in this case T = ^jjlKF Ehi- cost <f), and the couple at any angle 



dT 

 <j} is — j-tOV ^jjlKPE'u^ sin 2(f), this obviously has zero value for values 0° and 



90°, and a maximum value for eft = 45^. 



TKANS. EOY. DUB. SOC, N.S., VOL. VH., PABT XIV. 2 I 



